1EUKELEY 

LIPRARY 

UNIVERSITY   OP 
CALIFORNIA 


EARTH 

SCIENCES 

LIBRARY 


WILLIAM   DILLER   MATTHEW 


GIFT   OF 
WILLIAM  DILLER  MATTHEW 


THE 

EVOLUTION 

OF  THE 

VERTEBRATES 

AND 

THEIR  KIN 


PATTEN 


THE 

EVOLUTION 

OF   THE 

VERTEBRATES 

AND 

THEIR  KIN 


BY 

WILLIAM   PATTEN,  PH.  D. 

PROFESSOR    OF    ZOOLOGY,    AND    HEAD    OF    THE    DEPARTMENT    OF    BIOLOGY 
IN    DARTMOUTH    COLLEGE,    HANOVER,    N.    H. 


WITH  309  ILLUSTRATIONS 


PHILADELPHIA 
P.  BLAKISTON'S  SON  &,  CO. 

1012  WALNUT  STREET 
1912 


NO. 


COPYRIGHT,  1912,  BY  P.  BLAKISTON'S  SON  &  Co. 


Printed  by 

The  Maple  Press 

York,  Pa. 


INTRODUCTION. 


It  is  many  years  since  a  sustained  attempt  has  been  made  to  unite  the  various 
branches  of  the  animal  kingdom  into  a  natural,  coherent  system,  or  genealogical 
tree,  that  would  indicate  the  rise  and  decline  of  the  important  functions  and 
organs,  map  out  the  highways  of  organic  evolution,  and  assign  in  geological  terms 
the  approximate  dates  and  surroundings  for  the  critical  events  in  structural  in- 
novation and  readjustment  that  have  marked  its  progress. 

The  most  important  problem  involved  in  such  an  undertaking  is  to  discover 
which  one,  if  any,  of  the  many  existing  invertebrate  phyla  forms  the  trunk  line  of 
descent  from  the  lowest  vertebrates  to  the  ccelenterates,  and  through  them  to  the 
protozoa. 

The  vertebrates  abruptly  make  their  appearance  as  fully  formed  fishes,  at 
the  close  of  the  Silurian  or  the  beginning  of  the  Devonian.  They  were  evidently 
more  highly  organized  than  any  of  the  invertebrate  types  that  had  appeared  up  to 
that  time,  and  they  must  have  arisen,  either  by  a  marked  transformation  of  some 
of  the  known,  preexisting  types,  or  from  some  extinct  and  totally  unknown  ones. 
On  either  supposition,  the  apparent  absence  of  transitional  forms  is  surprising, 
since  the  relatively  large  size,  distinctive  form,  and  well  developed  skeleton  of 
primitive  vertebrates,  under  the  known  conditions,  should  leave  behind  some 
recognizable  traces  of  their  predecessors  in  the  form  of  fossils. 

The  real  missing  links  in  the  graded  series  of  animal  forms  that  most  concern 
the  morphologist  belong,  therefore,  to  the  Silurian  period.  There  the  main 
trunk  of  the  animal  kingdom,  upon  which  the  whole  vertebrate  stock  rests,  is 
lost,  leaving  without  reason  or  warning  a  vast  unknown  abyss  beside  which  the 
gap  between  man  and  his  immediate  predecessors  sinks  into  microscopic  in- 
significance. 

On  the  one  side  are  the  vertebrates,  including  a  long  series  of  animals,  from 
the  lowest  fishes  to  man.  All  of  them  agree  in  their  fundamental  plan  of  struc- 
ture and  mode  of  development;  the  principal  organs  of  any  member  of  the  series 
may  be  surely  identified  in  the  others,  and  the  general  trend  of  evolution  in  the 
phylum  is  clearly  indicated.  The  fishes,  for  example,  are  the  lowest  members  of 
the  series,  and  they  are  followed  by  the  amphibia,  reptiles,  and  mammals.  Com- 
parative anatomy  shows  the  gradual  evolution  of  form  and  structure  in  this 
series  as  a  whole,  and  its  evidence  is  corroborated,  in  the  main,  by  the  embryonic 
development  of  any  member  of  the  series,  while  the  geologic,  or  historic  record 
harmonizes  with,  and  confirms  the  testimony  of  the  other  two.  In  fact,  the 
vertebrates  clearly  constitute  a  common  stock,  a  single  phylum  of  the  animal 


730&05 


VI  INTRODUCTION. 

kingdom.  It  has  many  side  branches,  it  is  true,  but  comparative  anatomy, 
embryology,  and  paleontology  are  in  substantial  agreement  as  to  what  kind  of 
animals  and  what  organs  and  functions  came  first  in  time,  what  were  the  most 
highly  developed,  and  what  was  the  general  trend  of  evolution. 

Even  the  simplest  vertebrates,  that  stand  at  the  beginning  of  this  long 
series,  were  very  highly  organized  animals,  for  all  the  fundamental  systems  of 
organs  well  known  to  us  in  man,  such  as  the  sensory,  nervous,  skeletal,  circulatory, 
and  excretory,  were  there  fully  established  and  highly  efficient.  But  there  is  little  in 
the  structure  or  development  of  these  organs  that  gives  us  any  positive  information 
as  to  their  previous  history,  condition,  or  origin,  the  very  information  essential 
to  a  true  understanding  of  their  meaning. 

On  the  other  hand,  when  we  look  below  the  vertebrates  for  the  main  highway 
of  evolution,  we  are  bewildered  by  the  multiplicity  of  doubtful  trails  that  appear 
to  have  neither  beginning  nor  end,  that  lead  as  readily  in  one  direction  as  another. 
Each  invites  us  onward;  but  if  followed,  suspicion  soon  grows  to  conviction  that 
we  have  been  deceived,  that  some  other  road  is  after  all  the  right  one. 

The  familiar  cry,  "This  way,"  "I  have  it,"  that  rose  when  an  enthusiastic 
pioneer  struck  the  annelid,  tunicate,  balanoglossus,  or  some  other  promising  trail, 
would  for  a  time  rouse  great  expectations;  but  it  always  ended  in  disappointment, 
and  gradually  created  an  attitude  of  indifference,  and  the  feeling  that  the  solution 
of  this  great  problem  was  forever  beyond  our  reach.  The  conviction  grew  that 
one  or  more  large  classes  of  animals  that  once  constituted  the  living  trunk  of 
the  genealogical  tree  during  Silurian,  or  pre-Silurian  times,  were  entirely  extinct, 
and  had  left  no  traces  whatever  behind. 

With  the  historic  record  of  the  most  important  period  in  the  evolution  of  the 
higher  animals  completely  destroyed,  the  problem  did  indeed  appear  hopeless. 
What  was  lacking  in  actual  records  was  supplied  by  the  speculative  biologists, 
and  they  did  their  work  so  well,  and  reiterated  it  so  often,  that  it  finally  passed  for 
the  truth,  and  its  central  idea,  that  the  vertebrates  had  their  origin  in  the 
annelids,  in  some  more  or  less  roundabout  way,  became  a  dogma. 

We  then  witnessed  the  interesting  phenomenon,  common  enough  but  always 
profitable  to  contemplate,  as  lightning  that  strikes  near  by,  that  those  who  would 
not  make  the  annelid  theory  a  part  of  their  creed,  and  who  continued  the  search 
in  other  directions  for  a  substantial  body  of  facts  to  build  upon,  were  branded 
as  morphological  heretics  and  speculators,  or  as  the  victims  of  a  too  vivid  imagina- 
tion; and  it  was  always  the  most  "orthodox"  and  persistent  speculator  that 
waved  the  hottest  brand. 

But  when  the  annelid  dogma  passed  the  period  of  productivity  without  off- 
spring, even  the  orthodox  biologists  lost  all  hope  of  solving  the  real  problem  of  the 
origin  of  vertebrates,  as  well  as  many  other  large  problems  in  invertebrate  phy- 
logeny,  and  turned  their  attention  toward  the  Eldorado  of  cytology,  heredity, 
and  experimental  evolution,  where  "results"  were  easy  and  promised  to  carry  far. 

Sterility  has  often  turned  devotion  to  contempt,  and  it  is  not  suprising  that  the 


INTRODUCTION.  Vll 

biologist,  whose  theories  were  unfruitful,  wrecked  his  wrath  on  the  temple  of 
morphology  and  condemned  its  triune  god  to  the  consolation  of  his  more  credu- 
lous colleagues;  "Paleontology,"  he  cried,  "is  mute,  Comparative  Anatomy 
meaningless,  and  Embryology  lies." 

But  perhaps  the  fault  was  ours.  We  did  not  understand,  because  of  igno- 
rance and  over  confidence.  It  is  not  fifty  years  since  the  doctrine  of  evolution  has 
been  generally  recognized,  and  during  the  latter  half  of  that  period  surprisingly 
little  persistent,  or  concerted  work  has  been  done  on  the  larger  problems  of 
phylogeny,  and  there  is  but  little  to  justify  the  too  common  attitude  that  the  pos- 
sibilities of  morphology  are  exhausted.  Much  disconnected  fragmentary  work 
has  been  done,  but  how  little  is  known  about  the  evolution  of  any  one  organ  or 
system  of  organs;  how  very  few  animals,  if  indeed  there  are  any,  whose  structure, 
development,  and  paleontological  record  are  known  with  even  approximate 
fullness  or  accuracy.  What  large  class  of  animals  is  not  separated  from  its  next 
of  kin  by  a  gap  too  wide  to  be  bridged  by  any  known  forms  ?  Are  these  gaps  due 
merely  to  a  hiatus  in  the  available  records,  or  in  our  knowledge  of  them,  or  are 
they  realities,  representing  periods  of  unusually  rapid  transformation  due  to 
sudden  changes  in  the  methods,  or  conditions  of  growth?  If  the  gaps  between 
the  vertebrates  and  ostracoderms,  and  the  ostracoderms  and  arachnids  appear  to 
be  wide  ones,  are  they  really  any  wider  than  those  between  the  fishes  and  amphibia, 
the  reptiles  and  mammals,  or  the  ccelenterates  and  arthropods  ?  Are  not  the 
evidences  of  genetic  relationship  of  the  same  nature  and  value  in  one  case  as  in 
the  other  ?  Is  not  the  paleontological  record  more  precise  and  complete  than  we 
have  supposed  ?  Will  not  embryology  be  less  enigmatic  under  a  new  interpreta- 
tion ?  If  the  arachnids  are  indeed  the  next  of  kin  to  the  ostracoderms,  and  through 
them  to  the  vertebrates,  is  that  after  all  so  incredible  ?  With  this  gigantic  column  in 
position,  will  not  the  remaining  branches  readily  fall  into  their  natural  positions, 
and  the  entire  genealogical  tree  of  the  animal  kingdom  take  on  the  convincing 
symmetry  and  coherency  of  reality,  of  a  living,  growing  organism  that  contains 
the  story  of  its  own  creation  ? 

These  are  some  of  the  problems  bound  up  in  the  evolution  of  the  vertebrates. 
Clearly  it  is  not  merely  a  question  of  constructing  a  convenient  and  more  or  less 
satisfactory  genealogy  of  the  animal  kingdom.  The  whole  philosophy  of  creative 
evolution  is  involved  in  the  answer.  We  must  face  these^  problems  fairly,  without 
prejudice  and  without  arrogance  (surely  the  record  of  past  achievements  affords 
no  grounds  for  that  attitude),  and  with  a  full  recognition  of  their  significance. 
Facts  are  stubborn  things  that  will  not  be  ignored,  that  call  out  for  recognition, 
and  for  their  proper  location  in  a  well  ordered  scheme,  if  not  in  one,  then  in  some 
other  that  is  better. 

The  problem  is  of  the  utmost  importance  to  the  biologist,  for  the  answer 
should  determine  the  location  of  several  large  classes  of  animals,  now  completely 
isolated;  it  will  enable  us  to  reconstruct  the  broad  outlines  of  the  genealogical 


Vlll  INTRODUCTION. 

tree  of  the  animal  kingdom  where  the  main  branches  emerge  from  the  darkness 
of  the  pre- Cambrian  period;  it  will  furnish  us  the  only  means  by  which  we  can 
hope  to  solve  some  of  the  most  important  problems  in  vertebrate  morphology, 
such  as  the  meaning  of  vertebrate  cephalogenesis,  of  concrescence,  germ  layers, 
gastrulation,  and  the  structure  of  the  oldest  fossil  representatives  of  the  vertebrate 
series.  The  answer  to  such  problems  cannot  be  found  till  after  we  have  dis- 
covered the  immediate  ancestors  of  the  vertebrates  and  the  broad  outlines  of  their 
structure,  for  when  the  point  of  departure  is  determined,  and  only  then,  can  we 
determine  which  is  the  base  and  which  is  the  summit  of  a  series  of  changes,  which 
the  primitive,  which  the  derived;  in  short,  the  direction  in  which  evolution  is 
moving. 

The  arachnid  theory  of  the  origin  of  vertebrates  has  made  slow  progress. 
This  is  not  surprising  since  it  has  had  to  contend  against  the  fixed  ideas  of  the 
specialist  working  in  some  narrow  field  of  vertebrate  or  invertebrate  morph- 
ology, and  who  is  unfamiliar  with  the  multitude  of  facts  and  details,  intricate  in 
themselves  and  in  their  bearings,  upon  which  the  arachnid  theory  rests.  It  has 
also  had  to  contend  against  the  indifference  of  the  newer  school  of  biologists,  who 
look  on  morphology  as  an  exhausted  field,  and  who  attach  an  exaggerated  import- 
ance to  experimental,  or  statistical  work,  or  to  the  minute  structure  of  cells,  or  to 
the  analysis  of  protoplasm. 

This  is  largely  due  to  a  common  misconception  of  the  real  aim  of  the  mor- 
phologist;  for  it  is  evident  that  tracing  the  identity  of  structure  under  the  disguise 
of  new  forms  is  only  the  beginning  of  the  morphologist's  work.  His  real  problem 
is  to  measure  the  rate  of  these  changes,  and  to  seek  out  the  underlying  causes. 
Hence,  a  great  morphological  problem,  such  as  the  origin  of  vertebrates,  is  essen- 
tially a  problem  in  experimental  evolution,  an  experiment  performed  on  the  largest 
scale  of  any  in  the  history  of  organic  evolution.  But  here  the  problem  presents 
itself  in  a  different  form  from  the  ordinary  laboratory  experiment.  There  the  ex- 
perimenter fixes  the  conditions,  as  nearly  as  possible,  and  then  records  and  meas- 
ures the  events  as  they  appear.  Here  the  morphologist  records  and  measures  the 
events,  and  from  them  tries  to  discover  the  conditions.  I  believe  I  have  discovered 
the  main  events  in  this  experiment  of  Nature,  and  I  have  recorded  it,  in  terms  of 
systematic  zoology,  in  a  genealogic  tree  of  the  great  arthropod-vertebrate  stock. 
This  discovery  enables  us  to  see  clearly  some  of  the  factors  that  have  brought 
about  the  results.  They  are  mainly  internal  factors,  insignificant  in  themselves, 
but  acquiring  such  immense  transforming  power  by  persistent  and  prolonged 
action  that  it  is  unnecessary  to  invoke  the  agency  of  such  factors  as  external 
environment,  natural  selection,  and  heredity.  At  most,  it  seems  to  me,  these 
factors  can  account  only  for  the  superficial  details  of  the  essentially  completed 
body.  Morphology  teaches  us  that  the  foundations  of  anatomical  structure  are 
automatically  created  by  the  processes  of  growth  and  organic  readjustment,  and 
that  they  remain  essentially  unmoved  by  external  conditons. 

For  almost  a  quarter  of  a  century  the  problem  of  the  evolution  of  the  verte- 


INTRODUCTION.  IX 

brates  has  been  to  me  a  stimulus  and  a  guide.  What  appears  to  be  an  approxi- 
mate solution  of  it  has  been  tested  and  tested  again,  and  elaborated — in  itself  the 
severest  test  of  all — by  many  methods  and  from  many  points  of  view,  for  it  has 
seemed  to  me  the  one  great  problem  that  must  be  solved  before  the  biologist  can 
approach  the  problems  of  creative  evolution  on  a  reasonably  secure  footing.  To 
gain  this  end,  I  have  given  the  best  I  had;  whether  that  is  much  or  little  is  of  no 
consequence,  except  in  so  far  as  it  is  a  guarantee  of  serious  endeavor  and  of  good 
faith.  That  I  am  conscious  of  many  difficulties  and  imperfections  need  not  be 
emphasized.  I  would  gladly  make  them  less.  But  to  be  overconscious  of  the 
one,  unsteadies  the  hand  and  draws  the  eye  away  from  the  open  waters,  and  too 
long  a  delay  over  the  inevitable  defects  means  to  be  surprised  by  the  night,  and 
still  unprepared. 


CONTENTS. 


PAGE 

INTRODUCTION  ......    ......................... 


HISTORICAL  SKETCH     ............................        jfe»^»  /v  if 

LIST  OF  AUTHOR'S  PAPERS    .....................    ...   .  -*i  X/ 

CHAPTER  I.  xiv 

OUTLINE  OF  THE  ARACHNID  THEORY    .    ....    .   .   .    .    .   .    .    ......   .   .         1-26 

I.  Its  scope  and  relation  to  other  theories,  i.  II.  Nature  of  the  evidence  to  be  pre- 
sented, 3;  A.  cephalogenesis  in  arthropods,  3;  B.  embryology,  4;  C.  arachnid 
cephalogenesis  prophetic  of  the  vertebrate  head,  5;  D.  paleontology.  III.  The 
process  of  cephalization  in  the  arthropods,  7  ;  A.  The  grouping  and  the  increase  in 
number  of  metamers,  7;  B.  origin  of  the  linear  arrangement  of  unlike  cephalic 
functions,  8.  IV.  The  subdivisions  of  the  incipient  vertebrate  head  in  arachnids, 
ii  ;  mesoderm,  12.  i.  The  procephalon,  12;  insects,  13;  arachnids,  13;  sense 
organs,  13;  olfactory  lobes,  hemispheres  and  optic  ganglia,  13;  rostrum,  14;  exter- 
nal boundaries  of  the  procephalon  in  the  adult,  14.  2,  3.  The  dicephalon  and  the 
mesocephalon,  15;  endocranium,  16;  oral  arches,  16;  taste  buds,  slime  buds  and 
cranial  ganglia,  17;  segmental  sense  organs,  17;  the  diencephalon  and  the  mesen- 
cephalon,  18;  the  suprastomodaeal  commissure  and  the  cerebellum,  19.  4.  The 
metencephalon,  or  vagus  region,  19;  vagus  appendages,  19;  vagus  neuromeres,  19; 
vagus  nerves,  20.  5.  The  branchiocephalon,  20;  mesoderm,  20;  neuromeres,  21; 
nerves,  21;  the  endocranium,  21;  the  mesoderm,  22;  comparison,  23;  the  vascular 
area  and  concrescence,  24;  the  new  mouth,  cephalic  navel  or  haemostoma,  25;  the 
closure  of  the  old  mouth  or  neostoma,  26;  conclusion,  26. 

CHAPTER  II. 

OUTLINE  OF  THE  ARACHNID  THEORY;  CONTINUED     ..............       27-40 

I.  Comparison  of  adult  arthropods  with  adult  vertebrates,  27.  I.  Orientation  of 
neural  and  haemal  surfaces,  27.  II.  Comparison  of  adult  arthropods  and  verte- 
brates, 29;  bunodes,  29.  III.  Comparison  of  arthropod  and  vertebrate  embryos, 
33;  form  controlling  factors  in  the  early  stages,  34;  the  gustrula,  ccelenterate,  or 
trochosphere  stage,  34;  transition  from  radiate  to  bilateral  symmetry,  35;  telopore, 
35;  germ  wall,  35;  concrescence  of  the  germ  wall,  35;  the  nervous  system,  38;  the 
primary  sense  organs,  38;  cornua3  38;  vertebrate  stages,  38;  the  auditory  pit,  39; 
the  heart,  39;  cornua,  40;  the  oral  arches  and  the  haemostoma,  40;  cranial  flex- 
ure, 40. 

CHAPTER  III. 

EVOLUTION  OF  THE  NERVOUS  SYSTEM  IN  SEGMENTED  ANIMALS      ........       41-52 

I.  Meaning  of  the  term  brain,  41.  II.  The  sternodceal  nerves,  42.  III.  The 
framework  of  the  nervous  system,  43.  IV.  The  differentiation  of  the  peripheral 
nerves,  45;  factors  that  modify  the  arrangement  of  peripheral  nerves,  46;  segrega- 

xi 


Xll  CONTENTS. 

PAGE 

tion  of  nerve  fibers,  46;  elimination,  47;  union,  historic  factor.  V.  Neuromeres 
and  metamarism,  49.  VI.  Primitive  sense  buds,  50. 

CHAPTER  IV. 

THE  SUBDIVISIONS  OF  THE  BRAIN 53-70 

I.  The  prosencephalon,  or  forebrain,  53;  the  procephalic  lobes,  division  into  three 
metameres;  acilius,  the  hemispheres,  optic  ganglia  and  ocelli;  arachnids,  54;  the 
olfactory  lobes  of  the  first  segment;  the  hemispheres  of  the  second;  the  forebrain 
flexure.  II.  The  diencephalon,  57;  diencephalic  flexure,  cheliceral  nerves  and 
ganglia,  minute  structure  in  limulus,  58;  the  large  basal  association  neurones, 
the  large  association  neurones  on  the  hemispheres,  the  cortex  neurones;  the  cheli- 
ceral lobes;  the  forebrain  and  cheliceral  commissures;  the  stomodseal  ganglia  and 
the  supra-stomodseal  commissure,  60;  nerves  to  labrum;  association  with  the  coxal 
taste  organs;  comparison  with  vertebrates,  60;  summary,  64;  III.  The  mesen- 
cephalon,  65;  the  oral  and  hyoid  arch  neuromeres;  mesenccele,  66;  comparison  of 
thoracic  neuromeres  of  arachnids  with  the  midbrain  neuromeres  of  vertebrates,  66. 
IV.  The  metencephalon,  or  vagus  neuromeres,  and  V.  the  branchiencephalon, 
67;  the  vagus  zone  in  arthropods,  its  special  sensory  characters;  the  branchience- 
phalon, 69;  the  branchiencephalic  neuromeres,  number,  segregation  of  their  nerves 
into  branchial,  cardiac,  intestinal,  hypobranchial,  or  branchiothoracic,  70. 

CHAPTER  V. 

MINUTE  STRUCTURE  or  THE  BRAIN  AND  CORD  OF  CERACHNIDS 7I-93 

Methods,  71.  I.  The  branchial  neuromeres  of  limulus,  71;  development,  71; 
commissures,  72;  peripheral  nerves,  72;  cell  clusters,  73;  nerve  roots,  76;  branchial 
nerve  roots,  haemal  roots,  77;  commissures,  79;  the  neuropile  centers,  79.  II. 
The  cephalic  neuromeres,  80;  haemal  commissures,  81;  haemal  roots,  81;  cranial 
ganglia,  81;  motor  neurones,  84;  gustatory  nerves  and  tracts,  84.  III.  Longitudi- 
nal tracts,  85;  longitudinal  haemal  tracts,  85;  longitudinal  neural  tracts,  86;  the 
lateral  or  pedal  ganglion  tracts,  86;  the  general  cutaneous  tracts,  87;  comparison 
with  vertebrates,  87.  IV.  Commissures;  summary,  90.  V.  The  neuroccelia,  92. 
VI.  The  neuroglial  93, 

CHAPTER  VI. 

PERIPHERAL  NERVES  AND  GANGLIA 94-109 

I.  Components  of  a  neuromere,  94.  II.  Nerves  of  the  diencephalon  and  mesen- 
cephalon,  94;  A.  neural  nerves,  94;  the  flabellum,  94;  the  cranial  ganglia,  95;  the 
ganglia  of  the  cord,  97;  the  haemal  nerves,  97;  the  lateral  line  nerve  of  the  chele- 
ceral  neuromere,  97.  III.  Nerves  of  the  metencephalon,  98;  limulus,  neural  nerves, 
98;  chilarial,  opercular,  haemal  nerves;  scorpion,  101;  neural  and  haemal  nerves, 
longitudinal  abdominal,  104;  general  cutaneous,  105;  cardiac  and  hypobranchial 
longitudinal  abdominal,  104;  general  cutaneous,  105;  cardiac  and  hyppobranchial 
nerves.  V.  Relation  of  vagal  and  hypobranchial  nerves  in  arachnids  to  those  in 
vertebrates,  107. 

CHAPTER  VII. 

GENERAL  AND  SPECIAL  CTUANEOUS  SENSE  ORGANS 110-124 

I.  General  cutaneous  sense  organs,  no;  temperature  organs,  no;  free  nerve  ends, 
in.  II.  Special  cutaneous  sense  organs,  in;  the  gustatory  organs  of  limulus, 


CONTENTS.  Xlll 

PAGE 

in;  reactions  to  stimuli,  112;  structure,  113;  the  flabellum,  113;  the  branchial 
warts,  115;  the  slime  buds,  116;  the  auditory  organ,  120;  lateral  line  organs  of 
vertebrates;  summary,  121. 

CHAPTER  VIII. 

LARVAL  OCELLI  AND  THE  PARIETAL  EYE 125-148 

I.  The  different  kinds  of  eyes  in  arthropods  and  vertebrates,  125;  eyes  of  orthro- 
pods,  125;  larval  ocelli,  125;  frontal  ocelli  or  stemmata,  125;  parietal  eye,  lateral 
eyes,  cerebral  eyes  of  vertebrates,  126;  parietal  eye,  lateral  eye,  olfactory  organ, 
127.  II.  The  eyes  as  segmental  sense  organs,  127;  procephalic  and  thoracic  sense 
organs.  III.  The  larval  ocelli  of  insects,  128;  larval  ocelli,  stemmata.  IV.  The 
parietal  eye,  129;  the  parietal  eye  of  the  scorpion,  129;  the  parietal  eye  of  limulus, 
131;  development;  palial  fold;  epiphysis;  anterior  neuropore;  change  of  position; 
endoparietal  and  ectoparietal  eye;  nerves;  retinal  cells;  the  parietal  eye  of  bran- 
chipus,  136;  the  parietal  eye  of  apus,  138;  the  parietal  eye  of  vertebrates,  139; 
petromyzon,  140;  parietal  eye  vesicle,  140;  ganglia,  142;  asymmetry;  comparison 
with  arthropods;  the  lenses  of  the  parietal  eye,  144;  location  of  the  placodes,  146; 
minute  structure,  140;  summary,  147. 

CHAPTER  IX. 

THE  COMPOUND  EYES  or  ARTHROPODS  AND  THE  LATERAL  EYES  OF  VERTEBRATES  .  .  149-159 
I.  Compound  eyes  of  arthropods,  149;  A.  serial  location,  149;  B.  development, 
150.  II.  Lateral  eyes  of  vertebrates,  151;  location,  151;  origin  of  the  chroid  fis- 
sure and  the  blind  spot,  151 ;  the  retinal-cell  pattern,  153;  the  retinal  ganglion,  153; 
the  lens,  153;  origin  of  the  imperfections  of  the  vertebrate  eye,  154.  III.  The 
optic  ganglia,  154;  location,  154;  parietal  eye  ganglia,  155;  lateral  eye  ganglia, 
optic  lobes,  155;  minute  structure  in  limulus,  155.  IV.  Comparison  with  verte- 
brates, 157;  summary  and  conclusion,  158. 

CHAPTER  X. 

THE  OLFACTORY  ORGANS  OF  ARTHROPODS  AND  VERTEBRATES 160-173 

I.  The  olfactory  organ  of  limulus,  160;  structure  in  adult,  160;  gross  structure; 
minute  structure;  development  of  olfactory  organ  and  nerves,   162;  olfactory 
placodes;  lateral  olfactory  nerves;  median  olfactory  nerve;  summary,  163.     II. 
The  olfactory  lobes  of  arachnids,  164;  development;  scorpion  and  spiders;  limulus; 
development;  structure.     III.  The  olfactory  organs  in  phyllopods,  frontal  organs, 
p.  165;  branchipus,  p.  166;  apus,  167.     IV.  Comparison  of  olfactory  organs  of 
vertebrates  and  arthropods,  168;  i.  number  of  placodes,  168;  2.  number  of  nerves, 
169;  3.  structure  and  termination  of  the  nerves,  169;  4.  origin  of  the  olfactory 
ganglia,  170;  5.  position  of  placode  cells,  170;  6.  serial  location  of  the  placodes  and 
their  migration,  170;  7.  olfactory  lobes,  171;  8.  function,  171;  summary  and  con- 
clusion, 172. 

CHAPTER  XL 

FUNCTIONS  OF  THE  BRAIN 174-194 

Parti.  Introduction,  methods,  175;  description  of  experiments,  176-186.     Part 

II.  Summary  of  experimental  and  anatomical  results,  gustatory  reflexes,  structure 
of  gustatory  apparatus,  186;  the  nerve-muscle  chewing  apparatus,  188;  experi- 


XIV  CONTENTS. 

PAGE 

mental  results,  the  swallowing  reflexes,  189;  course  of  the  nerve  impulses  in  the 
gustatory,  chewing  and  swallowing  reflexes,  189.  II.  The  crossed  thoracic 
reflexes,  190.  III.  The  crossed  and  uncrossed  abdomino-thoracic  reflexes,  190. 
IV.  Locomotion,  191.  V.  Equilibrium,  191.  VI.  Respiration,  191;  respiratory 
reflexes,  192;  comparison  with  vertebrates,  193.  VII.  The  cerebral  hem- 
spheres,  194. 

CHAPTER  XII. 

THE  HEART 195-209 

I.  Location  of  the  heart,  195.  II.  The  development  of  the  heart,  195;  compari- 
son of  vertebrate  and  arachnid  heart.  III.  The  circulation,  198;  arachnids,  198; 
comparison  with  vertebrates,  198,  direction  of  blood  currents,  aortic  arches,  carotids, 
circle  of  Wellis,  aortae,  cardinals,  curvature  of  heart,  split  posterior  end,  reduction 
of  cardiac  area,  and  moulding  effect  of  blood  stream  on  structure  of  heart  walls. 

IV.  The  cardiac  nerves  and  ganglion,  200;  the  median  cord  or  ganglion,  200; 
the  cardiac  plexus,  the  lateral  cardiacs,  pericardials,  segmental  cardiacs.     V.  The 
minute  structure  of  the  cardiac  ganglion,  202;  small  multipolar  or  motor  cells, 
giant  bipolar  cells,  small  bipolar  cells,  motor  terminals,  205;  sensory  terminals, 
cardiac  ganglia  in  vertebrates.     VI.  Experiments  on  the  heart,  205.     VII.  Sum- 
mary and  conclusion,  208. 

THE  NERVOUS  SYSTEM  AND  SENSE  ORGANS  OF  VERTEBRATES  AND  ARACHNIDS.    GEN- 
ERAL SUMMARY  OF  CHAPTERS  I-XII 209-214 

CHAPTER  XIII. 

THE  EARLY  STAGES  OF  ARTHROPOD  AND  VERTEBRATE  EMBRYOS 215-248 

I.  Primary  causes  of  differential  growth,  215.  II.  Morphological  interpretation 
of  the  early  stages,  219.  III.  Embryology  of  limulus,  222;  i.  cleavage,  223;  com- 
parison with  vertebrates,  223;  2.  the  germ  disc  or  primitive  cumulus,  224;  3.  for- 
mation of  metameres,  225;  4.  the  gastrula,  227;  5.  the  germ  wall,  228;  6.  the  meso- 
derm,  230;  the  sources  and  kinds  of  mgsoderm:  (a)  procephalic,  (b)  postoral,  (i) 
axial  cord,  (2)  mesoblastic  somites,  (3)  lateral  plates;  the  fibre  cells,  232,  give  rise 
to:  (a)  inter-tergal,  branchial  section  of  branchio-thoracic,  and  numerous  scat- 
tered, muscles;  (b)  to  semi-amoeboid  wandering  cells  which  persist  in  adult  stages; 
Vascular  area,  236;  pellucid  area.  IV.  The  cephalic  navel,  dorsal  organ,  or  neos- 
toma,  238.  V.  Concrescence  and  the  caudal  navel  or  blastopore,  243. 

CHAPTER  XIV. 

THE  OLD  MOUTH  AND  THE  NEW;  LOCOMOTOR  AND  RESPIRATORY  APPENDAGES   .    .   249-273 
The  salient  features  of  the  mouth  and  appendages  in  arthropods  and  vertebrates, 
249;  the  argument,  250.     I.  The  closing  of  the  old  mouth,  251.     II.  The  new 
mouth,  253.     III.  The  jaws  or  oral  arches,  255;  development  of  the  oral  arches 
in  the  frog,  257;  conclusion,  260.     IV.  The  gill  arches  and  the  external  gills,  261. 

V.  The  gill  sacs,  the  thyroid  and  the  thymus,  263.     VI.  The  gut  pouches,  266. 
VII.  The  locomotor  appendages,  263;  conclusion,  271. 

CHAPTER  XV. 

VARIATION  AND  MONSTROSITIES 274-288 

Problem  stated.  I.  Invaginated  appendages,  275.  II.  Asymmetry,  276.  III. 
Degeneration,  277;  A.  median  fusion  and  antero-posterior  degeneration,  277; 


CONTENTS.  XV 

PAGE 

B.  hour-glass  embryos,  279;  C.  acephalic  and  acaudal  embryos,  280;  D.  final 
stages  of  degeneration,  281;     IV.  Double  embryos,  281.     V.  Triple  embryos, 
284.     VI.  Summary  and  conclusion,  287. 

CHAPTER  XVI. 

THE  DERMAL  SKELETON 289-305 

The  five  kinds  of  skeletal  structures  in  arthropods  and  vertebrates,  289.  I.  The 
dermal  skeleton  of  vertebrates,  289;  the  minute  structure  of  the  dermal  bones  in 
the  ostracoderms,  290;  tremataspis,  290;  pteraspis,  293;  ateleaspis,  295.  II. 
Dermal  skeleton  of  limulus,  296;  minute  structure,  297.  III.  Summary  and  com- 
parison, 302. 

CHAPTER  XVII. 

THE  ENDOCRANIUM,  BRANCHIAL  AND  NEURAL  CARTILAGES 306-322 

I.  The  endoskeleton  of  arachnids,  306.  II.  The  neural  arches,  307.  III.  The 
branchial  cartilages,  307;  development  of  branchial  cartilages  in  limulus,  308; 
minute  structure,  309.  IV.  The  endocranium,  312;  apus,  312;  mygale,  313;  lim- 
ulus, 314;  scorpion,  317;  telyphonus,  319.  V.  Summary  and  comparison,  319. 

CHAPTER  XVIII. 

THE  MIDDLE  CORD,  THE  LEMMATOCHORD  AND  THE  NOTOCHORD 323-336 

I.  The  middle  cord  of  insects,  324;  the  lemmatochord  of  lepidoptera,  326.  II. 
The  middle  cord  of  the  scorpion;  A.  neural  sinus,  merochord  and  bothroidal  cord 
of  the  adult,  328;  B.  development  of  the  lemmatochord,  329;  merochord,  330; 

C.  development  of  the  neural  sinus,  neuroglia  and  canalis  centralis,  331.     III. 
The  middle  cord  of  limulus,  334.     IV.  Summary  and  comparison,  335. 

CHAPTER  XIX. 

THE  OSTRACODERMS  AND  THE  MARINE  ARACHNIDS 337~347 

Nature  of  the  problem.  I.  The  marine  arachnids  and  their  origin,  338.  II. 
The  ostracoderms,  341;  historical  review,  342. 

CHAPTER  XX. 

THE  OSTRACODERMS 348-380 

Subdivisions  of  the  body,  349;  the  cephalic  appendages,  350;  jaws,  350;  skeleton, 
351;  trend  of  development  of  the  exoskeleton,  352;  the  eyes,  355;  olfactory  organ, 
356;  auditory  organ,  356;  cutaneous  sense  organs,  356.  I.  Aspidacephali,  358; 
cephalospidae,  358;  trematospidae,  359;  exoskeleton,  oral  region,  lateral  eyes, 
marginal  and  postorbital  openings,  lateral  line  organs,  appendages,  ateleaspidse, 
363.  II.  The  anaspida,  364;  ccelolepidae,  birkeniidae.  III.  The  pteraspida,  364; 
pterospidae,  psamostaedae.  IV.  Antiacha,  367;  exoskeleton,  367;  atrial  frill,  371; 
gills,  371;  viscera,  372;  jaws,  373;  hyoid  arches,  375;  mouth,  375;  eyes,  olfactory 
organs,  sensory  grooves,  376;  cephalic  appendages,  376;  preservation,  377;  loco- 
motion, 379;  food,  379. 

CHAPTER  XXI. 

THE  VERTEBRATES 381-392 

I.  The  cyclostomata,  383.     II.  The  elasmobranchii  and  holocephali,  384.     III. 
The  arthrodira,  teleostomii  dipnoi  and  amphibia,  386. 


XVI  CONTENTS. 

PART  II.     THE  ACRANIATA. 

CHAPTER  XXII. 

PAGE 

THE  CRANIATES  AND  THE  ACRANIATES 393-407 

Statement  of  the  problem,  393.  I.  The  craniates,  395.  II.  The  acraniates,  396; 
metamerism,  398;  appendages,  nervous  system,  399;  degeneration,  attachment, 
mantle,  400;  skeleton,  heart  and  circulation,  401;  sexual  organs,  development,  401; 
A.  molluscs  and  annelids,  403;  trochosphere,  gastrula,  blastopore;  B.  craniates, 
403;  modification  of  the  gastrula,  telopore,  concrescence;  C.  acraniates,  405;  mes- 
entocoel,  telopore,  mouth,  406;  the  naupula,  ccelom,  407. 

CHAPTER  XXIII. 

THE  ClRRIPEDS,  TUNICATES,   AND    ECHINODERMS 408-430 

I.  The  cirripeds,  408;  the  nauplius  and  the  naupula,  the  metamorphosis,  410; 
appendages,  alimentary  canal,  411;  ccelom,  excretory  organs,  412;  sexual  organs, 
degeneration,  413;  the  old  mouth  and  the  new.  II.  The  tunicates,  415;  meta- 
morphosis, heart  and  vascular  system,  417;  eyes,  418;  the  old  mouth  and  the  new, 
mantle,  419;  comparison  with  cirripeds,  summary,  420.  III.  The  echinoderms, 
421;  larva,  422;  ciliated  band,  423;  cephalic  appendages,  424;  attachments,  de- 
velopment, 425;  teloccel,  mesoderm  and  ccelom,  426;  thoracic  appendages,  427; 
excretory  organs,  disc,  vertebrate,  428;  asymmetry,  summary,  430. 

CHAPTER  XXIV. 

THE  ENTEROPNEUSTA,  PLEROBRANCHIA,    POLYZOA-BRACHIOPODA    PHORONIDA    AND 

CHALTOGNATHA 431-453 

IV.  The  enteropneusta  and  echinoderms,  431;  the  enteropneusta  and  the  arthro- 
pods, 432;  structure  and  development,  433;  cleavage,  433;  gastrula  and  teloccele, 
433;  mesoderm  and  ccelom,  434;  metamorphosis,  435;  cephalic  caecum,  435; 
late  larval  and  adult  stages,  435;  endocranium,  436;  muscles,  437;  ccelom,  437; 
nervous  system,  437  V.  The  pterobranchia,  439.  VI.  The  polyzoa,  440;  ento- 
procta,  441;  conclusion,  443;  ectoprocta,  443.  VII.  The  brachiopoda,  445. 
VIII.  The  phoronida,  446;  fusiform  cells,  447.  IX.  The  chaetognatha,  448;  de- 
velopment, 448;  adult,  449;  integument,  449;  excretory  organs,  449;  trunk,  450; 
head,  450;  endocranium,  450;  nervous  system,  450;  cephalic  sense  organs,  452; 
lateral  eyes,  452;  parietal  eye,  452;  olfactory  organ,  452;  conclusion,  453. 

CHAPTER  XXV. 

SUMMARY  AND  CONCLUSION 454-472 

I.  The  evolution  of  a  creative  environment,  454.  II.  Crises  in  organic  evolution, 
456;  A.  the  evolution  of  metamerism  and  bilateral  symmetry,  457;  B.  asymmetry 
as  a  creative  factor,  458;  C.  chiten  and  the  exoskeleton  as  creative  factors,  439; 
D.  the  increasing  volume  of  the  yolk  sphere  as  a  creative  factor,  461 ;  E.  the  increas- 
ing volume  of  the  brain  as  a  creative  factor,  461;  F.  the  creation  of  a  new  environ- 
ment for  the  eyes,  462;  the  significance  of  a  natural  system  of  classification,  463; 
the  various  aspects  of  evolution,  468. 

EXPLANATION  OF  THE  LETTERING 473 

INDEX 483 

' 


HISTORICAL  SKETCH. 


The  resemblance  between  vertebrates  and  arthropods  first  attracted  my 
attention  in  1884.  In  m7  paper  on  the  development  of  the  phryganids,  it  was 
stated,  page  594,  that  a  wonderful  analogy,  if  not  homology,  exists  between  the 
structure  and  mode  of  growth  of  the  medullary  plate,  the  neural  and  gastrular  in- 
vaginations,  and  the  neurenteric  canal  of  insects  and  the  corresponding  structures 
in  vertebrates. 

Three  years  later,  a  resemblance  between  the  minute  structure  of  the 
compound  eyes  of  arthropods  and  the  retina  of  vertebrates  was  recognized. 

In  1888,  it  was  shown  that  the  invagination  of  the  procephalic  lobes,  supposed 
by  writers  of  that  period  to  give  rise  to  a  two-layered  compound  eye,  in  reality 
gave  rise  to  the  optic  ganglion  only,  while  the  eye  itself  consisted  of  a  single  layer. 
Further  study  of  the  developing  brain  and  eyes  of  Acilius,  Buthus,  and  Limulus, 
showed  that  in  many  arthropods  the  procephalic  lobes  underwent  a  complex 
process  of  invagination,  accompanied  by  the  overgrowth  of  a  neural  crest,  or 
palial  fold,  the  result  being  the  formation  of  a  vesicular  forebrain,  and  the  transfer 
of  the  ocelli,  located  on  the  outer  margin  of  the  lobes,  to  the  end  of  a  tubular  or 
epiphysial  outgrowth  of  the  membranous  roof  of  the  forebrain  vesicle. 

Here  were  revealed,  for  the  first  time,  all  the  steps  in  the  transformation 
of  an  invertebrate  type  of  eye  into  the  type  of  eye  so  characteristic  of  ver- 
tebrates. This  apparently  simple  fact  was  in  reality  the  result  of  very  complex 
conditions,  and  it  seemed  incredible  that  they  could  be  duplicated  except  in 
animals  belonging  to  the  same  stock. 

These  discoveries,  therefore,  appeared  so  profoundly  significant  that  I  deter- 
mined to  follow  the  clue  to  the  end,  to  see  whether  further  analysis  of  the  eyes,  the 
brain,  and  other  systems  of  organs  would  not  confirm  the  obvious  conclusion  to  be 
drawn  from  them.  The  results  proved  to  be  so  surprisingly  in  accord  with  them, 
that  in  the  following  year,  1889,  a  definite  theory  was  formulated,  and  a  prelim- 
inary sketch,  or  outline,  of  it  was  published  under  the  title  "  On  the  Origin  of 
Vertebrates  from  Arachnids." 

This  theory  has  formed  the  basis  of  all  my  subsequent  work,  and  as  far  as  it 
went,  is  practically  the  same  as  the  one  presented  here.  In  that  paper  it  was 
maintained  that  the  vertebrates  are  descended  from  the  arachnid  division  of  the 
arthropods,  in  which  were  included  the  typical  arachnids,  the  trilobites,  and 
merostomes.  The  ostracoderms  were  regarded  as  a  separate  class,  uniting 
the  arachnids  with  the  true  vertebrates.  Limulus  and  the  scorpion  were  the 
types  most  carefully  studied,  because  they  were  the  nearest  and  most  available 

xvii 


XV111  HISTORICAL    SKETCH. 

living  representatives  of  the  now  extinct  merostomes,  or  giant  sea  scorpions,  that 
were  regarded  as  the  arachnids  standing  nearest  to  the  ostracoderms. 

Other  evidence  and  conclusions  were  as  follows :  i.  In  the  arachnids  a  forebrain 
vesicle  is  formed  by  the  same  process  of  marginal  overgrowth  as  in  the  vertebrates. 
From  the  floor  of  the  vesicle  arise  the  forebrain  and  optic  ganglia;  from  the 
membranous  roof,  a  tubular  outgrowth  is  formed  that  contains  a  parietal,  or  pineal 
eye,  similar  in  structure,  mode  of  origin,  and  innervation  to  the  pineal  eye  of 
vertebrates.  2.  The  kidney-shaped  compound  eye  of  arachnids  has  been  trans- 
ferred to  the  walls  of  the  cerebral  vesicle  in  vertebrates,  giving  rise  to  the  retina, 
which  still  shows  traces  of  ommatidia  in  the  arrangement  of  the  rod-and-cone 
cells.  Its  original  shape  is  temporarily  retained  in  vertebrates,  but  gives  rise 
ultimately,  by  adaptive  exaggeration,  to  the  choroid  fissure.  3.  The  arachnids 
have  a  cartilaginous  endocranium  similar  in  shape  and  location  to  the  primordial 
cranium  of  vertebrates.  4.  They  have  an  axial,  subneural  rod  comparable  with 
the  notochord.  5.  In  arachnids,  the  brain  contains  approximately  the  same  num- 
ber of  neuromeres  as  in  vertebrates.  It  is  also  divided  into  similar  regions,  each 
one  having  a  similar  number  of  neuromeres,  a  similar  distribution  of  nerves,  and  a 
similar  relation  to  cranial  ganglia  and  sense  organs,  to  those  in  vertebrates.  6. 
The  segmental  sense  organs  (median  and  lateral  eyes,  olfactory  and  auditory 
organs)  are  comparable  with  those  in  vertebrates.  The  coxal  sense  organs  are 
associated  with  special  sensory  nerves  and  ganglia,  comparable  with  the  cranial 
dorsal-root  nerves  and  ganglia  (suprabranchial  sense  organs)  of  vertebrates. 
7.  The  basal  arches  of  the  appendages  are  comparable  with  the  oral  and  branchial 
visceral  arches  in  vertebrates.  8.  The  tendency  toward  concentration  of  neuro- 
meres has  narrowed  the  passage  way  for  the  stomodeum  and  modified  the  mode  of 
life  in  the  arachnids.  This  ultimately  led  to  its  permanent  closure,  the  infundi- 
bulum  and  adjacent  nerve  tissues  in  vertebrates  representing  the  remnants  of 
the  old  stomodaeum  with  its  nerves  and  ganglia.  10.  The  progressive  degeneration 
of  haemal  thoracic  muscles,  the  fusion  of  thoracic  metameres,  the  position  of  the 
oral,  or  neural  surface,  in  .swimming  and  crawling,  were  identified  with  corre- 
sponding conditions  in  vertebrates,  u.  The  eye  muscles  of  vertebrates  arose 
from  a  special  group  of  haemo-neural  muscles  belonging  probably  to  the  first 
two  or  three  thoracic  segments.  12.  The  process  of  gastrulation  in  vertebrates 
and  arachnids  is  confined  to  the  procephalic  lobes,  in  the  place  where  at  a  later 
period  the  primitive  stomodaeum  appears.  The  so-called  "gastrulation"  of  verte- 
brates and  arachnids  is  an  entirely  different  and  independent  process,  that  is,  the 
process  of  adding  by  apical  or  teloblastic  growth  a  segmented,  bilaterally  sym- 
metrical body  to  a  primitive  radially  symmetrical  head.  13.  The  arachnids 
resemble  the  vertebrates  in  more  general  ways,  as  in  the  minute  structure  of 
cartilage,  muscle,  nerves,  digestive,  and  sexual  organs. 

In  the  following  paper,  '93,  the  structure  of  the  forebrain  of  Limulus,  with  its 
lobes  and  cavities  was  compared  in  detail  with  the  brain  of  vertebrates.  The 
coxal  sense  organs  were  described  and  shown  to  be  gustatory  organs  comparable 


HISTORICAL    SKETCH.  XIX 

with  the  suprabranchial  organs  of  vertebrates.  The  remarkable  structure  of  the 
olfactory  organs  in  Limulus  was  also  described  for  the  first  time. 

In  1894  it  was  shown  that  the  exoskeleton  consisted  of  a  complicated,  and 
for  an  invertebrate,  a  very  remarkable  system  of  chitenous  trabeculae  resembling 
a  primitive  form  of  dermal  bone. 

In  1896  was  published  a  paper  on  the  "Variations  in  the  Development  of 
Limulus."  It  was  undertaken  in  the  hope  that  it  might  throw  some  light  on  the 
normal  development,  or  give  some  indications  of  the  kind  of  variations  that  have 
led  to  the  higher  types. 

In  1899  and  'oo,  in  cooperation  with  Mr.  Redenbaugh  and  Miss  Hazen,  a  de- 
scription of  the  peripheral  nervous  system,  endocranium,  and  coxal  glands  was 
published.  The  work  was  begun  with  the  purpose  of  furnishing  a  detailed 
account  of  the  various  systems  of  organs  in  arachnids  as  a  basis  for  further  com- 
parisons with  vertebrates. 

In  1901  advantage  was  taken  of  a  six  months  leave  of  absence  from  college 
duties  to  study  the  principal  collections  of  ostracoderms  in  European  museums. 
It  was  rarely  possible  to  make  use  of  such  collections  for  anything  more  than  a 
superficial  examination.  An  effort  was  therefore  made  to  obtain  material  that 
could  be  sectioned,  or  used  in  any  manner  that  seemed  desirable,  in  order  to  get 
at  the  anatomical  structure.  A  valuable  collection  of  Tremataspis  and  Thy- 
estes  was  obtained  in  the  island  of  Oesel  in  the  Baltic  Sea,  and  a  few  cephalaspids 
and  pteraspids  were  obtained  by  gift  and  purchase  in  England.  In  the  next  four 
or  five  years  an  effort  was  made  to  obtain  ostracoderms  in  the  vicinity  of  Dal- 
housie,  N.  B.,  Canada,  at  first  with  little  success.  Finally,  I  obtained  a  very  large 
number  of  specimens  in  a  beautiful  state  of  preservation,  from  which  it  was 
possible  to  work  out  the  anatomy  in  great  detail.  The  structure  of  the  eyes,  jaws, 
and  internal  organs  afforded  a  striking  confirmation  of  our  conclusion  that  the 
ostracoderms  form  a  new  class  of  animals  standing  between  the  vertebrates  and 
arachnids. 

In  1888,  '89,  and  '90,  Gaskell  published  his  first  papers  on  the  Origin  of  the 
Central  Nervous  System  of  Vertebrates.  The  basis  of  his  theory  was  that  "the 
central  nervous  system  of  a  crustacean  ancestor  had  grown  round  and  enclosed 
the  alimentary  canal."  "  The  ventricles  of  the  brain  were  the  old  cephalic  stomach 
and  the  canalis  centralis  of  the  spinal  cord,  the  long  straight  intestine  which  led 
originally  to  the  anus."  The  vertebrate  develops  a  new  heart,  alimentary  canal, 
and  other  organs  to  take  the  place  of  those  enclosed  in  the  central  nervous  system. 

In  its  conception  and  mode  of  analysis  of  the  conditions  in  the  vertebrates  and 
arthropods,  this  theory  is  entirely  different  from,  and  wholly  irreconcilable  with  my 
own.  In  my  judgment,  the  foundations  on  which  it  is  built  are  totally  wrong. 
The  fundamental  error,  which  is  inextricably  interwoven  in  all  his  conclusions, 
making  a  detailed  criticism  of  them  unnecessary,  is  the  assumption  that  the  neural 
surface  of  an  arthropod  is  the  same  as  the  haemal  surface  of  a  vertebrate.  In  this 
confusion  of  opposite  surfaces,  which  is  like  starting  on  a  voyage  of  discovery  with 


XX  HISTORICAL    SKETCH. 

the  notion  that  north  is  south,  and  east  is  west,  the  nerve  cords  are  transferred 
from  one  side  of  the  body  to  the  other,  turning  them  literally  upside  down  and 
inside  out,  annihilating  the  most  fundamental  systems  of  organs,  such  as  the 
heart  and  entire  alimentary  canal,  and  necessitating  the  creation  ude  novo"  of 
whole  systems  of  organs  to  take  their  place.  In  this  process  the  axes  of  growth 
and  differentiation  are  reversed,  or  ignored,  and  no  attempt  is  made  to  reconcile 
these  assumptions  with  the  actual  conditions  that  are  so  familiar  in  the  embryonic 
development  of  both  vertebrates  and  arthropods. 

LIST  or  THE  AUTHOR'S   PAPERS  CONCERNING  THE  EVOLUTION  OF  THE  VERTEBRATES 

1884.     The  Development  of  Phryganids  with  a  preliminary  note  on  the  development  of 
Blatta  Germanica.     Quart.  J.  M.  Sc.,  Vol.  XXIV,  N.  S. 

1886.  Eyes  of  Molluscs  and  Arthropods.     Mitth.  aus.  d.  Zool.  Stat.  zu.  Neapel.  Vol.  VI. 

1887.  Eyes  of  Molluscs  and  Arthropods.     Journ.  Morphol.  Vol.  I,  April  n. 

1887.  Studies    on    the   Eyes    of   Arthropods.     I.   Development  of  the  Eyes   of  Vespa, 
with  Observations  on  the  Ocelli  of  some  Insects.     Journ.  of  Morphol.,  Vol.  I, 
April  ii. 

1888.  Studies   on  the  Eyes  of  Arthropods.     II.  Eyes  of  Acilius.     Journ.  of  Morphol., 
Vol.  II. 

1888.     Segmental  Sense  Organs  of  Arthropods.     Journ.  of  Morphol.,  Vol.  II. 
1890.     On    the    Origin    of    Vertebrates    from    Arachnids.      Quart.    Journ.    Micr.    Sc., 
Vol.  XXXI,  Part  3. 

1893.  On  the  Morphology  and  Physiology  of  the  Brain  and  Sense  Organs  of  Limulus. 
Quart.  Journ.  Micr.  Sc.,  Vol.  XXXV,  No.  137. 

1894.  On  Structures  Resembling  Dermal  Bones  in  Limulus.     Anat.  Anz.,  Bd.  9,  No.  14. 
1896.     Variations  in  the  Development  of  Limulus    Polyphemus.     Journ.    of  Morphol., 

Vol.  XII,  No.  i. 
1896.     The  Visual  Centres  of  Arthropods  and  Vertebrates.     Morph.  Soc.  Sc.,  Vol.  V, 

P-  431- 
1899.     Patten  Wm.,  and   Redenbaugh,  W.    A.     The    Endocranium  of   Limulus,  Apus, 

and  Mygale.     Journ.  Morphol.,  Vol.  XVI,  No.  i. 
1899.     Patten,    Wm.,  and     Redenbaugh,    W.    A.     The    Nervous    System    of    Limulus 

Polyphemus.     Ibid. 
1899.     Gaskell's  Theory  of  the  Origin  of  Vertebrates.     Am.   Naturalist,  Vol.  XXXIII, 

April,  pp.  360-9. 
1900      Patten,  Wm.,   and    Hazen,    A.    P.     The    Development    of    the     Coxal    Gland, 

Branchial  Cartilages,  and  Genital  Ducts  of  Limulus.     Journ.  Morphol.,  Vol.  XVI, 

No.  3. 

1901.  On    the    Origin  of  Vertebrates,  with    Special   Reference    to    the    Ostracoderms. 
Address  before  the  V.  International  Congress  of  Zoologists,  Berlin. 

1902.  On   the   Structure   and    Classification  of   the  Tremataspidae.     Amer.  Nat.,  Vol. 
XXXVI,  No.  425. 

1903.  On   the   Structure   and  Classification  of  the  Tremataspidae.     Mem.  Acad.  Imp. 
Sci.  St.  Petersbourg,  Vol.  XIII,  No.  5. 

1903.  On  the  Appendages  of  Tremataspis.     Amer.  Nat.,  Vol.  XXXVII,  No.  436. 

1903.  The  Structure  of  the  Ostracoderms.     Science,  Vol.  XVII,  No.  430. 

1903.  On  the  Structure  of  the  Pteraspidae  and  Cephalaspidae.     Am.  Nat. 

1904.  New  Facts  Concerning  Bothriolepis.     Biol.  Bui.,  Vol.  VII.,  July. 


HISTORICAL    SKETCH.  XXI 

1904.  The  Structure  of  Bothriolepis,  with  Exhibition  of  Specimens  of  Devonian  Fishes 
of  Canada.  Read  before  the  Am.  Soc.  of  Zoologists,  Phila. 

1907.  On  the  Origin  of  Vertebrates.  I.  The  Conditions  Controlling  the  External 
Morphology  of  Primitive  Vertebrates.  Lantern  Slides.  Read  before  Section 
VII,  General  Zool,  VII.  International  Zool.  Congress,  Boston,  August. 

1907.  On  the  Origin  of  Vertebrates.  II.  The  Interpretation  of  the  Structure  of  Echino- 
derms,  Ascidians,  Balanoglossus,  and  Cephalodiscus.  Lantern  Slides.  Ibid. 

1907.  International  Congress,  Boston,  M creator  Projections  of  Vertebrate  and  Arachnid. 
Embryos. 

Exhibits.     A.     Collection    of    Bothriolepis   from   the   Devonian   rocks   of    New 
Brunswick. 

B.  Fifty  Models  Illustrating  the  Structure  and  Embryology  of  Primitive  Verte- 
brates and  Related  Forms.     Reviewed  in  Amer.  Nat.,  Vol.  XLI,  No.  490. 


THE 

EVOLUTION  OF  THE  VERTEBRATES 
AND  THEIR  KIN, 


CHAPTER  I. 

OUTLINE  OF  THE  ARACHNID  THEORY. 

In  the  two  following  chapters  we  shall  present  a  brief  outline  of  the  arachnid 
theory,  showing  the  broad  foundations  upon  which  it  rests  and  the  relation  of 
the  principal  organs  in  the  arachnids  to  those  in  the  vertebrates. 

I.  ITS  SCOPE  AND  RELATION  TO  OTHER  THEORIES. 

The  arachnid  theory,  like  every  other  large  problem  in  descent,  should  be 
based  on  comparative  physiology,  anatomy,  embryology,  and  paleontology, 
and  should  be  constructed  in  accordance  with  the  established  principles  of  these 
sciences.  This  particular  theory  has  the  additional  task  of  reconciling,  eliminating, 
or  absorbing  the  claims  of  strongly  entrenched  rival  theories,  some  of  which  con- 
tain certain  elements  of  truth,  It  is  important,  therefore,  to  at  once  determine 
which  supplies  the  greatest  volume  of  evidence;  which  draws  its  evidence  from 
the  widest  fields;  which  can  eliminate  the  others,  or  include  the  others  within 
itself. 

We  shall  show  that  in  these  respects  the  arachnid  theory  stands  in  a  class 
by  itself,  for  it  is  the  only  one  that  is  securely  built  on  the  natural  science  trinity 
of  structure,  function,  and  historic  sequence.  It  not  only  has  its  own  distinctive 
merits  upon  which  it  claims  recognition,  but  it  is  the  only  theory  that  can  either 
eliminate  the  others,  or  incorporate  them  within  itself,  where  they  become  rein- 
forced and  revitalized. 

The  essential  features  of  the  annelid  theory,  for  example,  are  included  in 
the  arachnid  theory,  because  both  arachnids  and  annelids  agree  in  the  funda- 
mental nature  of  their  metameric  structure.  But  when  standing  alone,  the  anne- 
lid theory  ceases  to  be  of  value  as  a  working  hypothesis,  or  as  a  touchstone  to 
solve  the  problems  of  vertebrate  morphology,  because  we  find  no  traces  in  the 
annelids  of  those  illuminating  modifications  of  metamerism  so  characteristic  of 
the  arachnids,  and  that  afford  us  the  required  data  for  filling  in,  and  explaining, 
the  enormous  gap  between  the  unspecialized  metameres  of  an  annelid  and  the 
groups  of  highly  specialized  metameres  in  the  head  of  a  vertebrate.  The  annelid 


2  OUTLINE    OF    THE   ARACHNID    THEORY. 

theory,  therefore,  in  the  form  in  which  it  is  generally  understood,  could  be  in- 
corporated into  the  arthropod  theory,  but  it  is  evident  that  the  conditions  could 
not  be  reversed,  for  no  resemblance  of  annelids  to  vertebrates  could  either  elimi- 
nate, or  account  for,  the  resemblance  of  arthropods  to  vertebrates. 

The  tunicate,  echinoderm,  balanoglossus,  amphioxus,  etc.,  theories  have 
similar  inherent  weaknesses,  indicating  that  they  must  be  subordinated  to 
some  larger  view.  The  baffling  resemblances  between  the  embryonic  stages  of 
these  form?  and  vertebrates  do  not  help  us  to  explain  vertebrate  cephalogenesis, 
or  to  account  for  the  origin  of  the  most  characteristic  vertebrate  structures;  and 
so  long  as  their  own 'origin  Is  unknown,  and  they  have  no  fixed  location  in  a 
general  system  of  classification,  they  can  throw  no  light  on  the  origin  of  verte- 
brates, or  on  the  still  broader  problems  of  the  origin  and  inter-relations  of  the 
other  great  subdivisions  of  the  animal  kingdom. 

All  this  is  changed,  however,  as  soon  as  we  recognize  that  the  echinoderms 
tunicates,  balanoglossus,  and  cephalodiscus  are  degenerate  offshoots  of  a  common 
arthropod-vertebrate  stock.  In  the  light  of  this  interpretation,  the  arachnid 
theory  not  only  recognizes  and  explains  the  resemblance  of  the  echinoderms, 
tunicates,  and  other  acraniates  to  the  vertebrates,  but  it  fixes  approximately 
their  position  in  the  animal  kingdom,  and  elucidates  the  salient  features  of  their 
morphology.  It  supplies,  in  the  evolution  of  the  arthropod  cephalothorax,  the 
key  to  the  analysis  of  the  vertebrate  head.  It  unites  the  apex  of  the  arthropod 
stock  with  the  base  of  the  vertebrate  stock,  and  welds  the  entire  series  of  seg- 
mented animals  into  one  homogeneous  group.  It  shows  that  the  great  verte- 
brate-ostracoderm-arthropod  phylum  forms  the  main  trunk  of  the  genealogical 
tree  of  the  animal  kingdom;  that,  emerging  from  unsegmented,  ccelenterate-like 
animals,  as  though  driven  by  some  mysterious  internal  power,  moves  with  aston- 
ishing precision,  through  broad,  predetermined  channels — from  which  neither 
habit,  nor  environment,  nor  heredity,  can  cause  it  to  diverge — toward  its  goal. 
And  finally  it  lays  before  us  in  their  historic  order  the  critical  events  of  these  age- 
long periods,  the  succession  of  structural  and  functional  changes  that  have  fol- 
lowed them,  and  that  have  in  turn  given  rise  to  still  other  changes  of  form  and 
new  conditions  of  growth.  It  thus  reveals  to  us,  as  only  the  true  science  of  mor- 
phology can  reveal,  the  important  agents  that  have  directed  the  course  of  evolution, 
and  that  have  determined  the  organic  forms,  or  shapes,  in  which  it  is  expressed. 

The  arachnid  theory  thus  not  only  unites  and  harmonizes  these  apparently 
conflicting  views  as  no  other  interpretation  can,  but  it  will,  in  my  judgment,  go 
a  long  way  toward  restoring  morphology  to  its  former  dominant  position  as  the 
expounder  and  prophet  of  the  biological  sciences.  Morphology  reduced  to  a 
barnyard  science,  without  its  vast  resources  in  comparative  anatomy,  its  per- 
spective in  geological  time,  and  its  world- wide  laboratory  of  Nature,  is  robbed  of 
its  chief  glory  and  power. 

One  naturally  looks  on  the  arthropods  as  the  probable  ancestors  of  the 
vertebrates,  because  they  are  the  most  highly  organized  of  segmented  invertebrates 


NATURE    OF    THE    EVIDENCE.  3 

and  because  the  histological  structure  of  their  muscles,  nerves,  sense  organs, 
cartilages,  etc.,  closely  resembles  that  of  the  vertebrates.  This  view  was,  there- 
fore, the  first  to  be  entertained  by  the  older  anatomists  (Leydig  and  Dohrn); 
but  in  more  recent  years  it  has  not  been  regarded  with  favor. 

So  far  as  I  have  been  able  to  determine,  most  zoologists  of  to-day,  who  make 
any  attempt  to  justify  their  deep  rooted  prejudice  against  the  arthropod  theory, 
base  their  objections  on  the  a  priori  ground  that  the  arthropods,  being  highly 
specialized  animals,  cannot  have  given  rise  to  the  vertebrates,  because  the  verte- 
brates must  have  come  from  some  generalized  type.  This  objection  clearly  has 
but  little  weight,  for  the  general  application  of  such  a  law  would  exclude  the 
possibility  of  any  evolution.  Every  animal  is  a  specialized  one  when  compared 
with  its  ancestors,  and  at  the  same  time  a  generalized  one  when  compared  with 
its  descendants.  Even  the  most  primitive  vertebrate  is  a  highly  specialized 
animal,  and  its  immediate  ancestors  were  also  highly  specialized.  It  is  clear, 
therefore,  that  in  order  to  solve  our  problem  we  must  discover  not  some  gener- 
alized ancestor  but  a  specialized  one,  and  the  only  evidence  of  value  in  deter- 
mining whether  we  have  found  the  right  one  or  not,  is  the  degree  to  which  its 
particular  kind  of  specialization  agrees  with  that  of  a  vertebrate. 

II.  NATURE  OF  THE  EVIDENCE  TO  BE  PRESENTED. 

Our  problem  then  is  a  perfectly  simple  one  in  principle,  although  it  is  one 
that  involves  an  enormous  amount  of  detail  in  its  application.  We  have  merely 
to  strip  off  the  superficial  disguise  of  our  hypothetical  arachnid  ancestors  and  see 
whether  either  their  underlying  structure,  their  mode  of  growth,  the  general 
direction  and  historic  sequence  of  their  evolution,  does  or  does  not  harmonize 
with  the  assumption  that  they  are  the  ancestors  of  the  vertebrates.  We  venture 
to  state  at  the  outset,  that  in  our  judgment  they  do  harmonize  with  this  assump- 
tion, and  so  fully  and  in  such  detail  as  to  leave  no  other  conclusion  open  than  that 
the  vertebrates  arose  from  arachnid-like  arthropods. 

A.  Cephalogenesis  in  Arthropods. — We  shall  show,  with  the  aid  of  com- 
parative anatomy,  that  the  process  of  cephalizing  the  anterior  part  of  the  body, 
that  is,  the  transformation  of  a  large  number  of  independent  metameres  into  a 
compact,  organized  group  of  unlike  structures  that  may  be  called  a  "head,"  is 
the  dominant  process  in  the  evolution  of  arthropods,  and  that  this  process  has 
already  definitely  established  in  the  higher  forms  the  more  characteristic  features 
of  the  vertebrate  head.  The  process  is  initiated  in  primitive  arthropods  either 
by  the  division  of  the  anterior  part  of  the  body  into  regions,  or  by  the  addition 
from  time  to  time  of  distinct  groups  of  like  metameres,  or  tagmata.  The  succes- 
sive appearance  of  new  groups  of  metameres  at  the  tail  end  of  the  body  marks 
distinct  epochs  in  the  evolution  of  the  arthropods,  and  they  constitute  the  under- 
lying basis  for  the  characteristic  subdivision  of  the  body  into  pre-oral,  oral,  tho- 
racic, vagus,  abdominal,  and  caudal  regions.  We  shall  call  them  the  procephalon, 


4  OUTLINE    OF    THE   ARACHNID    THEORY. 

dicephalon,  mesocephalon,  metacephalon,  and  branchiocephalon.  Each  region 
usually  consists  of  a  certain  number  of  metameres,  modified,  or  specialized,  in  a 
very  constant  and  definite  manner  in  respect  to  its  sense  organs,  nerves,  and  other 
characters.  In  the  higher  arachnids  they  unite  in  various  ways  to  form  larger 
aggregates,  such  as  the  cephalo-thoracic-branchial  region.  In  the  vertebrates 
they  have  become  still  more  compactly  united  to  form  the  head,  the  subdivisions 
of  which  still  consist,  as  nearly  as  may  be  determined,  of  the  same  number  of 
metameres,  modified  in  the  same  characteristic  manner  as  the  corresponding 
subdivisions  of  the  arthropod  trunk  and  cephalothorax.  (Figs,  i,  3  and  5.) 


ol.o. . 
prosen. 
gust.o. 
die.enc 
mesen.c 

meten.c. 


ec.pa.e. 
en.pa.e. 
dor.o. 
.I.e. 


FIG.   i. — Plan  of  a  marine  arachnid,  based  in  part  on  Limulus.     Designed  to  show  the  principal  body  regions 
and  their  characteristic  organs.     A,  Neural,  or  oral  surface;  B,  haemal,  or  cardiac  surface. 

B.  Embryology. — We  shall  show  that  arachnid  and  vertebrate  embryos, 
from  the  very  beginning  of  their  development,  are  fundamentally  alike  in  structure 
and  mode  of  growth,  and  that  this  likeness  is  continued  through  successive, 
parallel  stages,  up  to  a  point  where  the  arachnid  stages  cease;  then  the  vertebrate 
embryo,  entering  on  its  particular  phases  of  development,  carries  them  to  com- 
pletion. We  shall  show  that  the  similarity  between  them  consists:  a.  in  the 
origin  of  the  germ  layers;  b.  in  the  general  form  and  segmentation  of  the  neural 
plate;  its  flexures,  mode  of  enclosure,  and  the  location  of  its  principal  parts;  c. 


CEPHALOGENESIS.  5 

in  the  serial  location  and  subsequent  migrations  of  the  primary  cephalic  sense 
organs  (median  and  lateral  eyes,  olfactory,  and  auditory  organs);  d.  in  the 
degree  of  development  of  the  cephalic  mesoblast,  and  in  the  direction  and  extent 
of  its  growth  in  the  several  regions;  e.  in  the  development  of  the  heart;/,  in  the 
concrescence  of  the  so-called  "lips  of  the  blastopore,"  and  in  the  growth  of  the 
margins  of  the  embryonic  area  (" germ  wall") ;  g.  in  the  formation  of  the  head  fold. 
C.  Arachnid  Cephalogenesis  Prophetic  of  the  Vertebrate  Head. — We 
shall  show  that  the  continuation,  or  the  exaggeration,  of  the  processes  already 
initiated  in  the  arachnids  inevitably  leads  to  the  establishment  of  the  conditions 
now  seen  in  the  vertebrates.  For  example:  a.  The  further  withdrawal  of  the 


noto. 


C 


per.c 


card.g. 


FIG.  2. — Semi-schematic  cross- sections  of  a  marine  arachnid,  showing  location  of  principal  organs.   A,  abdominal 
region;  B,  branchial  region;  C,  mesocephalic,  or  thoracic  region. 

principal  alimentary  and  urogenital  organs  of  the  arachnids  into  the  postcephalic 
regions,  would  produce  the  condition  in  vertebrates,  b.  The  continued  enlarge- 
ment and  closer  union  of  the  thoracic  neuromeres,  and  their  more  precocious 
development  during  embryonic  periods,  aided  possibly  by  the  further  overgrowth 
of  the  labrum  and  optic  ganglia,  would  lead  to  a  further  narrowing,  and  over- 
growth of  the  passage  for  the  esophagus,  and  ultimately  to  the  permanent  closure 
of  the  old  mouth,  as  in  vertebrates,  c.  The  continued  increase  in  the  size  of  the 
yolk  sphere,  the  absence  of  mesodermic  structures  on  the  haemal  side  of  the  tho- 


6  OUTLINE    OF    THE   ARACHNID    THEORY. 

racic  and  cephalic  region,  and  the  increasingly  precocious  development  of  the  fore- 
brain,  would  inevitably  lead  to  the  formation  of  a  more  pronounced  head  fold, 
with  a  disproportionately  shortened  or  diminished  haemal  surface,  and  would  force 
the  bases  of  the  more  anterior  oral  appendages  forward  and  haemally  tilt  they 
meet  on  the  opposite  side  of  the  head,  thus  giving  rise  to  the  premaxillary,  maxillary, 
and  mandibular  arches  of  the  vertebrate  head.  d.  This  shortening  of  the  ante- 
rior haemal  surface  of  the  head  inevitably  draws  the  heart,  with  its  neighboring 
muscles  and  nerves  derived  from  the  vagal  and  branchial  segments,  farther  for- 
ward into  the  head  region,  thus  producing  that  remarkable  forward  dis- 
location of  the  heart,  hypobranchial  muscles  and  nerves  so  familiar  in  vertebrates. 
(Figs.  17,  19,  33,  77.)  e.  Finally  the  readjustment  of  the  whole  head,  in  response 
to  these  changes,  leads  to  that  new  condition  of  architectural  stability  that  marks 
the  true  vertebrates. 

The  preliminary  stages  that  lead  up  to  these  readjustments  were,  no  doubt, 
gradual  and  more  or  less  tentative,  for  they  did  not  in  themselves  create  sufficiently 
altered  conditions  to  upset  the  balance  of  organic  power.  But  the  later  stages 
of  the  readjustment,  especially  the  final  stages  in  the  transfer  of  the  oral  arches  to 
the  haemal  side,  appear  to  have  been  rapidly  accelerated  for  a  period  and  then 
checked  by  their  approaching  reunion  on  the  haemal  side  of  the  head  and  by  the 
creation  there  of  a  new  condition  of  organic  stability. 

The  closing  of  the  old  mouth,  the  formation  of  a  new  one,  the  transfer  of  the 
oral  arches  to  the  haemal  side,  and  the  appearance  of  true  gill  clefts  must  have 
taken  place  during  the  embryonic,  or  larval  period,  the  increasing  volume  of  the 
yolk  sphere  making  such  a  cataclysmic  metamorphosis  possible.  Hence  it  is 
probable  that  the  transition  from  the  arthropod  to  the  vertebrate  type  will  never 
be  completely  bridged  by  the  discovery  of  new  animals.  The  gap  between  the 
two  classes  is  a  real  one,  representing  a  comparatively  short  period  of  rapid  trans- 
formation from  the  old  condition  in  the  arthropods  to  a  new,  approximately 
stable  condition  in  the  vertebrates. 

D.  Paleontology.— Nevertheless,  we  shall  show  that  the  wide  gulf  which  now 
separates  the  arachnids  and  vertebrates,  in  some  important  respects,  was  bridged 
in  early  paleozoic  times  by  a  large  and  varied  class  of  animals  known  as  the 
ostracoderms.  They  constitute  the  only  great  class  of  animals  that  have  flour- 
ished for  a  comparatively  short  period  and  then  become  totally  extinct;  a 
fact  that  in  itself  testifies  to  the  unstable,  transitory  character  of  their  anatomical 
structure. 

Heretofore  it  has  been  assumed  that  the  ostracoderms  were  highly  specialized 
vertebrates,  in  spite  of  the  fact  that  they  possessed  a  very  simple  and  primitive 
structure,  and  were  the  first  vertebrate-like  animals  to  appear  on  the  geological 
horizon.  They  were  contemporaneous  with  the  highest  and  most  dominant 
type  of  arthropods  then  in  existence,  the  marine  arachnids,  or  sea  scorpions,  of 
the  Silurian  period.  There  is  a  striking  resemblance  between  these  early  verte- 
brates and  the  contemporaneous  arachnids,  not  only  in  their  form  and  <reneral 


PALEONTOLOGY.  7 

appearance,  but  in  the  minute  structure  of  their  exoskeleton,  the  character  of 
their  appendages,  the  arrangement  of  their  median  ocelli,  and  in  the  structure 
of  their  jaws.  (Figs.  232  to  265.)  For  a  long  time  the  ostracoderms  were 
supposed  to  be  jawless  fishes,  but  a  special  investigation  of  this  point  was  made 
and  it  was  demonstrated  that  Bothriolepis,  the  best  known  member  of  the  class, 
possesses  well  developed  maxillae  and  mandibles,  quite  unlike  those  of  typical 
vertebrates,  but  precisely  like  those  demanded  by  the  arachnid  theory. 

Thus,  in  the  light  of  the  arachnid  theory,  these  ancient  and  remarkable 
animals,  that  have  been  repeatedly  mistaken  for  arthropods  and  for  vertebrates, 
but  which  are  neither  wholly;  which  have  withstood  the  keen  scrutiny  of  Agassiz, 
Huxley,  Ray  Lankester,  and  Smith  Woodward,  take  on  a  new  meaning.  We  can 
now  clearly  see  that  they  belong  neither  to  the  vertebrates  nor  to  the  invertebrates, 
but  form  a  class  by  themselves,  intermediate  between  the  two;  presenting  on  the 
one  hand,  in  their  appendages,  jaws,  eyes,  skeleton  and  gills,  affinities  with  the 
marine  arachnids,  and  on  the  other,  in  their  tail,  dermal  skeleton,  and  dorsal  fins, 
affinities  with  true  vertebrates. 

III.     THE  PROCESS  OF  CEPHALIZATION  IN  THE  ARTHROPODS. 

If  we  trace  the  evolution  of  cephalization  in  the  arthropods  and  analyze  the 
causes  that  have  brought  it  about,  we  shall  see  that  it  reaches  its  highest  expression 
in  the  arachnids  and  that  it  was  brought  about  by  the  same  kind  of  changes  that 
have  taken  place  in  the  vertebrates. 


A.  The  Grouping  and  the  Increase  in  Number  of  Metameres. — The  domi- 
nant process  in  the  evolution  of  the  arthropods  is  the  spasmodic  generation  of  new 
groups  of  terminal  metameres,  the  gradual  specialization  of  each  group,  and  its 
more  intimate  union  with  the  older,  more  anterior  members  of  the  series.  The  in- 
crease in  the  total  number  of  metameres,  from  the  first  three  that  are  characteristic 
of  the  nauplius,  to  the  seven  found  in  the  ostracods,  eleven  in  the  cladocera,  and 
the  twenty-one  or  -two  so  commonly  present  in  the  higher  forms,  goes  hand  and 
hand  with  the  specialization  and  union  of  the  more  anterior  groups  into  an  increas- 
ingly complex  organic  unit  that  in  the  vertebrate  sense  may  be  properly  called  a 
"head." 

While  this  process,  in  a  variable  degree,  occurs  in  all  arthropods,  it  is  only 
in  the  arachnids  that  it  takes  place  in  the  particular  manner  that  is  characteristic 
of  vertebrates.  In  the  more  typical  representatives  of  that  class,  the  first  fifteen 
or  sixteen  metameres  are  divided  into  unlike  groups  that  have  a  similar  sequence, 
consist  of  a  similar  number  of  metameres,  and  present  a  similar  morphological 
and  physiological  specialization  of  organs  to  that  seen  in  the  corresponding  regions 
of  the  vertebrate  head. 

It  is  evident,  therefore,  that  the  ancestral  history  of  the  vertebrate  head  is  con- 
tained in  the  first  fifteen  or  sixteen  arachnid  metameres,  and  that  in  the  arachnids 


s 


OUTLINE    OF    THE   ARACHNID    THEORY. 


we  may  study  this  process  of  cephalization  in  detail.     At  one  end  of  the  body  we 
mav  observe  the  birth  of  new,  independent  metameres,  and  at  the  other  the  gradual 


P>:  C       Di.C. 


st.co. 


Pr  C     DiC.        Ms  C.         Mt.C.  Er.C. 


D 


FIG.  3. — Diagrams  showing  the  five  characteristic  body  regions  of  arthropods,  and  their  progressive  concen- 
tration to  form  the  head  of  a  vertebrate.  The  principal  points  illustrated  are:  a,  The  early  location  of  the  prin- 
cipal functions;  b,  the  concentration  of  the  cardiomeres  in  the  branchial  region;  c,  the  enlargement  and  concentra- 
tion of  the  anterior  cephalic  neuromeres;  d,  the  change  in  position  of  the  optic  ganglia  and  oral  arches;  e,  the 
closure  of  the  old  mouth  and  the  formation  of  the  new  one;/,  the  transfer  of  locomotor  organs  from  the  meso- 
cephalon  to  the  postbranchial  metameres.  A  and  B,  Insect;  C ,  arachnid;  D,  vertebrate. 

decline  of  metamerism,  and  the  incorporation  of  the  old  metameres,  as  specialized 
subordinate  parts,  into  a  new  and  more  highly  organized  unit. 


B.  Origin  of  the  Linear  Arrangement  of  Unlike  Cephalic  Functions. —It  is 

frequently  assumed  that  the  primitive  vertebrate  head  consisted  of  a  considerable 


ORIGIN    OF    THE   ARRANGEMENT    OF    CEPHALIC    FUNCTIONS.  9 

number  of  like  metameres,  each  one  complete  in  itself,  that  is,  having  all  the 
organs  of  an  ideal  metamere.  This  assumption  is  untenable.  A  considerable 
number  of  cephalic,  or  anterior  metameres,  even  approximately  complete  or 
perfect,  rarely,  if  ever,  occur  in  any  animal  outside  those  pictured  in  text-book 
diagrams.  It  is  certain  that  no  such  condition  occurs  in  the  arachnids.  While 
it  may  be  assumed  that  metameric  growth  tends  to  produce  a  linear  series  of  like 
parts,  it  is  clear  that  it  does  not  do  so  in  reality.  The  first  products  of  apical 
growth  must  necessarily  differ  from  the  last,  because  different  conditions  are  cre- 
ated by  apical  growth  at  each  successive  stage  of  its  progress.  The  actual  result, 
therefore,  is  a  linear  sequence  of  unlike  structures  and  functions  for  a  given  number 
or  generation  of  metameres.  This  particular  sequence  becomes  unbalanced  and 
remodelled  with  the  appearance  of  the  next  generation.  But  on  the  whole  a 
definite  linear  succession  of  unlike  organs  becomes  established  at  a  very  early 
period  in  the  evolution  of  segmented  animals;  and  it  follows  a  logical,  inherently 
necessary  order,  that  is  never  completely  lost  or  disguised. 

With  the  elongation  and  increase  in  size  of  the  primitive  trunk  the  ingestive, 
gustatory,  locomotor,  cardiac,  and  respiratory  functions  become  more  localized, 
their  position  being  determined,  in  part,  by  the  necessary  conditions  for  their 
activities,  and  in  part  by  the  historic  order  in  which  they  became  established;  for 
the  location  of  any  new  function  is  limited  to  the  territory  that  is  not  already  pre- 
empted by  other  organs.  For  that  reason  we  find  that  the  most  essential  organs 
are  the  first  to  develop,  and  they  arise  from  the  oldest  parts  of  the  body,  that  is, 
from  the  more  anterior  and  median  neural  surface;  the  organs  of  more  recent 
origin  arise  on  the  haemal  and  caudal  sides  of  the  older  ones. 

The  primary  sense  organs,  i.e.,  the  parietal  and  lateral  eyes,  the  olfactory 
organs,  and  the  coordinating  centers  (forebrain)  are  already  definitely  located  in  the 
procephalon  of  the  nauplius,  which  probably  represents,  in  part,  the  remnants 
of  a  trochosphere.  These  organs  are,  therefore,  of  very  great  antiquity.  They 
retain  their  original  position  throughout  the  entire  range  of  the  arthropod-verte- 
brate phylum,  and  by  the  root-like  extension  of  their  nerve  fibers  establish  re- 
lations with  the  new  metameres  as  fast  as  they  are  formed. 

Hence  the  primary  sense  organs  and  the  primary  coordinating  centers  are 
located  at  the  anterior  end  of  the  body,  not,  as  is  frequently  asserted,  because  the 
body  moves  head  first,  or  because  of  any  necessary  correlation  between  the  location 
of  the  brain  and  sense  organs  (Parker),  but  because  the  head  is  the  oldest  part  of 
the  animal,  and  because  these  particular  sense  organs  and  nerve  centers  were,  in 
a  historic  sense,  the  first  ones  to  be  definitely  established,  taking  their  origin  back 
to  a  period  when  the  primitive  head  was  the  whole  body. 

With  the  appearance  of  the  first  postcephalic  metameres,  arose  the  first 
gustatory  organs,  and  the  first  swimming,  grasping,  crushing,  and  chewing  ap- 
pendages. They  were  necessarily  located  immediately  behind  the  primitive 
head,  in  the  oral  region.  With  the  addition  of  another  generation  of  metameres, 
the  body  became  heavier  and  larger,  and  the  appendages  on  the  new  metameres 


10 


OUTLINE    OF    THE   ARACHNID    THEORY. 


were  used  as  supplementary  swimming,  or  respiratory  appendages,  or  for  crawling 
or  walking,  and  the  circulatory  organs  appeared  in  the  haemal  region.  (Fig.  308.) 
The  internal  organs,  such  as  the  stomach,  digestive  glands,  gut  pouches, 
organs  of  excretion  and  generation,  establish  their  relations  to  the  rest  of  the  body, 
if  at  all,  through  the  circulation.  They  are  less  dependent  on  location,  or  on 


FIG.  4. — Diagrams  showing  location  of  principal  organs  in  Bothriolepis  and  a  primitive  vertebrate.     A,  B,  Haemal; 
view;   C,  neural  view.    A,  Bothriolepis;   B,  C,  primitive  vertebrate. 


specialization  in  form,  for  effective  action,  hence  they  are  eventually  crowded  into 
the  more  posterior  metameres,  or  they  atrophy  and  new  ones  arise  farther  back 
to  take  their  places.  (Figs.  307,  308.) 

In  the  typical  arachnids,  a  definite  linear  arrangement  of  unlike  functions,  in 
accordance  with  the  above  principles,  is  established  at  an  early  period.  The 
order  is  essentially  the  same  as  that  in  the  vertebrate  head,  and  is  as  follows: 
olfactory,  coordinating,  visual,  swallowing,  gustatory,  auditory,  locomotor,  equili- 
bratory,  cardiac,  and  respiratory.  (Figs.  5,  57  and  114.) 

In  the  posterior  cephalic  regions,  the  digestive,  excretory,  and  genital  organs 
are  closely  associated  with,  or  overlap,  the  branchial  and  cardiac  organs,  this 
arrangement  forming  a  conspicuous  feature  in  the  arachnids.  It  appears  to  be 
retained,  to  a  large  extent,  in  Bothriolepis  and  other  ostracoderms.  (Fig.  5.)  In 
the  vertebrates,  this  arrangement  is  further  modified  by  the  atrophy  of  the  pre- 
branchial  locomotor  appendages,  by  the  formation  of  new  ones  behind  the  gills, 


THE    SUBDIVISIONS    OF   THE  HEAD.  II 

and  by  the  gradual  transfer  of  the  digestive  and  urinogenital  system  still  farther 
back  into  the  newly  developing  trunk. 

We  need  not  follow  in  detail  the  further  progress  of  these  changes  in  the 
higher  vertebrates;  the  atrophy  of  the  gills  and  the  development  of  the  lungs 
behind  them;  the  atrophy  of  head  kidneys,  and  the  development  of  new  ones 
farther,  and  then  again  farther  back;  and  the  final  shifting  of  locomotor  func- 
tions to  the  pelvic  appendages,  are  all  familiar  manifestations  of  the  same 
process. 

Thus  the  evolution  of  the  arthropod-vertebrate  stock  consists:  i.  in  the 
successive  generation  of  groups  of  like  metameres,  each  group  being  from  the 
beginning  somewhat  different  from  the  preceding  one;  2.  in  the  subsequent  en- 
largement, diminution,  or  elimination  of  segmental  organs  and  the  consequent  re- 
adjustments that  follow  these  changes;  the  result  always  leading  toward  a  more 
successful  linear  coordination  of  unlike  organs,  the  process  attaining  its  highest 
expression  in  man.  Hence,  broadly  speaking,  the  progress  of  organic  evolution 
in  segmented  animals  may  be  measured  by  the  extent  to  which  the  linear  coordi- 
nation of  unlike  organs  replaces  the  linear  succession  of  like  metameres. 


IV.  THE  SUBDIVISIONS  OF  THE  INCIPIENT  VERTEBRATE  HEAD  IN 
THE  ARACHNIDS. 

The  five  main  divisions  of  the  anterior  part  of  the  body  in  the  arachnids  are 
as  follows:  (Figs.  3,  5,  14-21.)  i.  The  procephalon,  or  primitive  head,  consists 
of  three  pre-oral  segments,  the  principal  organs  contained  in  it  being  the  rostrum, 
olfactory  lobes,  cerebral  hemispheres,  the  visual  and  the  olfactory  organs.  2. 
The  dicephalon  consists  of  two  or  three  metameres  immediately  surrounding  the 
mouth,  and  includes  the  stomodaeum  with  its  appropriate  nerve  centers,  the  leg- 
jaws,  principal  gustatory  organs,  and  the  anterior  part  of  the  endocranium.  3. 
The  mesocephalon  consists  of  three  or  four  posterior  thoracic  metameres  and  in- 
cludes the  principal  locomotor  appendages,  auditory  and  excretory  organs,  and 
the  posterior  part  of  the  endocranium.  4.  The  metacephalon,  or  vagus  region, 
consists  of  from  two  to  four  greatly  modified  metameres,  the  appendages  being 
either  very  small  and  standing  close  to  the  median  line,  or  absent,  or  converted 
into  sense  organs.  The  neuromeres  and  their  ganglia  are  large,  but  very  compact. 
Other  components  of  the  metameres  are  absent,  or  small  and  degenerate.  The 
whole  region  forms  a  highly  specialized,  constricted  intermediate  zone  lying  be- 
tween the  mesocephalon  and  the  next  following  division.  5.  The  branchio- 
cephalon  consists  of  four  or  five  metameres,  in  which  are  located  the  principal 
respiratory  organs,  branchial  cartilages,  and  the  heart. 

The  Brain. — The  structure  and  grouping  of  the  neuromeres  reflect  the  condi- 
tions characteristic  of  these  subdivisions  of  the  body,  thus  laying  the  foundations 
for  the  subdivisions  of  the  brain  in  vertebrates.  In  the  latter,  the  original  appearance 


12 


OUTLINE    OF    THE   ARACHNID    THEORY. 


of  the  arachnid  brain  is  modified  by  the  closure  of  the  old  mouth,  and  by  the  loca- 
tion of  the  optic  ganglia  over  the  diencephalic  and  mesencephalic  neuromeres, 
instead  of  over  the  prosencephalic  ones,  to  which  they  really  belong.  (Figs.  46, 
47,  57  and  58.) 

The  Mesoderm.  (Fig.  138,  A  and  B). — The  procephalic  mesoderm  is  scanty 
and  in  the  early  embryonic  stages  forms  a  single,  thin-walled  ccelomic  chamber. 
In  the  dicephalon  and  mesocephalon,  six  pairs  of  ccelomic  chambers  are  formed, 
constituting  true  somites,  or  head  cavities;  but  segmented  lateral  plates  are  con- 


oc. 


ol.o.        pa.ey.  Olfactory. 
Pro.C.  ---._    >^T^\         Coord™ 


FIG.  5. — Diagrams  showing  the  probable  relations  between  the  subdivisions  of  the  head  and  trunk,  and  the  location 
of  the  principal  organs  in  an  insect,  merostome  and  ostracoderm  (Bothriolepis)  seen  from  the  neural  side. 

spicuously  absent.  In  the  metacephalon  and  branchiocephalon,  distinct  somites 
and  lateral  plates  are  developed  in  each  metamere. 

The  Middlecord,  or  lemmatochord  (notochord  of  vertebrates),  extends  through 
the  posterior  sections  of  the  head.  In  the  older  stages  it  may  terminate  in  an 
enlargement  in  the  mesocephalon,  but  it  never  extends  beyond  the  dicephalon, 
ending  abruptly  just  behind  the  stomodaeum  (infundibulum). 

Let  us  examine  these  subdivisions  of  the  future  head  more  carefully. 

i.  The  Procephalon. 

The  procephalon  is  the  primitive  head.  In  the  adult  arachnids,  it  is,  exter- 
nally, an  irregular,  ill  defined  area  of  ectoderm  within  which  lie  the  rostrum,  and 
the  primitive  visual  and  olfactory  organs.  (Figs.  149-155,  p.c.)  In  the  early 
embryonic  stages,  it  is  represented  by  the  procephalic  lobes,  from  which  the  fore- 


THE    PROCEPHALON. 


Y.TYlt. 


Br.C. 


brain  with  its  olfactory  lobes,  hemispheres,  and  its  appropriate  sense  organs  are 
derived.  (Figs.  14-21.)  The  structure  of  the  procephalic  lobes,  their  main 
divisions,  and  the  relations  of  the  three  sets  of  primary  sense  organs  to  them, 
are  practically  identical  throughout  the  arthropod  series.  In  the  higher  arachnids, 
their  structure  and  mode  of  development,  and  that  of  their  associated  sense  organs, 
resembles  that  of  the  vertebrates. 

In  Insects  (Acilius),  the  procephalic  lobes  consist  of  three  segments,  each  one 
containing  a  neuromere,  an  optic  ganglion,  a  segment  of  the  marginal  plate,  and 
two    pairs    of  segmental  sense  organs,  or 
ocelli.      (Fig.  14.)     Three  infoldings  occur 
on  the  margins  of  the  lobes,  between  the 
optic  plate   and   the  optic    ganglia,    iv1'3, 
but  they  soon  close  without  involving  the 
marginal  sense  organs,  and  without  form- 
ing a  common  cerebral  vesicle. 

In  the  Arachnids  (scorpion),  the  lobes 
are  at  first  similar  to  those  of  Acilius;  later 
they  are  depressed,  and  a  thin  marginal 
fold,  or  neural  crest,  advances  over  them, 
converting  the  entire  forebrain  into  a  hol- 
low vesicle  that  for  a  long  time  opens  to 
the  exterior  through  an  anterior  neuropore. 
(Figs.  15,  16,  18,  46,  47,  an.p.  and  eph.) 

Sense  Organs. — Meantime  the  anterior 
pair  of  marginal  sense  organs  move  forward 
and  unite  in  the  median  line  to  form  the 
anlage  of  the  olfactory  organ  (Limulus). 
(Figs.  38,  39,  141,  142,  153,  ol.o.)  The  two 
pairs  of  sense  organs  on  the  second  seg- 
ment (ocellar  placodes,  parietal  eyes)  are 
ingulfed  in  the  palial  overgrowth  and  carried 
to  the  middle  of  the  roof  of  the  forebrain 
vesicle.  Here  a  tubular  outgrowth  is  for- 
med, on  the  end  of  which  the  ocellar  placodes  are  located,  after  the  manner 
of  a  typical  parietal  eye.  (Figs.  46,  47,  57,  141,  142,  pa.e.)  In  the  arach- 
nids, the  sense  organs  of  the  third  segment  (lateral  eye)  lie  for  a  time  on  the 
outer  margin  of  the  neural  crests,  but  later  they  move  away  from  them, 
so  they  are  not  ingulfed  in  the  palial  overgrowth.  (Fig.  16,  A. I.  e.)  The  lateral 
eyes  of  insects,  crustaceans,  and  arachnids  appear  to  belong  to  the  fourth  neuro- 
mere  (antennal  or  cheliceral),  that  is,  to  the  first  metamere  of  the  next  division 
of  the  head. 

Olfactory  Lobes,  Hemispheres  and  Optic  Ganglia. — During  the  formation  of 
the  palial  overgrowth,  the  first  forebrain  segment  becomes  deeply  infolded  to 


FIG.  6. — Mesonacis  (Olenellus)  vermontana 
(Hall).  Lower  Cambrian,  showing  body  regions, 
and  groups  of  like  metameres,  or  tagmata. 


j^  OUTLINE    OF    THE   ARACHNID    THEORY. 

form  the  olfactory  lobes.  The  cerebral  hemispheres  arise  from  the  median  part 
of  the  second  segment,  the  optic  ganglia  of  the  parietal  eye  (ganglion  habenula), 
from  the  lateral  margin  of  the  second  segment,  and  the  ganglion  of  the  lateral 
eyes  (tectum  opticum),  from  the  lateral  lobes  of  the  third  or  fourth  segment. 

(Figs.  15,46,  47>  oil) 

The  Rostrum  (labrum)  in  insects  arises  as  a  pair  of  small  cephalic  appen- 
dages, on  the  very  anterior  median  margin  of  the  cephalic  lobes.  (Fig.  14.)  In 
the  arachnids  it  forms  an  unpaired,  immovable  process,  which  in  the  later  stages 
lies  on  the  anterior  margin  of  the  mouth.  (Figs.  15,  17,  18,  43^  47-)  It  differs 
from  all  other  arthropod  appendages,  in  that  it  receives  its  nerves  from  ganglia 


FIG.  7. — Primitive  Crustacea  seen  from  the  neural  surface,  showing  various  arrangements  of  the  procephalic  sense 
organs.     A,  Sida;  B,  Limnadia  larva;  C,  Branchipus  larva. 

situated  on  the  median  side  of  each  nerve  cord,  that  is,  from  the  stomodaeal  ganglia 
and  commissure,  which  are  situated  near  the  fourth,  or  first  post-oral,  segment. 
(Figs.  38  and  39,  st.g.) 

External  Boundaries  of  the  Procephalon  in  the  Adult. — The  margins  of  the 
ectodermic  area  covering  the  outer  surface  of  the  forebrain,  after  the  palial  over- 
growth is  formed,  mark  the  boundaries  of  the  primitive  head.  The  latter  becomes 
greatly  distorted  by  the  forebrain  flexure,  which  carries  the  anterior  part  of  the 
forebrain  round  the  end  of  the  egg  onto  the  future  haemal  surface,  while  the  pos- 
terior part  is  drawn  a  long  way  backward  by  the  caudad  migration  of  the  mouth 
and  rostrum.  (Figs.  3,  17,  43,  44,  46.)  It  thus  happens  that  the  neural  surface 
of  the  procephalon  is  the  only  one  that  is  actually  developed.  The  haemal  surface 
is  not  formed  from  procephalic  tissue,  but  by  the  extension  of  the  lateral  and 
anterior  margins  of  the  procephalic  lobes  around  the  anterior  end  of  the  ovum, 


THE    DICEPHALON   AND    MESOCEPHALON.  15 

and  by  their  union  there  with  the  haemal  end  of  the  first  thoracic  metameres. 

(Fig.  17-) 

In  this  way  the  original  area  of  the  procephalic  ectoderm  has  been  greatly 
extended.  In  the  adult  Limulus,  it  is  divided  into  two  isolated  parts:  that  which 
has  been  carried  onto  the  haemal  surface  of  the  carapace,  and  that  which  remains 
on  the  neural  surface.  (Figs.  141-155.)  The  latter  portion  may  be  approxi- 
mately defined  as  an  elongated  area,  with  the  olfactory  organ  at  its  anterior  end 
and  the  apex  of  the  rostrum  at  its  posterior  end;  it  is  drawn  out  laterally  by  the 
migration  of  the  lateral  eyes  toward  the  posterior  haemal  surface.  (Fig.  153,  pr.c.) 

In  the  scorpion,  there  is  a  neural  and  haemal  section  of  the  procephalon,  as  in 
Limulus.  (Comp.  Figs.  16,  17,  18,43.)  The  original  neural  surface  of  the  em- 
bryonic procephalon  has  been  doubled  over  in  the  adult  so  that  its  anterior  edge 
lies  on  the  haemal  surface,  directed  backward  instead  of  forward.  (Figs.  17,  22.) 


FIG.    8. — Primitive    crustaceans  (Cladocera). 
A,  Neural  surface;  B,  haemal. 


FIG.  9. — Same  in  side  view  and  in  median 
section. 


It  is  important  to  bear  these  facts  in  mind,  since  where  these  changes  have 
taken  place,  the  linear  arrangement  of  the  segmental  sense  organs  appears  to  be 
the  reverse  of  what  it  is  when  the  procephalon  remains  largely  on  the  neural 
surface,  as  it  does  in  many  phyllopods  and  vertebrates.  (Figs.  7,  8,  9,  34.) 

2-3.  The  Dicephalon  and  the  Mesocephalon. 

The  dicephalon  and  the  mesocephalon  include  the  first  six  or  seven  post-oral 
metameres,  frequently  spoken  of  as  the  thorax.  It  is  generally  divided  into  two 
regions.  The  anterior  one,  the  dicephalon,  consists  of  two  or  three  circum-oral 
metameres  whose  appendages  may  be  smaller  than  the  others,  and  specially 
modified  to  serve  as  leg-jaws  for  testing,  holding,  tearing,  or  crushing  food,  and 
conveying  it  to  the  mouth.  It  includes  the  stomodaeum  and  the  stomodaeal 
ganglia,  the  latter  being  intimately  associated  with  the  gustatory  and  swallowing 
reflexes.  The  posterior  division,  or  mesocephalon,  comprises  three  or  four  well- 
developed  metameres  whose  appendages  serve  for  walking  or  swimming. 


i6 


OUTLINE    OF    THE    ARACHNID    THEORY. 


In  the  insects,  the  first  four  metameres  fuse  with  each  other,  and  with  the 
procephalon,  to  form  the  so-called  "head,"  the  last  three  metameres  usually 
remaining  separate.  (Figs.  3, ,4,  5, A.) 

In  phyllopods  (Branchipus)  the  first  two  metameres,  and  possibly  an  evanes- 
cent third,  or  premandibular,  fuse  with  each  other  and  with  the  procephalon. 
The  remaining  three  metameres,  the  mandibular  and  two  maxillary,  fuse  with 
each  other,  forming  a  group  by  themselves  distinct  from  the  anterior  division. 

In  arachnids,  such  as  the  scorpions,  spiders,  trilobites,  and  merostomes, 
all  six  thoracic  metameres  unite  with  one  another  and  with  the  procephalon  to 
form  the  cephalothorax,  leaving  on  the  haemal  side  little  or  no  indication  of  the 
larger  divisions,  or  of  the  more  primitive  division  into  metameres. 

On  the  neural  side,  the  metameric  structure  is 
always  retained.  The  reduction  in  size,  and  the 
modification  of  the  first  two  or  three  pairs  of  appen- 
dages for  feeding  purposes,  are  usually  clearly  indica- 
ted, the  last  three  or  four  pairs  serving  mainly  for 
locomotion. 

In  Limulus,  the  subdivision  of  the  thorax  into 
an  anterior  and  a  posterior  division  at  first  sight  does 
not  appear  to  exist;  but  I  have  shown  that  in  abnor- 
mal embryos  the  first  two  or  three  thoracic  metameres 
act  as  a  unit,  in  that  they  frequently  separate  from 
the  posterior  ones  opposite  the  large  thoracic  sense 
organs,  or  they  fuse  with  each  other,  or  disappear 
entirely,  or  otherwise  manifest  a  distinct  independence 
in  their  development.  The  large  thoracic  placodes, 
the  forerunners  of  the  auditory  placodes  of  verte- 
brates, mark  this  latent  cleavage  line  between  the 
FIG.  io.— Mesothyra  (after  Haii  and  group  of  oral  metameres  of  the  diacephalon,  and  those 

Clark).     Upper  Devonian.  ,      , 

belonging   to    the    mesocephalon.      (Figs.     141,    142, 
184-188.) 

The  endocraninm  arose  primarily  in  association  with  the  dicephalic  met- 
ameres, but  in  the  higher  forms  takes  its  origin  from  the  mesocephalic  meta- 
meres also.  With  the  concentration  of  all  the  cranial  neuromeres,  the  endo- 
cranium  embraces,  or  underlies  all  of  them  except  the  more  posterior  ones  of 
the  branchiocephalon. 


Oral  Arches.— The  basal  joints  of  the  thoracic  appendages,  especially  in 
the  arachnids,  are  greatly  expanded  where  they  join  the  body,  forming  oblong 
arches  to  which  the  slender,  more  movable  part  of  the  appendage  is  attached.  In 
the  arachnids,  these  basal  arches  may  be  located  some  distance  from  the  median 
line,  on  the  lateral  wall  of  the  head.  At  least  four  or  five  of  these  anterior  thoracic 


THE  DICEPHALON  AND  MICROCEPHALON.  17 

arches  persist  as  the  circumoral,  visceral  arches  of  vertebrates,  that  is,  as  the 
pre-maxillary,  maxillary,  mandibular,  and  hyoid  arches,  and  possibly  the  first 
gill  arch.  (Figs.  32-34,  160-172.) 


Taste  Buds,  Slime  Buds,  and  Cranial  Ganglia. — In  the  typical  appendicular 
arches  of  arachnids,  there  is  a  lobe  on  the  median  or  neural  side  that  forms  the 
mandibular  or  coxal  spurs,  and  in  which  are  located  important  groups  of  sense 
organs,  i.e.,  gustatory  buds  and  slime  buds.  They  are  the  forerunners  of  the 


.pa.e 


FIG.    ii.  pIG.    I2. 

FIG.  ii. — Diagrams  of  marine  arachnids,  to  illustrate  the  relations  of  their  organs  to  those  in  the  ostracoderms. 
FIG.   12. — C,  hypothetical  form,  intermediate  between  a  merostome  and  an  ostracoderm  (Cephalaspis) ;  D,  is 
an  accurate  restoration  of  a  small  cephalaspid  (sp.  nov.  ?)  from  Scaumenac  Bay,  P.  Q.,  except  the  external  gill, 
ex.  g.  which  are  hypothetical. 

" epibranchial  organs,"  "lateral  line  organs,"  and  "gustatory  organs"  of  verte- 
brates. At  an  early  embryonic  period,  in  the  wide  zone  between  the  nerve  cord 
and  the  coxal  and  gustatory  spurs,  and  in  close  connection  with  the  latter,  immense 
oblong  ganglia  (pedal  ganglia)  are  developed  from  thickenings  of  the  overlying 
ectoderm.  (Figs.  36-39,  134-137.)  These  ganglia  arise  independently  of  the 
medullary  plates.  Later,  they  unite  the  proximal  end  of  the  pedal  nerve  with  the 
corresponding  neuromere.  They  are  the  forerunners  of  the  cranial  ganglia  of 
vertebrates. 

Segmental  Sense  Organs. — In  the  scorpion,  each  appendicular  arch,  except 
the  first,  has,  on  its  lateral  margin,  close  to  the  base  of  the  coxa,  two  sensory  cups, 
in  form  and  in  minute  structure  very  similar  to  the  conspicuous  pits  on  the  outer 
surface  of  the  neuromeres.  (Figs.  15-16,  74,  s.so.)  All  these  pits  quickly  lose 
their  sensory  character  and  later  apparently  disappear  or  are  converted  into  gan- 
glion cells. 


i8 


OUTLINE    OF    THE   ARACHNID    THEORY. 


Similar  segmental  sense  organs  are  seen  in  Limulus,  but  farther  removed  from 
the  bases  of  the  appendages.  The  one  that  develops  into  the  so-called  "  dorsal 
organ"  (auditory  pit  of  vertebrates)  lies  opposite  the  fourth  appendage.  (Figs. 
140-153,  s.o. 4)  Later  it  becomes  greatly  enlarged  and  is  a  conspicuous  feature 
on  the  haemal  margin  of  the  thorax  till  after  the  last  moult  of  the  trilobite  stage. 
At  the  height  of  its  development,  it  is  a  disc-shaped  thickening,  slightly  pigmented 

and  sensory  in  appearance.  (Fig.  131.) 
The  four  remaining  pits  (Fig.  140)  are 
very  faint  and  transitory,  although  in  the 

^:,v  corresponding  regions  of  the  adult,  there 

*<.'.          J*  are  patches,  or  knobs  of  skin  that  are  highly 

4*ili  sensitive  and  richly  supplied  with  nerves. 

The  thoracic  segmental  sense  organs  of 
Limulus  and  the  scorpion  lie  nearly  in  line 
with  the  cephalic  sense  organs,  and  are 
probably  serially  homologous  with  them. 


The   Diencephalon   and  the    Mesencep- 
halon. — We  may  recognize  two  groups  of 
thoracic  neuromeres,  the  diencephalon  and 
mesencephalon,  approximately  correspond- 
ing with  the  external  divisions  of  the  thorax. 
The    diencephalon,   or  tween-brain, 
consists  of  the   first  one,  or  two  or  three, 
neuromeres  that  surround  the  oesophagus. 
It  includes  the  large,  lateral  stomodaeal 
ganglia  that  are  attached  to  the  median  wall 
of  the  cheliceral  neuromere,  but  which  arise 
as   thickenings,  or  evaginations,   from  the 
side  walls  of  the  oesophagus.     These  neu- 
FIG.  13— Bunodesiunuia.    Restoration  from    romeres  contain  the  swallowing  center  and 

numerous     specimen     in    the    author's    collection     an   imnnrfonf  renter  fnr  all   tVip  tasfp  nraanc 
obtained  from  the  island  of  Oesel,  Russia.     Photo-      &U     mP°rtant  Center  lor  all  tJlC  tEStC  Organs 

of  the  more  posterior  thoracic  appendages. 
(Fig.  114.) 

The  enlargement  and  closer  union  of  the  thoracic  neuromeres,  and  the  back- 
ward overgrowth  of  the  rostrum  and  the  optic  ganglia,  ultimately  lead  to  the 
closure  of  the  mouth.  After  it  closes,  the  inner  end  of  the  stomodaeum  persists 
in  vertebrates  as  the  epithelium  of  the  saccus  vasculosus,  the  passageway  between 
the  circum-oesophageal  neuromeres  becomes  the  infundibulum,  and  the  stomo- 
daeal ganglia,  arising  from  its  deeper  side  walls,  the  lobi  inferiori.  (Figs.  43  and 
44.)  The  last  position  occupied  by  the  arachnid  mouth  mav  be  identified  in 


graph  of  enlarged   plaster  model   by   the   author 
X  about  2. 


THE    METACEPHALON.  1 9 

vertebrates,   as  the  opening  behind  the  cerebellum,  now  closed  by  the  choroid 
plexus  of  the  fourth  ventricle.     (Figs.  3,  43,  44,  46,  58.) 

The  mesencephalon  consists  of  the  last  three  or  four  thoracic  neuromeres; 
they  are  usually  conspicuous  for  their  distinctness,  great  breadth  and  volume, 
and  for  the  large  size  of  their  ganglia.  In  the  vertebrates,  they  form  the  pos- 
terior portion  of  the  crura  cerebri,  and  are  still  further  accentuated  as  one  of  the 
principal  divisions  of  the  brain,  by  the  migration  of  the  optic  ganglia  of  the  lateral 
eyes  backward  and  upward  till  they  come  to  overlie  them  as  the  tectum  opticum. 
The  parietal  eye  ganglia  overlie  the  diencephalon  as  the  ganglia  habenulae.  (Figs. 

43>  44,  57,  58-) 

The  Suprastomodceal  Commissure  and  the  Cerebellum. — In  all  arthropods, 
the  lateral  stomodaeal  ganglia  are  united  by  a  large  commissure  that  forms  a 
prominent  arch  over  the  anterior  or  neural  surface  of  the  stomodaeum.  This  com- 
missure is  one  of  the  most  conspicuous  and  constant  landmarks  in  the  arthropod 
brain.  (Fig.  3,  st.co.)  In  the  insects,  it  contains  a  large,  median  mass  of  gan- 
glion cells,  arising  as  an  evagination,  or  as  a  thickening,  in  the  anterior,  median 
wall  of  the  stomodaeum,  close  to  its  external  opening.  (Fig.  3,  a.)  The  projecting 
arch  of  the  commissure  becomes  crowded  backward  by  the  backward  migration 
of  the  mouth  and  rostrum,  and  by  the  increasing  size  of  the  lateral  eye  ganglia, 
forming  in  vertebrates  the  rudiment  of  the  cerebellum.  (Figs.  3,  D,  and  46.) 

Thus  the  median  stomodaeal  ganglion  of  arthropods  and  the  cerebellum  of 
vertebrates  are  the  only  brain  structures  that  may  be  said  to  arise  originally  in  the 
median  line  above  the  neural  surface  of  the  brain;  the  parietal  eyes,  the  ganglia 
habenulae,  and  the  optic  lobes  being  originally  paired  structures  arising  from  the 
lateral  margins  of  the  medullary  plate. 

4.  The  Metacephalon,  or  Vagus  Region. 

The  metacephalon,  or  vagus  region,  forms  a  remarkable  intermediate  zone 
between  the  mesocephalon  and  the  branchiocephalon.  It  consists  of  from  one 
to  four  metameres  that  usually  atrophy,  or  fuse  with  one  another  at  an  early 
period,  leaving  little  or  no  external  trace  of  their  existence  in  the  adult.  Their 
feeble  development  is  the  principal  cause  of  the  sharp  constriction  which,  in  many 
insects  and  arachnids,  separates  the  thorax  from  the  abdomen.  (Figs.  3,  6,  14, 
15,  16,  46,  47,  57,  M.  c.  or  I'g.1"4.) 

The  vagus  appendages  rarely  serve  as  locomotor  or  respiratory  organs.  They 
show  a  marked  tendency  to  become  unpaired;  they  may  dwindle  into  insignifi- 
cance, or  they  may  be  retained  as  highly  specialized  sense  organs  (chilaria  and 
metastoma  of  merostomes;  genital  papillae  and  pectens  of  scorpions). 

The  vagus  neuromeres,  on  the  other  hand,  are  well-developed,  but  they  fuse 
with  one  another  so  quickly  that  it  is  very  difficult  to  distinguish  their  boundaries 
after  the  early  embryonic  stages.  Their  motor  elements  are  greatly  reduced  and 
the  sensory  ones  correspondingly  enlarged,  owing  to  the  reduction  of  the  corre- 


20 


OUTLINE    OF    THE   ARACHNID    THEORY. 


spending  trunk  muscles  and  the  absence  of  appendages,  or  their  conversion  into 
sense  organs.  In  them  is  located  an  important  decussation  of  the  longitudinal 
tracts  passing  from  the  cord  to  the  brain,  and  vice  versa;  and  the  vagus  neuro- 
meres  are  the  most  anterior  ones  in  which  such  a  crossing  takes  place.  (Figs. 

65,  66,  114,  v. dec.) 

The  vagus  nerves  have  special  relations  with  the  heart,  intestine,  and  integu- 
ment. They  are  the  only  segmental  nerves  that  are  persistently  directed  back- 
ward into  foreign  territory,  a  result  that  is  due  in  part  to  the  forward  concentration 
of  the  vagus  neuromeres,  and  in  part  to  the  backward  growth  of  the  nerves  and 
the  atrophy  of  their  native  metameres.  (Figs.  38,  42,  57,  70,  71.) 


FIG.  14.- — Diagram   of   an   insect  embryo  (Acilius)  in  FIG.  15. — Scorpion  embryos  in  mercator 

mercator  projection.  projection. 

The  interpolation  of  the  vagus  neuromeres  between  the  mesencephalon  and 
the  branchiencephalon  is  a  very  important  and  striking  feature  in  the  morphology 
of  the  arachnids.  They  form  a  compact  group,  or  distinct  brain  region,  which  in 
its  anatomical  and  physiological  characters,  and  in  the  distribution  of  its  nerves, 
is  very  similar  to  the  vagus  region  of  vertebrates. 

5.  The  Branchiocephalon. 

This  group  of  metameres,  four  or  five  in  number,  is  the  least  specialized  of 
any  so  far  considered.  The  appendages  may  be  well-developed  (Limulus  and 
many  Crustacea)  or  they  may  be  rudimentary.  In  the  higher  forms,  this  region 
is  chiefly  notable  as  the  site  of  the  respiratory  organs,  i.e.,  the  tracheae,  gills,  lung- 
books,  and  heart.  (Fig.  3,  br.c.) 

The  mesoderm  is  complete,  each  metamere  containing  well  developed  somites 


THE    ENDOCRANIUM.  21 

and  lateral  plates.  (Figs.  16,  142.)  The  most  characteristic  organs  found  in 
these  metameres,  such  as  the  cartilaginous  branchial  bars  and  the  segments  of 
the  heart,  arise  from  the  mesoderm. 

The  neuromeres  usually  remain  separate,  but  there  is  a  tendency  for  the 
anterior  ones  to  move  forward  and  join  the  vagus  group.  In  vertebrates,  the 
entire  group  has  joined  the  vagus  neuromeres,  forming  the  most  posterior  part  of 
the  medulla.  (Fig.  58.) 

Nerves. — In  the  arachnids,  the  great  complex  of  vagus  and  branchial  nerves 
has  already  made  notable  progress  in  that  separation  of  components  from  the 
primary  segmental  nerves,  and  in  their  regrouping  into  compound  nerves  whose 
constituent  parts  have  a  common  function,  that  is  so  characteristic  of  vertebrates. 

We  may  recognize,  for  example,  the  beginning  of  the  lateral  line  nerve  in 
the  combined  sensory  components  of  the  first  three  vagal  appendages  of  the 
scorpion.  In  Limulus,  the  primitive  condition  of  the  vertebrate  cardiac  nerves 
is  seen  in  the  eight  pairs  of  segmental  cardiac  nerves  that  arise  from  the  vagal  and 
branchial  neuromeres.  (Figs.  59,  78,  c7~14.)  The  visceral  arch  nerves  are  repre- 
sented by  the  branchial  nerves,  and  the  hypoglossal,  by  the  combined  group  of 
motor  components  that  supply  the  great,  branchio-thoracic  muscles.  (Fig.  77.) 
The  intestinal  nerves  are  also  indicated;  iI~I°. 

It  is  only  necessary  to  unite  the  vagal  and  branchial  neuromeres  into  a 
more  compact  mass,  and  to  complete  the  union  of  the  sensory,  branchial,  hypo- 
branchial,  cardiac,  and  intestinal  components  into  compound  nerves,  to  realize 
the  characteristic  condition  so  familiar  in  vertebrates.  (Compare  Figs.  57  and  58.) 

It  will  be  observed  that  the  similarity  exists,  not  only  in  the  union  of  the 
originally  separate  components  into  the  same  physiological  groups,  but  that  the 
number  of  neuromeres  and  components  is  approximately  the  same;  that  their 
topographical  position  is  the  same;  and  that  the  general  course  and  distribution 
of  the  resulting  nerves  is  the  same. 

The  Endocranium. 

All  the  higher  arachnids  are  provided  with  a  cartilaginous  endocranium  that 
is  the  forerunner  of  the  primordial  cranium  of  vertebrates.  It  may  be  traced 
back  to  such  primitive  arthropods  as  Branchipus,  Apus,  and  other  phyllopods.  In 
Branchipus,  it  is  a  small  plate  of  cartilage,  lying  on  the  haemal  side  of  the  mesen- 
cephalon,  and  serving  for  the  attachment  of  the  mandibular  muscles. 

In  the  higher  arachnids,  it  is  more  voluminous,  serving  mainly  for  the  at- 
tachment of  the  leg  and  jaw  muscles,  and  for  the  great  longitudinal  muscles  that 
move  the  cephalothorax  on  the  branchial  section  of  the  body.  Its  structure  is 
similar  to  that  of  the  primordial  cranium  of  vertebrates,  and  it  has  the  same  topo- 
graphical relation  to  the  brain  and  to  the  alimentary  canal.  The  rudiments  of 
the  following  parts  may  be  recognized :  occipital  ring,  trabeculse,  pituitary  fora- 
men, and  palato-pterygoid  arch.  (Figs.  209-220.) 


22 


OUTLINE    OF    THE   ARACHNID    THEORY 

The  Mesoderm. 


Origin  of  the  mesoderm.  To  understand  the  peculiarities  of  the  cephalic 
mesoderm,  we  must  consider  its  origin  as  a  whole. 

In  Limulus,  the  mesoderm  arises  in  part  from  the  telopore,  a  shallow,  terminal 
depression  overlying  a  confused  mass  of  proliferating  nuclei  destined  to  form  meso- 
derm, yolk  cells,  and  endoderm.  (Figs.  128  and  140.) 

As  the  embryo  elongates,  the  depression  maintains  its  terminal  position, 
changing  to  a  longitudinal  groove,  and  finally  taking  the  form  of  a  typical  primi- 


FIG.  1 6. — Scorpion  embryos  in  mercator  projection. 

tive  streak.  (Figs.  129,  130,  140,  t.p.}.  From  the  primitive  streak,  a  sheet  of 
mesoderm  extends  forward  and  laterally,  finally  breaking  up  into  somites  and 
lateral  plates. 

In  the  abdominal,  or  branchiocephalic  and  vagal  regions,  the  somites  are 
hollow,  contain  true  coelomic  cavities,  and  are  quite  distinct  from  the  overlying 
ectoderm.  The  corresponding  lateral  plates  are  sharply  segmented,  and  they 
are  united,  for  a  short  period,  with  the  overlying  ectoderm.  They  appear  to  be 
formed  from  the  ectoderm  by  a  local,  inward  proliferation,  that  takes  place,  not 
only  in  the  region  of  the  germ  wall,  but  along  the  lines  that  mark  the  anterior 
and  posterior  boundaries  of  the  lateral  plates.  (Fig.  128,  a.) 

On  the  peripheral  margins  of  the  expanding  mesodermic  area,  no  segmenta- 


THE    MESODERM.  23 

tion  of  any  kind  is  visible.  Ectoderm,  mesoderm,  and  yolk  cells  form  a  common, 
thickened  rim,  or  germ  wall,  similar  in  general  appearance  to  the  early  stages  of 
the  primitive  streak,  and  extending  along  the  entire  lateral  margins  of  the  germinal 
aiea.  (Figs.  140-142.) 

The  post-oral  mesoderm  therefore  arises  from  three  distinct  sources.  The 
axial  portion,  consisting  of  the  double  line  of  mesoblastic  somites,  arises  from  the 
primitive  streak;  it  represents  the  trail  of  mesoderm  cells  left  behind  as  the  telo- 
blasts  of  the  primitive  streak  migrate  backward.  The  greater  part  of  the  lateral 
plate  mesoderm  is  formed  from  the  proliferating  cells  of  the  germ  wall,  as  it 
spreads  over  the  surface  of  the  yolk  in  a  lateral  direction.  But  on  the  median 
side  of  the  germ  wall,  the  definitive  ectoderm  continues  to  proliferate  inward  for 
a  considerable  distance  along  the  lines  that  separate  the  lateral  plates.  The  cells 
thus  produced  form  a  part  of  the  lateral  plates,  and  the  proliferating  lines  break 
the  lateral  sheet  of  mesoderm  into  distinct  segments. 

The  dicephalic  and  mesocephalic  (thoracic)  mesoderm  of  arachnids  presents 
a  most  important  modification.  It  forms  at  first,  a  well  defined  band  on  either 
side  of  the  nerve  cord.  Each  band  then  becomes  divided  into  distinct  ccelomic 
chambers  or  somites;  but  segmented  lateral  plates  are  absent,  the  mesoderm  of 
that  region  consisting  of  scattered  cells  that  are  not  visible  in  surface  views. 
(Figs.  15,  1 6,  19-21.)  From  the  thoracic  somites,  or  head  cavities,  arise  the 
muscles  of  the  appendages,  the  cartilaginous  cranium,  and  the  secreting  cells  of 
the  coxal  gland,  or  head  kidney. 

The  procephalic  mesoderm  is  scanty  and  unsegmented,  forming  a  thin 
walled,  unpaired  coelomic  vesicle  that  breaks  down  into  scattered  cells.  The 
procephalic  mesoderm  appears  to  arise  from  the  primitive  cumulus  before  apical 
growth  begins. 

Comparison. — With  the  progress  of  cephalization  in  the  arthropods,  there 
has  been,  therefore,  a  steady  decrease  in  the  volume  of  mesodermic  structures. 
In  the  higher  arachnids,  mesoderm  is  almost  absent  in  the  procephalon,  and  the 
lateral  plates  are  absent  in  the  dicephalic  and  mesocephalic  regions.  The  result, 
or  cause,  if  you  will,  is  the  absence  of  the  thoracic  sections  of  the  heart  and  of  the 
longitudinal,  intersegmental  muscles;  the  shortened  thoracic  tergites  then  fuse 
with  one  another  and  with  the  procephalon  to  form  a  continuous  unsegmented 
shield,  or  cephalic  buckler. 

In  the  vertebrates,  the  decrease  in  volume  of  the  cephalic  mesoderm  is  carried 
still  further,  affecting  the  anterior  head  regions,  as  well  as  the  more  posterior 
ones,  that  in  the  arthropods  are  usually  well  equipped  with  mesodermic  structures. 
This  decrease  is  due  chiefly  to  the  progressive  atrophy,  or  fusion,  or  condensation 
of  what  were  originally  freely  movable  parts,  and  the  consequent  reduction  in  the 
number  and  volume  of  cranial  muscles.  For  example,  practically  all  the  haemal, 
longitudinal,  intersegmental  muscles  disappear  with  the  fusion  of  the  branchial 
region  with  the  head.  The  several  pairs  of  originally  separate  leg-jaws  fuse  into 
unpaired  oral  arches,  only  one  of  which  is  freely  movable.  The  mesocephalic 


24  OUTLINE    OF    THE   ARACHNID    THEORY. 

locomotor  appendages  and  their  voluminous  muscles  disappear,  and  also  numerous 
endocranial  muscles,  owing  to  the  union  of  the  endocranium  with  the  dermal 
skeleton.  Finally  the  branchial  appendages  lose  a  part  of  their  muscles  in  their 
conversion  into  lung-book-like  gill  pouches. 

This  progressive  degeneration  of  the  cephalic  mesoderm,  from  before  back- 
ward, has  been,  therefore,  an  ever  present  factor,  exercising  a  persistent  and  power- 
ful influence  over  the  form  of  the  head  and  the  structure  of  the  brain  thoroughout 
the  whole  arthropod-vertebrate  phylum. 

The  Vascular  Area  and  Concrescence. 

Vascular  Area. — The  mode  of  growth  of  the  extra  embryonic  area,  the 
concrescence  of  the  germ  wall,  and  the  character  of  the  mesoderm  in  the  various 
regions  is  shown  in  Fig.  138, 


FIG.  17.— A-C,  Scorpion  embryos  in  side  view,  semi-diagrammatic.     The  thoracic  appendages  are  removed  in  B 
and  C.     D,  Diagram  indicating  relations  of  the  cephalic  organs  in  arachnids  to  those  in  vertebrates. 

The  margin  of  the  germinal  area  belonging  to  the  thoracic  metameres  is 
greatly  thickened,  forming  large  masses  of  spherical  or  oval  cells  containing  a 
small  excentric  nucleus,  and  a  brilliantly  refractive,  colorless  thread,  usually 
coiled  with  great  regularity  in  the  long  axis  of  the  cell.  (Fig.  131.) 

Some  of  these  cells  are  ultimately  converted  into  muscles,  others  remain  as 
free  amoeboid  cells,  and  in  the  adult  may  be  found  in  great  numbers  scattered 
among  the  connective  tissue  lacunae,  in  the  anterior  part  of  the  cephalothorax. 

Whether  the  degenerating  muscle  cells  of  the  cephalothorax  are  to  be  re- 
garded as  true  blood  corpuscles  or  not  is  doubtful;  but  it  is  evident  that  owing 
to  the  increase  in  size  of  the  yolk  sphere,  there  is  already  established,  in  the  higher 


THE    CEPHALIC    NAVEL.  25 

arachnids,  an  extra  embryonic  germinal  area,  and  that  certain  parts  of  this  area 
may  be  regarded  as  the  beginning  of  an  extra  embryonic  vascular  area.  The 
peripheral  ends  of  the  vagus  and  abdominal  lateral  plates  give  rise  to  the  heart, 
pericardium,  longitudinal  haemal  muscles,  and  to  blood  corpuscles. 

Concrescence. — As  the  lateral  margins  of  the  germinal  area  grow  faster 
than  the  median  portion,  concrescence  of  the  germinal  wall  will  ultimately  occur 
in  the  precephalic  and  post  caudal  regions.  In  very  large  yolked  eggs,  pre- 
cephalic  concrescence  will  tend  to  bring  the  cardiomeres  into  conjunction,  either 
in  front  of,  or  underneath  the  procephalon,  that  is  in  their  characteristic  position 
invertebrates.  (Figs.  17-23,  138,  140,  141.) 

The  post  caudal  concrescence  will  tend  to  unite  the  posterior  margins  of 
the  germ  wall  behind  the  real  apex  of  the  body,  giving  rise  to  the  various 
phenomena  in  vertebrates  that  have  been  confused  with  "gastrulation,"  "con- 
crescence of  the  lips  of  the  blastopore,"  and  with  apical  growth. 

The  New  Mouth,  Cephalic  Navel,  or  Haemastoma. 

In  the  arachnids,  there  is  a  special  area  on  the  anterior  haemal  surface, 
just  in  front  of  the  procephalon,  that  we  shall  call  the  cephalic  navel.  It  prob- 
ably occurs  in  all  arthropods,  under  various  modifications,  as  the  so-called  dorsal 
organ.  It  is  primarily  a  thickening  of  the  haemal  blastoderm,  entirely  outside,  or 
beyond  the  germinal  area.  In  the  arach- 
nids, the  thickened  blastoderm  gives  rise 
to  an  immense  mass  of  proliferating  cells 
that  are  ultimately  invaginated  into  the 
yolk,  where  they  degenerate  and  are  absor- 
bed. This  infolded  area  of  degenerating 
cells  forms  the  central  point  toward  which 
all  the  surrounding  organs  converge;  the 
germ  wall,  with  its  appropriate  structures 

advancing  toward  its  sides  and  posterior    \   ***  1!J^-^^^^^—^    i 
margin,  and   the  procephalon  toward  the  <S2sr~^    I^T^^ 

anterior  one.    There  is  thus  formed,  either 

in     front    Of,    Or    below,    the    procephalon,    a  FIG.  i8.— Anterior  end  of  an  embryo  scorpion, 

j         -i  •    i          n     ,v  showing  forebrain  completely  covered  by  the  palial 

vortex   center   toward  which  all  the  sur-  fold 

rounding  organs  move,  and  into  which  is 

infolded  the  remnants  of  the  haemal  blastoderm   (Figs.  23,  127,  138,  139,  c.nv.) 

The  opening  between  the  enteron  and  the  exterior,  thus  virtually  established  on 

the  haemal  surface,  finally  closes  in  the  arthropods,  but  in  the  vertebrates  a 

permanent  opening  is  established  at  this  point,  that  becomes  the  new  mouth,  or 

the  hcemastoma. 

Although  the  cephalic  navel  ultimately  closes  in  the  arthropods,  prophetic 
signs  of  its  future  function  are  not  lacking,  for  on  the  site  where  it  is  formed, 


26  OUTLINE    OF    THE   ARACHNID    THEORY. 

there  are  frequently  developed  adhesive  discs  (phyllopods),  or  root-like  out- 
growths (cirripeds,  copepods)  that  serve  as  organs  of  attachment,  or  for  the  ab- 
sorption of  nutriment. 

The  cephalic  navel  of  arthropods  may  be  regarded  as  one  of  the  inevitable 
products  of  apical  growth  on  a  spherical  yolk  surface,  just  as  the  belly  navel  of 
vertebrates  is  a  product  of  the  peculiar  method  of  closing  up  the  haemal  surface. 
The  center,  around  which  the  converging  lips  of  the  cephalic  navel  are  formed, 
is  the  degenerating  area  of  haemal  blastoderm,  often  called  the  dorsal  organ. 

The  Closure  of  the  Old  Mouth  or  Neostoma. 

In  the  arachnids,  there  is  a  progressive  enlargement  and  fusion  of  the  an- 
terior cephalic  neuromeres,  that  gradually  leads  toward  the  narrowing  of  the 
passageway  for  the  stomodaeum,  and  ultimately  to  the  closing  of  the  mouth.  The 
backward  growth  of  the  rostrum  and  the  transfer  of  the  optic  ganglia  to  the 
region  overlying  the  mouth,  due  apparently  to  remote,  but  persistent  and  cumu- 
lative causes,  are  contributory  factors  in  bringing  about  this  result. 

These  conditions  at  first  lead  to  a  profound  modification  of  the  mode  of  life, 
making  a  liquid,  or  finely  divided  diet  a  necessity,  and  ultimately  to  the  utilization 
of  the  cephalic  navel  as  a  new  entrance  to  the  alimentary  canal. 

Conclusion. 

In  the  arachnids,  the  body  is  built  up  by  successive  generations  of  new  groups 
of  metameres,  or  tagmata,  at  definite  historic  periods  in  the  evolution  of  the  phylum. 

The  process  of  cephalizing  the  anterior  regions  of  the  body  consists  in  the 
gradual  and  extensive  elimination  of  motor  elements  and  the  establishment  of  a 
definite  sequence  of  functions  and  organs,  according  to  an  inherently  necessary 
order. 

The  first  five  tagmata  embrace  the  first  sixteen  metameres  and  lay  the  founda- 
tions for  the  head  in  vertebrates.  Each  tagma  is  characterized  by  a  special  number 
of  metameres,  by  peculiarities  in  the  number  and  structure  of  its  neuromeres, 
sense  organs,  ganglia,  nerves,  mesoderm  and  endo-skeleton,  and  by  their  sequence 
and  mode  of  growth,  that  are  in  essential  agreement  with  those  in  the  corre- 
sponding divisions  of  the  vertebrate  head. 

The  arachnid  body  consists  of  metameres  added  to  the  primitive  head,  which 
represents  the  remnants  of  the  coelenterate  body.  The  greater  part  of  the  arachnid 
body  and  its  primitive  head  forms  the  vertebrate  head.  Nearly  all  the  vertebrate 
body  consists  of  a  new  generation  of  metameres,  not  represented  in  arachnids. 

The  conditions  created  by  apical  growth,  by  cephalization,  and  by  the  increase 
in  the  volume  of  the  yolk  sphere,  lead  to  the  closure  of  the  old  mouth,  and  to  the 
formation  of  a  new  one  on  the  haemal  surface,  the  primitive  dorsal  organ  forming 
the  starting  point  for  the  cephalic  navel,  that  ultimately  becomes  the  new  mouth. 


CHAPTER  II. 

OUTLINE  OF  ARACHNID  THEORY;  CONTINUED. 

I.  COMPARISON  OF  ADULT  ARTHROPODS  WITH  ADULT  VERTEBRATES. 

The  preceding  analyses  have  shown,  that  beneath  a  heavy  disguise  of  con- 
tour and  surface  detail,  the  structural  plan  of  an  arachnid  and  of  a  primitive 
vertebrate  is  after  all  the  same.  Let  us  now  consider  several  types  of  adult 
arthropods  and  see  how  they  compare  with  vertebrates. 

I.  Orientation  of  Neural  and  Haemal  Surfaces. 

It  will  be  seen  that  although  the  location  of  the  eyes  and  the  shape  of  the 
body  indicate  the  usual  position  of  the  animal  during  locomotion,  they  afford  no 
certain  evidence  as  to  which  is  the  neural  and  which  the  haemal  surface,  for  the 
pattern  formed  by  the  sense  organs  on  the  neural  surface  of  the  cephalothorax 
of  some  arthropods  may  be  very  similar  to  that  on  the  haemal  surface  in  others, 
and  this  fact  must  be  borne  in  mind  when  comparing  them  either  with  ostraco- 
derms  or  with  true  vertebrates. 

In  the  phyllopods,  and  in  many  other  Crustacea  that  swim  neural  side  up  by 
means  of  oar-like  cephalic  appendages,  the  center  of  gravity  usually  lies  below 
the  attachments  of  the  swimming  appendages.  In  such  cases  the  parietal  ocelli 
and  lateral  eyes  lie  near  their  original  embryonic  position,  on  the  upper,  or  neural 
surface,  as  they  do  in  vertebrates.  (Figs.  7,  9,  244,  247  and  260.) 

Where  locomotion  is  effected  either  side  up,  as  in  Limulus,  the  prevalent  mode 
of  life  may  be  indicated  by  the  position  of  the  eyes  and  legs,  and  by  the  shape  of 
the  body.  Limulus,  for  example,  uses  its  sixth  pair  of  legs  as  pushing  poles, 
as  it  moves  over  soft  bottoms,  or  crawls  along  partly  buried  in  sand,  with 
little  more  than  the  median  and  lateral  eyes  exposed.  During  the  adult  stage, 
however,  it  frequently  swims,  neural  side  up,  for  considerable  periods,  and  per- 
sistently does  so  in  the  larval  or  trilobite  stages,  the  sloping,  anterior  margin  of 
the  shield,  like  a  well  turned  bow  of  a  boat,  holding  the  head  up  and  the  body 
properly  balanced.  The  same  modes  of  life  and  dual  methods  of  locomotion 
undoubtedly  occurred  in  many  trilobites  and  merostomes,  and  when  the  free 
swimming  life  predominates,  one  or  more  pairs  of  appendages  are  enormously 
enlarged  to  form  heavy,  oar-like  swimming  appendages.  The  lateral  eyes  may 
then  lie  well  forward  on  the  head,  between  the  neural  and  the  haemal  surfaces 

(Fig.  5). 

27 


28 


OUTLINE    OF    THE   ARACHNID    THEORY. 


In  Bothriolepis  (Figs.  247  and  248),  we  have  a  similarly  shaped  body,  with 
similar  oar-like  cephalic  appendages,  and  from  the  various  positions  in  which 
they  are  found  in  the  deposits,  there  can  be  no  doubt  that  they  crawled,  partly 
buried  in  soft  mud,  with  the  ocular,  or  neural  side  up,  but  swam  with  the  neural 
side  down,  the  center  of  gravity  lying  below  the  attachment  of  the  arms  toward 
the  bottom  of  the  boat-shaped  head.  The  same  was  probably  true  of  Cyathaspis 
(Fig.  244),  Tremataspis  (Fig.  236),  Pteraspis,  and  probably  to  a  less  extent  of 
Cephalaspis  (Fig.  232). 


FIG.  19. — Embryos  of  a  spider  in  side  view. 

The  prevailing  position  among  vertebrates  is  unquestionably  with  the  neural 
side  uppermost,  although,  as  we  have  just  seen,  the  most  primitive  vertebrates 
may  move  about  with  either  side  up.  It  is  by  no  means  true  that  the  prevailing 
position  of  the  invertebrates  is  with  the  neural  side  down.  In  many  annelids, 
there  appears  to  be  no  fixed  position  for  the  neural  and  haemal  surfaces.  In 
most  crawling  arthropods  (insects  and  spiders),  the  neural  side  is  directed  down- 
ward, but  probably  in  the  vast  majority  of  phyllopods,  cladocera,  copepods, 
merostomes,  and  trilobites,  and  in  the  larvae  of  decapods  and  cirripeds,  the  pre- 
vailing position,  when  swimming  freely,  is  with  the  neural  side  uppermost,  and 
that  is  the  approximate  position  in  practically  all  the  adult  cirripeds. 


BUNODES.  29 

It  is  thus  clear  that  the  position  of  the  animal  during  locomotion  has  no 
morphological  value  whatever.  It  is  necessary  to  emphasize  this  point,  because 
the  ancient  superstition,  to  the  effect  that  it  is  always  the  same  surface  of  a  verte- 
brate or  of  an  invertebrate  that  points  heavenward,  or  that  it  is  the  baptismal  name 
of  a  surface  that  determines  its  identity,  is  still  deeply  rooted  in  the  minds  of  an 
incredible  number  of  zoologists. 

II.  COMPARISON  OF  ADULT  ARTHROPODS  AND  VERTEBRATES. 

Bunodes. — The  form  that  perhaps  most  nearly  realizes  the  generalized 
arachnid  type  we  have  tried  to  portray  is  Bunodes,  a  small,  silurian  merostome 
from  the  island  of  Oesel,  Russia.  (Fig.  13.)  In  my  visit  to  this  island  in  1901, 
a  large  collection  of  these  forms  was  obtained  from  which  I  have  made  a  large 
scale  model,  showing  in  detail  the  essential  features  of  the  haemal  surface.  This 
animal  is  remarkable  for  the  fact  that  it  has  no  recognizable  exoskeleton.  The 
fossils  consist  of  well  denned,  but  very  thin,  carbonaceous  films,  in  a  fine  chalky 
matrix.  They  are  found  side  by  side  with  small  eurypterids  that  are  covered 
with  a  delicate  chitenous  membrane,  still  retaining  apparently  its  original,  chem- 
ical structure,  and  close  to  fragments  of  Tremataspis,  consisting  of  perfectly 
preserved,  calcareous,  dermal  plates.  It  is  therefore  probable  that  Bunodes  had 
neither  a  chitenous  nor  a  calcareous  exoskeleton. 

The  general  form  of  the  body  is  intermediate  between  that  of  Limulus  and 
that  of  a  trilobite,  or  of  a  typical  merostome.  All  the  five  head  divisions,  except 
the  diacephalon,  are  clearly  indicated,  and  they  are  surprisingly  like  those  in 
larval  Limuli  (Fig.  152).  There  is  a  distinct  procephalon,  six  thoracic,  two  vagal 
(chelarial  and  opercular?),  and  five  branchial  metameres.  The  most  remarkable 
feature  is  a  pair  of  short,  slender  antennae  clearly  seen  in  one  specimen. 

For  the  sake  of  exposition  we  may  picture  to  ourselves  the  manner  in  which 
an  adult  arachnid,  or  other  arthropod,  might  be  moulded  into  a  vertebrate,  although 
it  is  manifestly  impossible  for  any  adult  animal  to  be  converted  into  another. 
We  may  start  with  a  form  like  Limulus,  or  Bunodes,  or  an  eurypterid,  or  with  an 
adult  phyllopod,  like  Branchipus,  or  a  cladoceran,  or  cirriped. 

In  practically  all  these  animals,  extensive  lateral,  or  pleural  folds  develop 
on  the  sides  of  the  cephalothorax,  that  either  extend  in  a  nearly  horizontal  plane, 
to  form  a  broad,  shield-shaped  cephalothorax,  with  backwardly  directed  cornua, 
as  in  the  marine  arachnids  (Fig.  155),  or  the  folds  may  be  directed  toward  the 
neural  surface,  forming,  in  extreme  cases,  the  bivalve  shield,  or  mantle,  of  phyllo- 
pods  (Fig.  273),  ostracoda  (Fig.  307),  cladocera  (Figs.  8  and  9),  and  cirripeds 
(Fig.  275).  It  may  enclose  the  head,  or  the  entire  body,  in  a  large  peribranchial, 
or  atrial  chamber,  which  contains,  or  into  which  opens,  the  nutrient,  excretory, 
respiratory,  and  genital  organs.  Another  characteristic  feature  is  the  often 
enormous  labrum,  or  rostrum,  that  shows  a  persistent  tendency  to  migrate  back- 
ward, forming  an  overhanging  lid  to  the  mouth  (Fig.  7).  The  rostrum  and  the 


OUTLINE    OF    THE   ARACHNID/THEORY. 


FIG.  20.— Spider  embryos  in  mercator  projection.     Camera-outline 


FIG.  2 1. -Spider  embryo  in  metcator  projection.     Camera-outline.. 


COMPARISON  OF  ADULT  ARTHROPODS  AND  VERTEBRATES.          31 

atrial  folds,  together  with  the  branchial  and  oral  appendages,  thus  tend  to  enclose 
the  mouth  in  an  ever  deepening  chamber.  When  this  condition  approaches  its 
extreme  development — cirripeds,  cladocera,  etc.  (Figs.  273-275) — the  mouth  be- 
comes very  inaccessible,  and  food  can  only  reach  it  in  a  finely  divided  condition, 
carried  there  by  roundabout  ways,  in  the  currents  of  water  produced  by  the  swim- 
ming, the  oral,  or  the  branchial  appendages.  Or  the  mouth  may  become  com- 
pletely closed,  as  in  many  dwarf,  or  parasitic  cirripeds.  (Figs.  280  and  281.) 
Under  these  conditions  the  form  and  general  appearance  of  a  phyllopod-like 
arthropod,  with  its  large  branchial,  or  atrial,  chamber,  and  its  oar-like  cephalic 
appendages,  approaches  that  of  some  simple  ostracoderms,  like  Cyathaspis,  or 
Pteraspis.  (Comp.  Figs.  176  and  244.) 

If  we  compare  an  adult  Limulus  viewed  from  the  neural  surface,  with  Cephal- 
aspis  seen  from  the  same  surface  (Figs,  n  and  12),  it  will  be  seen  that  such  an 
arachnid  could  be  made  into  an  ostracoderm  by  the  union  and  backward  growth 


to. 


Pr.C. 


FIG.  22. — Young  spider,  showing  the  procephalon,  transferred  from  the  neural  to  the  haemal  surface,  and  the  loca- 
tion of  the  thoracic  appendages,  mouth,  heart,  and  respiratory  organs.     Thoracic  appendages  removed. 

of  the  anterior  margins  of  the  cephalothorax,  thus  enclosing  the  mouth  and 
appendages  in  a  large  branchial  chamber,  like  that  in  some  phyllopods  (Figs.  9- 
10).  The  eyes  and  olfactory  organs  could  remain  in  their  original  embryonic 
position  near  the  center  of  the  head;  the  olfactory  organs  in  front,  the  three 
parietal  ocelli  in  the  center,  and  the  lateral  eyes  on  either  side.  (Fig.  12.)  The 
enlarged  coxal  joints  of  the  anterior  thoracic  appendages,  extending  on  to  the 
haemal  surface,  would  form  the  visceral  arches  about  the  mouth,  the  free  append- 
ages forming  the  external  gills  and  the  jointed,  oar-like  arms.  A  varying  number 
of  infolded  branchial  appendages,  similar  to  the  lung  books  of  arachnids,  would 
initiate  the  true  gill  pouches,  and  finally  the  elongated  post-abdomen  would  form 
the  beginning  of  the  flexible  trunk,  with  its  pleural  or  lateral  folds,  from  which 
the  post-cephalic  appendages  later  arise. 

From  the  cephalaspids  we  may  easily  derive  the  remaining  ostracoderms. 
In  Bothriolepis,  the  old  cephalo-thoracic  portion  remains  comparatively  small, 
while  the  abdominal  buckler  has  become  greatly  enlarged  and  closed  on  its  neural 


32 


OUTLINE    OF    THE   ARACHNID    THEORY. 


surface  to  form  a  true  atrial  chamber  that  encloses  the  gills  and  cloaca.  (Fig.  5.) 
The  jointed,  oar-like  appendages,  which  belong  to  one  of  the  posterior  meso- 
cephalic  segments,  are  attached  to  the  angle  of  the  cornua,  that  are  here  very  small 
compared  with  those  of  Cephalaspis. 

Bothriolepis  retains  the  hinge-like  joint  in  the  vagus  region,  which  is  such 
a  prominent  feature  in  trilobites,  merostomes,  and  other  arachnids.  The  same 
joint  is  a  conspicuous  feature  in  Dinichthyes,  Coccosteus,  etc.,  a  group  of  primitive, 
fish-like  animals  that  probably  unite  the  typical  ostracoderms  with  the  true 
vertebrates.  (Fig.  250.) 

In  Tremataspis  (Figs.  236  and  237),  there  are  probably  several  pairs  of 
small  cephalic  appendages,  comparable  with  external  gills,  that  protruded  from 
the  openings  on  the  oral  surface;  the  larger,  oar-like  pair,  at  the  beginning  of  the 
series,  being  especially  noteworthy.  The  exhalent  branchial  currents  and  the 
excretory  products,  no  doubt  pass  out  of  the  posterior  end  of  the  atrial  chamber, 
as  in  Bothriolepis. 

The  assumed  changes  above  described  affect,  in  the  main,  the  external  form 
of  the  animal.  The  internal  structure  might  remain  essentially  as  it  now  is  in 
arachnids,  and,  except  for  certain  organs,  it  would  harmonize  with  the  structural 
plan  in  vertebrates.  For  example,  it  would  be  necessary,  in  order  to  complete  the 
transformation  of  an  arachnid  into  a  vertebrate,  to  close  the  old  mouth  and  to 
connect  the  new  one  and  the  gill  pouches  with  the  enteron.  The  factors  involved 
in  these  changes  are  described  elsewhere.  For  the  present,  it  is  enough  to  recog- 
nize the  fact  that  these  events  have  taken  place,  in  some  way  and  at  some  time, 
whatever  the  method  or  cause  may  have  been. 

On  the  other  hand  it  will  be  observed  that  many  internal  organs,  that  we  are 
accustomed  to  consider  as  characteristic  of  vertebrates,  are  already  present  in 
the  arachnids,  in  their  proper  position  and  relations,  and  merely  have  to  be  en- 
larged or  improved,  or  even  left  as  they  are,  to  agree  with  those  in  vertebrates  or 
ostracoderms. 

For  example,  there  is  already  present  in  Limulus,  in  addition  to  the  brain, 
sense  organs,  and  other  structures  that  have  been  considered,  a  head  kidney, 
cox.  o.,  heart,  h,  aortic  arches,  a.  o.,  and  cardinal  sinuses,  card.  s.  foreshadowing 
those  in  vertebrates.  (Fig.  2.)  There  are  infolded  gill  sacs  and  gut  pouches  in 
arachnids,  that  are  precursors  of  the  gill  clefts,  thyroids,  and  other  enteric  diver- 
ticula  in  vertebrates.  (Figs.  179-182.)  There  is  in  Limulus  and  other  arachnids 
a  large  cartilagenous  endocranium  and  gill  bars,  so  similar  in  form,  location,  and 
histological  structure  to  those  of  vertebrates,  that  they  might  readily  pass  for 
those  of  some  primitive,  unknown  member  of  that  class.  (Figs.  210-220.)  There 
is,  in  Limulus,  an  internal,  dermal  skeleton,  made  of  chiten,  it  is  true,  but  never- 
theless consisting  of  a  network  of  trabeculae,  cancellae,  Haversian  canals,  lacunae, 
and  canaliculae,  so  much  like  those  of  certain  ostracoderms  (Pteraspis)  that  it  is 
doubtful  whether  fossilized  fragments  of  one  skeleton  could  be  distinguished  from 
those  of  the  other,  if  their  real  origin  was  unknown.  (Figs.  196-207.)  And 


COMPARISON    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS.  33 

finally,  there  is  present  in  all  arthropods  that  have  been  carefully  studied  in  regard 
to  this  organ,  a  median,  subneural  cord  agreeing  in  position,  development,  and  in 
some  cases  in  function,  with  the  notochord  of  vertebrates.  (Figs.  221-231.) 

It  is  evident,  therefore,  that  the  resemblance  in  form  and  general  appearance 
between  the  ostracoderms  and  the  marine  arachnids  is  not  a  fanciful  one,  to  be 
classed  as  a  meaningless  coincidence,  or  as  due  to  mimicry,  to  parallelism,  or  to 
a  particular  mode  of  life.  The  resemblance  is  real,  and  pervades  the  whole 
organism,  and  can  be  satisfactorily  explained  only  on  the  assumption  that  there 
is  a  close  genetic  relationship  between  the  two  classes. 

III.  COMPARISON  OF  ARTHROPOD  AND  OF  VERTEBRATE  EMBRYOS. 

A  comparison  of  adult  arachnids  with  adult  vertebrates  helps  us  to  see  the 
morphological  relations  that  exist  between  the  two  types,  but  it  cannot  tell  us  how 
one  arose  from  the  other.  That  is  the  function  of  comparative  embryology,  for 
the  rise  of  one  great  class  from  another  takes  place  during  the  malleable  embry- 
onic periods,  when  transitional  stages  are  created  by  a  slow  yielding  to  the  impact 
of  successive  readjustments  between  organs  developing  under  unequal  and  un- 
stable conditions. 

Hence  the  supreme  test  of  any  broad  theory  of  phylogeny  is  its  ability  to  pre- 
sent an  unbroken  series  of  embryonic  stages,  naturally  or  inevitably  leading  from 
one  type  to  the  other,  and  to  point  out  the  efficient  causes  for  them.  This  embryonic 
series  should  include,  at  the  proper  period,  the  characteristic  anatomical  structures 
of  both  types.  The  established  direction  of  growth  shown  by  various  systems  of 
organs,  and  the  general  conditions  that  control  growth  in  the  lower  type,  should 
persist  in  the  higher,  supplying  a  past  cause  for  the  creation  of  the  fundamental 
features  of  the  new  type,  and  a  present  one  for  those  now  appearing  in  it.  There 
should  be  no  changes  demanded  that  necessitate  the  sudden  destruction  of  old 
organs,  or  the  abrupt  creation  of  new  ones;  that  interrupt  the  continuity  of  in- 
dividual life,  or  that  break,  or  entangle,  the  necessary  morphological  relations  of 
one  organ  to  another.  This  dual  embryonic  series  should  run  parallel  with, 
and  should  supplement  and  elucidate  the  series  of  adult  forms  left  along  the  trail 
made  by  the  two  types  in  their  slow  process  of  evolution. 

I  have  made  a  series  of  models  that  show  how  the  embryos  of  vertebrates  and 
arachnids  conform  to  these  requirements.  (Figs.  24-34.)  They  are  intended 
to  illustrate  the  principal  stages  in  the  development  of  a  primitive  vertebrate 
supposed  to  be  descended  from  arachnids.  The  series  begins  as  an  arachnid 
embryo  and  leads,  without  any  greater  changes  than  are  found  in  the  develop- 
ment of  the  higher  animals,  through  the  typical  embryonic  stages  of  forms  like 
Limulus  and  scorpion,  into  a  vertebrate  embryo  of  the  fish-like  or  amphibian  type. 

The  series  shows  us  that  the  early  stages  of  vertebrate  embryos,  in  all  essen- 
tial respects,  run  parallel  to,  or  are  identical  with,  those  of  arachnids;  and  that  the 
same  morphogenic  forces  which  created  the  cephalothorax  of  arachnids  find  their 
full  expression  in  the  head  of  vertebrates.  It  shows  that  both  embryos  begin 

3 


34 


OUTLINE    OF    THE   ARACHNID    THEORY. 


pc.mx 
.  mx 


their  growth  from  the  same  surface  of  the  egg  and  spread  over  the  yolk  in  the 
same  directions,  enclosing  the  opposite  side  in  the  same  manner;  that  there  has 
been  no  transfer  of  the  nerve  cord  from  one  surface  to  the  other,  as  claimed  by 
Gaskell,  and  that  the  medullary  plates  in  both  types  are  homologous,  and  not,  as 
claimed  by  C.  L.  Herrick,  one  dorsal,  the  other  ventral. 

Form  Controlling  Factors  in  the  Early  Stages. — Apical  growth  and  the 
volume  and  composition  of  the  yolk  sphere  are  impor- 
tant factors  in  the  development  of  the  embryo,  because 
the  physical  and  chemical  composition  of  the  yolk 
sphere  controls  the  rate  of  radial  growth,  while  the 
circumference  of  the  yolk  sphere,  and  the  ratio 
between  the  rate  of  apical  and  bilateral  growth,  deter- 
mines the  relative  time  and  place  at  which  certain 
organs  arise,  and  the  physical  conditions  under  which 
they  develop.  Owing  to  the  relatively  large  volume 
of  the  yolk  sphere  in  the  arachnids,  neither  the  ccelente- 
rate  nor  trochosphere  stages  can  assume  the  form  of 
the  ancestral,  free  swimming  animal,  i.e.,  a  nearly 
spherical  body  growing  in  each  of  three  dimensions  at 
about  an  equal  rate,  for  they  are  reproduced  in  the 
arachnid  egg  under  totally  different  conditions.  They 
appear  at  a  time  when  cell  growth  is  beginning  on  the 
outer  surface  of  a  relatively  large  sphere  of  inert  ma- 
terial, and  the  various  organs  must  be  mapped  out  in 
one  plane,  like  a  Mercator  projection  of  the  earth's 
surface.  Moreover  at  these  early  stages,  the  develop- 

FIG.  23. -Diagram  illustrating  ment  of  the  deePer  lying  organs  is  delayed,  owing  to 

a  hypothetical,  transitional  condi-  the  impenetrability  of  the  yolk  and  the  lack  of  respira- 
tion, between  the  embryo  of  a 

marine    arachnid    and    that    of   a  tory  facilities. 

primitive  vertebrate      It  shows  the  Th  jj    th  j          .  t    fe       expressed   jn 

convergence  of  procephalon,  appen-  J 

dicuiar  arches,  and   mesodermic    the  form  of  a  film  in  which  the  rate  of  growth,  in  the 

lateral    plates    around    the     dorsal        ,  , .  .  .  „.,.,,,. 

organ  to  form  the  cephalic  navel,  tnree  dimensions,  is  very  unequal.  This  film  increases 
or  the  aniage  of  the  h^mostoma.  jn  iength  by  a  process  of  apical  growth,  which  takes 

The   uncovered  yolk,   that   is   sur-  J 

rounded  by  the  concrescing  germ    place  at  one  end  only;  in  breadth  by  bilateral  growth, 

walls   and  cardiomeres,  constitutes  i     •       ,  r  •    i  i  ^•    ^  ^i  rni  i 

the  beiiy  navel,  s.  TV.  anc*  m  thickness  by  radial  growth.     These  early  con- 

ditions are  identical  for  all  segmented  animals,  and  it 

is  only  necessary  to  fix  the  location  of  the  growing  apex,  and  the  direction  of 
bilateral  growth,  in  order  to  fix,  beyond  question,  the  identity  of  the  head  and 
tail  ends  and  the  neural  and  haemal  surfaces. 

The  Gastrula,  Coelenterate,  or  Trochosphere  Stage.— The  first  stage 
after  cleavage  (Fig.  24,  A),  shows  the  primitive  cumulus,  the  primary  center  for  the 
origin  of  the  germ  layers.  The  central  depression,  gst,  marks  an  area  of  inward 
proliferation  which  gives  rise  to  the  endoderm,  yolk  cells,  and  procephalic  meso- 


GASTRULATION   AND    CONCRESCENCE.  35 

derm.  This  stage  represents  the  radiate  or  coelenterate  phase,  and  is  to  be  re- 
garded as  the  true  gastrula  of  the  arthropod-vertebrate  stock.  The  central  de- 
pression deepens  and  later  forms  the  stomodaeum,  the  outer  opening  being  the 
neurostoma.  The  stomodaeum,  at  a  very  early  period,  is  enclosed,  or  surrounded 
by  a  nerve  ring  consisting  of  the  stomodaeal  ganglia  and  their  commissures,  which 
probably  represents  the  remnants  of  the  circumoral  nerve  ring  of  the  coelenterates. 
The  stomodaeum  is,  therefore,  caught  in  a  trap,  the  bars  of  which  are  continually 
growing  stronger,  and  from  which  it  never  escapes. 

Transition  from  Radiate  to  Bilateral  Symmetry. — The  blastodisc,  or 
cell  layers  covering  the  primitive  cumulus,  gradually  spreads  out  over  the  surface 
of  the  yolk  in  all  directions.  Then,  on  the  posterior  side  of  the  disc  and  independ- 
ently of  the  central  depression,  a  second  thickening  appears,  in  which  cell  growth 
and  proliferation  is  especially  active.  It  marks  the  beginning  of  apical  growth 
and  of  bilateral  symmetry,  and  lays  the  foundations  for  the  first  metameres.  The 
anterior  portion  of  the  primitive  cumulus  gives  rise  to  the  procephalic  lobes. 
(Figs.  24,  25.) 

The  Telopore. — The  rapid  cell  division  at  the  apex  of  the  developing  trunk 
may  give  rise  either  to  an  elongated  axial  groove  (insects),  or  to  a  terminal  in- 
folding, or  telopore  (arachnids),  the  so-called  ^blastopore"  of  authors.  Later 
it  may  be  changed  to  a  typical  primitive  streak  (Limulus  and  scorpion),  or  there 
may  be  no  infolding  whatever  (Cymothoa).  This  axial  or  terminal  ingrowth 
(Fig.  25),  is  not  to  be  regarded  as  a  modification,  or  as  an  extension  of  the  process 
of  gastrulation  in  the  procephalic  lobes.  It  is  merely  a  local  exaggeration  of  the 
marginal  growth  of  the  blastodisc.  The  infolding  is  a  secondary  result  of  the 
rapid  tangential  proliferation  that  takes  place  at  the  head  of  the  comet-like  out- 
growth. It  may  or  may  not  be  present. 

The  Germ  Wall. — At  the  close  of  the  primitive  cumulus  stage,  a  germ  wall, 
g.  w.,  is  formed  on  the  lateral  and  posterior  margins  of  the  blastodisc.  With  the 
formation  of  the  trunk,  it  forms  the  lateral  boundaries  of  the  developing  metameres. 
(Figs.  25,  26).  The  germ  wall,  which  is  merely  a  thick  band  of  proliferating 
cells,  similar  to  the  teloblasts  at  the  caudal  end,  spreads  laterally  over  the  yolk 
surface  (Figs.  31,  32),  leaving  behind,  in  addition  to  the  ectoderm  and  yolk 
cells,  a  sheet  of  underlying  mesoderm  that  gradually  breaks  up  into  somites  and 
lateral  plates. 

Differentiation,  therefore,  takes  place  along  two  main  axes;  from  the  median 
line  laterally,  and  from  the  head  end  backward;  hence  the  anterior  median  part 
of  the  germinal  area,  which  now  includes  the  primitive  head  and  the  new  body, 
is  always  the  oldest  and  shows  the  greatest  histological  differentiation;  the  marginal 
and  caudal  part  is  the  youngest  and  is  the  least  differentiated. 

Concrescence  of  the  Germ  Wall. — As  the  gradually  widening  germinal 
area  advances  over  the  yolk,  the  germ  walls  of  the  more  posterior  segments  form 
a  A  ?  the  arms  of  which  gradually  unite,  forming  a  double,  primitive  streak-like 
band  of  nuclei  behind  the  telopore.  (Figs.  16,  21,  25,  26,  27,  138.) 


OUTLINE    OF    THE   ARACHNID    THEORY, 


c.h 


gst. 


c.y.c; 
ci.-k 


B 


24        XP 


tp.'- 


C 


cK     ro 


27 


FIGS.  24  to  34. — A  hypothetical  series  of  arachnid  and  vertebrate  embryos.  The  purpose  of  the  series  is  to 
show  the  continuity  in  the  methods  of  growth  and  organic  differentiation  in  vertebrates  and  arachnids.  It  begins 
with  the  typical  arachnid  stages  and  leads  up  to  those  characteristic  of  primitive  vertebrates,  where  without  inter- 
ruption they  are  carried  on  to  completion. 

FIG.  24. — A  shows  the  radially  symmetrical  germ  disc,  or  primitive  cumulus,  with  its  centrally  located  gas- 
trula  ingrowth;  B,  beginning  of  apical,  or  teloblastic,  growth,  and  the  appearance  of  bilateral  symmetry;  C,  the 
formation  of  the  medullary  plate;  the  unequal  expansion  of  the  thickened  margin  of  the  germ  disc,  or  germ  wall, 
g.  iv.,  and  the  infolding  of  the  teloblast,  to  form  the  telopore,  t.  p. 

FIG.  25. — The  open  medullary  plate  stage,  with  its  neural  crests,  the  marginal  infoldings  that  mark  the  beginning  of 
the  forebrain  vesicle,  and  the  forebrain  sense  organs  on  the  outer  slope  of  the  neural  crest. 

FIG.  26. — Shows  the  appearance  of  the  thoracic  appendages;  the  segmentation  of  the  lateral  plate  mesoderm 
in  the  abdomial  region;  the  beginning  of  the  postanal  concrescence  of  the  germ  wall;  and  the  infoldings  for  the 
middle  cord,  or  notochord.  The  telepore  is  replaced  by  a  primitive  streak. 

FIG.  27. — Shows  the  appearance  of  the  gustatory  lobes,  the  vagus  and  abdominal  appendages,  and  the  eleva- 
tion of  the  caudal  lobe.  The  olfactory  organs  have  moved  forward,  in  front  of  the  head,  and  the  median  eyes 
have  been  transferred  to  the  inner  limb  of  the  neural  crest.  The  cerebellum  appears  as  the  suprastomodaeal 
commissure. 

FIG.  28. — The  palial  fold  has  covered  nearly  the  whole  of  the  forebrain,  and  the  optic  ganglia  are  crowded 
backward  and  upward  toward  the  oral  region.  The  pleural  folds  appear,  and  the  thoracic  folds,  or  the  thoracic 
shield,  extend  over  the  posterior  thoracic  appendages,  forming  the  beginning  of  the  opercular  or  branchial  fold. 

FIG.  29. — The  optic  ganglia  have  united  over  the  stomodaeum  to  form  the  tectum  opticum.  The  neural  crests 
of  the  branchial  region  have  closed,  except  over  the  mouth  which  lies  just  behind  the  stomodaeal  commissure,  or 
cerebellum.  The  uncovered  space  marks  the  location  of  the  choroid  plexus  of  the  fourth  ventricle. 


GASTRULATION  AND  CONCRESCENCE. 


37 


at,  co. 


bnv. 


FIG.  30. — The  embryo  now  presents  typical  vertebrate  conditions.  The  old  mouth  is  practically  closed.  The 
posterior  thoracic  and  vagus  appendages  appear  as  the  external  gills,  and  the  pleural  fold  as  the  postcephalic 
lateral  fold  that  gives  rise  to  the  pectoral  and  pelvic  appendages. 

FIGS.  31  and  32. — Side  views  of  Figs.  26  and  27,  showing  the  extension  of  the  germ  wall  over  the  yolk,  and  the 
elevation  of  the  procephalic  lobes  above  the  general  level  of  the  yolk. 

FIG.  33. — Side  view  of  Fig.  29.  It  shows  the  projection  of  the  procephalic  lobes  beyond  the  anterior  surface 
of  the  yolk,  thus  allowing  the  oral  arches,  or  the  basal  arches  of  the  anterior  thoracic  appendages,  to  approach 
the  haemal  surface  of  the  forehead.  The  lateral  eye  has  been  carried  into  the  brain  vesicles,  appearing  through  the 
skin  as  the  kidney-shaped  retina. 

FIG.  34. — Side  view  of  Fig.  30.  At  least  four  pairs  of  oral  arches  have  united  on  the  haemal  suface,  giving  rise 
to  the  premaxillary,  maxillary,  mandibular.  and  hyoid  arches.  Vestiges  of  the  free  appendages  persist  as  the 
oral  arch  papillae,  tentacles,  or  balancers.  The  large  thoracic  segmental  sense  organ  opposite  the  fourth  thoracic 
appendage  and  behind  the  hyoid  arch  has  now  become  the  auditory  placode. 


^ 8  OUTLINE    OF    THE   ARACHNID    THEORY. 

Thus  the  embryo  elongates,  apparently,  in  two  different  ways;  by  true  apical 
growth  at  the  original  apex  of  the  embryo,  and  by  the  concrescence  of  the  adjacent 
parts  of  the  germ  wall,  behind  the  apex.  Failure  to  recognize  the  meaning  of 
these  two  processes  in  vertebrates  has  led  to  much  confusion. 

As  the  telopore  is  merely  a  locally  exaggerated  marginal  growth,  its  products 
are  not  primarily  different  from  those  of  the  germ  walls  that  concresce  behind  it. 
But  it  will  be  observed  that  in  their  derivation  and  in  their  serial  arrangement, 
the  two  sets  of  products  stand  in  totally  different  relations  to  one  another,  and  to 
their  surroundings.  (Figs.  138,  157 .) 

In  vertebrates,  as  well  as  in  arachnids,  neither  the  telopore,  nor  the  concres- 
cing  margins  of  the  germ  wall  have  anything  to  do  with  a  true  gastrula,  nor  is  their 
mode  of  growth  comparable  with  the  ccelenterate  method  of  gut  formation.  As 
indicated  elsewhere,  post-anal  concrescence  is  the  inevitable  result  when  a  living 
film,  extending  by  apical  and  marginal  growth,  spreads  over  a  spherical  surface. 

The  Nervous  System  follows  in  the  paths  first  laid  down  by  the  expanding 
germ  layers.  The  procephalic  lobes,  developing  from  the  territory  of  the  primi- 
tive cumulus,  and  the  lateral  nerve  cords  on  either  side  of  the  line  of  apical  giowth. 
A  slipper-shaped  medullary  plate  is  thus  formed  that  in  both  arthropod  and  verte- 
brate embryos  has  essentially  the  same  structure,  location,  and  mode  of  growth. 
In  both  types  we  may  recognize  the  marginal  sense  organs  of  the  procephalic 
lobes  and  the  central,  stomodaeal  infolding.  This  infolding,  with  a  rostrum-like 
elevation  on  its  anterior  margin,  is  frequently  a  conspicuous  marking  in  the  middle 
of  the  cephalic  lobes  of  amphibia  (Rana,  and  Necturus)  (Fig.  25.) 

In  this  stage,  the  olfactory  lobes  make  their  appearance  as  a  deep  fold  across 
the  anterior  border,  and  the  edges  of  the  neural  crest  begin  to  grow  over  the 
lateral  margins  of  the  medullary  plate.  A  little  later,  the  thoracic  appendages 
appear  as  gill  arches  and  external  gills.  (Figs.  26-28.) 

The  Primary  Sense  Organs.— In  the  following  stages  (Figs.  27-34),  the 
primary  sense  organs  on  the  margins  of  the  cephalic  lobes  move  into,  or  toward, 
their  final  position,  and  the  gustatory  organs  make  their  appearance  on  the  inner 
margins  of  the  basal  lobes  of  the  thoracic  appendages.  The  olfactory  organs, 
ol,  o,  move  toward  the  anterior  median  line  but  remain  in  the  surface  ectoderm, 
outside  the  brain  chamber;  the  two  pairs  of  ocelli  are  caught  in  the  palial 
fold  and  carried  onto  the  membranous  roof  of  the  brain  vesicle  to  form  the  parietal 
eye;  the  kidney-shaped  lateral  eyes  lie  on  the  outer  edge  of  the  fold,  not  quite 
inside  of  the  brain  chamber.  (Fig.  27.) 

Meantime  the  cornua,  c,  of  the  thoracic  shield  and  the  edges  of  the  abdominal 
pleurites  appear.  (Figs.  28,  33.) 

The  haemal  side  of  the  cephalothorax  is  unsegmented.  The  greatly  thickened 
germ  wall  in  this  region  concentrates  around  a  point  between  the  anterior  end  of 
the  heart  and  the  forebrain,  where  a  great  mass  of  cells,  the  remnants  of  the 
haemal  blastoderm,  are  engulfed  in  the  yolk  and  absorbed.  (Fig.  33,  c.nav.} 

Vertebrate  Stages.— Up  to  this  point,  our  vertebrate-arachnid  embryo  has 


CHARACTERISTIC   VERTEBRATE    STAGES.  39 

been  passing  through  the  coelenterate  and  arthropod  stages  of  its  development,  the 
later  ones,  as  we  have  represented  them,  being  essentially  like  those  of  Limulus  and 
the  scorpion,  although  not  presenting  any  characters  foreign  to  a  vertebrate.  In 
the  following  stages,  the  vertebrate  characters  appear. 

The  changes  that  most  affect  the  general  shape  and  appearance  of  the  brain 
are  the  transfer  of  the  lateral  eye  placodes  to  the  inside  of  the  cerebral  vesicle, 
and  the  union  of  the  optic  ganglia  over  the  neural  surface  of  the  brain  to  form  the 
ganglion  habenulae  and  the  tectum  opticum,  or  optic  lobes.  (Fig.  28.)  As  the 
latter  increase  in  size,  they  crowd  the  stomodaeal  commissure  and  its  ganglion 
backward,  over  the  posterior  part  of  the  midbrain  region,  where  they  form  the 
rudiment  of  the  cerebellum.  (Figs.  29,  30,  107  and  108.) 

When  the  convex,  kidney-shaped  lateral  eye  placode  is  transferred  to  the 
brain  wall,  it  becomes  a  concave,  horseshoe-shaped  retina.  Still  later,  it  becomes 
a  circular  one,  with  a  median  fissure  and  centrally  located  nerve,  both  conditions 
being  the  direct  result  of  its  ancestral  shape  and  mode  of  growth.  (Fig.  106.) 

The  Auditory  Pit  develops  from  a  prominent,  disc-like  placode,  or  segmental 
sense  organ,  which  in  Limulus  lies  on  the  cephalothoracic  shield,  opposite  the 
third  or  fourth  thoracic  segment.  (Figs.  29-30.)  With  the  concrescence  of  the 
anterior  oral  arches  on  the  haemal  side  of  the  head,  the  disc  shifts  its  position  to 
that  part  of  the  head  where  it  makes  its  first  appearance  in  vertebrate  embryos. 
(Figs.  33-340 

The  Heart  has  been  formed  in  typical  arthropod  fashion,  by  the  concrescence 
of  the  lateral  plates  of  the  vagus,  and  the  anterior  abdominal  metameres,  which 
accounts  for  the  fact  that,  in  both  arthropods  and  vertebrates,  the  anterior  heart 
nerves  arise  from  the  corresponding  vagus  and  branchial  neuromeres.  (Figs.  32 
and  33.) 

In  the  formation  of  the  vertebrate  heart,  new  factors  may  arise  in  the  greatly 
increased  volume  of  the  yolk  sphere.  In  this  case  the  branchial  metameres,  lying 
near  the  equator  of  the  egg,  must  extend  their  lateral  plates  completely  round  the 
yolk  before  a  heart  segment  can  be  formed.  Hence,  in  the  larger  yolked  eggs,  it 
is  only  the  vagal  and  anterior  branchial  metameres  that  are  in  a  position  to  form 
heart  segments  in  time  to  nourish  the  growing  head  structures.  If  the  younger 
and  shorter  caudal  metameres  produced  heart  segments,  they  would  necessarily 
arise  later  and  would  be  separated  from  the  head  by  the  barrier  of  the  abdominal 
yolk  navel.  (Fig.  34.)  The  heart  would  then  be  a  single  tube  at  either  end  and 
a  paired  tube  in  the  middle.  (Figs.  23,  139,  C.) 

The  thoracic,  and  the  post  branchial  cardiomeres  have  been  eliminated  in 
vertebrates,  as  they  have  been  in  arachnids,  and  the  heart  develops,  as  nearly  as 
one  may  determine,  from  about  the  same  group  of  vagus  and  branchial  metameres 
in  both  cases.  But  the  posterior  end  of  the  vertebrate  heart  still  extends  partly 
round  the  yolk  navel;  hence  the  divided  posterior  end  and  divergent  vitelline 
veins;  and  it  is  crowded  into  an  area  on  the  haemal  surface  that  is  growing  shorter 
while  the  heart  is  growing  larger,  hence  the  auriculo-ventricular  curvature. 


40  OUTLINE    OF    THE   ARACHNID    THEORY. 

The  Cornua  of  the  cephalothoracic  shield  are  retained  as  the  opercular 
fold,  which  extends  over  the  posterior  appendages,  as  suggested  for  Cepha- 
laspis,  to  form  a  respiratory,  or  atrial  chamber.  The  projecting  margins  of  the 
abdominal  segments  or  pleurites  are  retained  as  the  lateral  fold,  from  which  the 
paired,  post-branchial  appendages  arise.  (Figs.  29-34.) 

The  Oral  Arches  and  the  Haemostoma. — Both  the  formation  of  the 
haemostoma,  or  new  mouth,  and  the  transfer  of  the  basal  joints  of  the  anterior 
thoracic  appendages  to  the  haemal  side  of  the  head  to  form  the  oral  arches,  are 
the  inevitable  results  of  the  processes  that  have  been  steadily  going  on  during  the 
phylogeny  of  the  arachnid  cephalothorax.  These  processes  are:  the  increased 
size  of  the  yolk  sphere;  the  increased  size  of  the  forebrain  neuromeres;  and  the 
progressive  degeneration  of  the  cephalic  mesoderm.  The  way  in  which  these 
changes  affect  the  location  of  the  jaws  and  the  shape  of  the  head,  during  the  early 
stages  of  development,  is  shown  in  Figs.  31-34.  It  will  be  seen  that  as  the  fore- 
brain  increases  in  volume  and  in  precocity,  the  apex  of  the  head  is  elevated  and 
thrust  forward  off  the  surface  of  the  egg.  As  the  haemal  ends  of  these  anterior 
metameres  are  greatly  reduced  in  volume,  or  absent,  the  other  head  structures, 
which  were  originally  neural  or  lateral  in  position,  such  as  the  anterior  meso- 
blastic  somites  and  the  appendages,  are  drawn  toward  the  haemal  side  of  the  head. 
Here  they  converge  around  the  ingrowing,  haemal  surface,  or  cephalic  navel 
(dorsal  organ)  that  represents  the  beginning  of  the  buccal  infolding,  the  basal 
joints  of  the  appendages  forming  the  beginning  of  several  pairs  of  oral  arches,  i.e., 
premaxillae,  maxillae,  mandibles,  and  hyoids. 

The  more  posterior  arches  are  not  subject  to  these  conditions;  hence  they 
tend  to  remain  in  their  original  position  on  the  neural  or  lateral  surface.  But 
in  the  later  stages  of  the  higher  vertebrates,  even  they  may  be  transferred  to  the 
haemal  surface.  (Figs.  307,  308.) 

The  mouth  parts  of  our  embryo  are  now  in  the  ostracoderm  and  cyclostome 
stage,  one  that  is  seen  temporarily  in  all  higher  vertebrate  embryos  (see  develop- 
ment of  the  jaws  in  the  frog)  (p.  257),  and  permanently  in  the  adult  cyclostomes 
and  ostracoderms.  (Figs.  159-174.)  The  true  vertebrate  condition  is  attained 
by  the  union  of  two  pairs  of  arches  to  form  a  single,  fixed,  upper  jaw,  and  the 
union  of  a  third  pair  to  form  a  movable,  unpaired,  under  jaw,  or  mandible. 


The  older  stages  of  our  arachnid-vertebrate  embryo,  Fig.  34,  are  character- 
ized by  an  increase  of  the  cranial  flexure,  bringing  the  heart  close  under  the 
anterior  end  of  the  brain,  and  producing  that  forward  dislocation  of  the  hypo- 
branchial  muscles,  so  characteristic  of  vertebrates;  by  the  opening  of  the  gut  pouches 
into  the  lung-books;  by  the  appearance  of  true  vertebrate  appendages  as  post- 
cephalic  outgrowths  of  the  marginal  fold;  by  the  increasing  size  of  the  cartilage 
cranium  and  gill  bars;  by  the  substitution  of  a  subdermal  skeleton  for  an  epidermal 
one,  and  by  the  conversion  of  the  arthropod  lematochord  into  the  notochord. 


CHAPTER  III. 

EVOLUTION  OF  THE  NERVOUS  SYSTEM  IN  SEGMENTED  ANIMALS. 

I.  MEANING  OF  THE  TERM  BRAIN. 

In  the  vertebrates,  the  term  "brain"  vaguely  signifies  the  specialized  an- 
terior end  of  the  neuron.  In  the  invertebrates,  the  term  may  be  even  more  vague, 
in  that  it  is  often  used  to  signify  only  that  part  assumed  to  lie  originally  in  front 
of  the  oesophagus,  that  is,  the  supra-oesophageal  ganglion.  Or  the  term  may 
signify  that  ganglion,  plus  a  varying  number  of  post-oral  neuromeres. 

The  lack  of  precise  definition  in  both  cases  is  significant,  and  justifies  the  use 
of  the  term,  as  we  shall  use  it  here,  namely,  to  signify  a  varying  number  of  neuro- 
meres consolidated  in  the  region  of  the  primitive  mouth. 

The  number  of  neuromeres  thus  set  apart,  their  specialization,  and  the 
intimacy  of  their  union,  gradually  increases  throughout  the  arthropod-vertebrate 
series,  and  furnishes  an  impressive  picture  of  persistent,  progressive  specializa- 
tion. 

In  the  arthropods,  there  are  many  oscillations  in  the  total  number,  and  in 
the  grouping,  of  the  brain  neuromeres.  The  primary  causes  of  their  union  are 
too  complex  to  be  analyzed,  except  in  the  broadest  way;  but  we  may  readily  recog- 
nize a  steady  progression  toward  a  definitely  organized  collection  of  neuromeres 
that  it  is  entirely  proper  to  call  a  brain  in  the  vertebrate  sense,  for  it  contains 
approximately  the  same  total  number  of  neuromeres  as  the  vertebrate  brain;  and 
it  is  divided  into  similar  groups  of  neuromeres,  each  of  which  is  associated  with 
nerves,  sense  organs,  and  other  structures  similar  to  those  in  vertebrates. 

The  evolution  of  the  brain  cannot  be  effectively  studied  apart  from  the  body 
regions  to  which  it  belongs,  for  each  moulds  the  other  and  reflects  the  other's 
changes.  The  events  that  created  the  vertebrate  brain,  and  whose  influence  is 
still  effective  in  moulding  its  form  and  function,  are  to  be  found  in  the  arthropods. 
There,  all  the  initial  phases  in  the  successive  incorporation  of  one  region  of  the 
trunk  after  another  into  a  more  complex  "head,"  and  of  one  part  of  the  cord  after 
another  into  a  more  and  more  complex  "brain,"  have  taken  place,  and  probably 
nearly  all  the  more  important  steps  in  the  process  are  there  crystallized  into 
recognizable  form. 

The  five  groups  of  neuromeres  included  in  the  first  fifteen  or  twenty  that 
make  up  the  vertebrate  brain  may  be  definitely  identified  with  the  corresponding 
divisions  of  the  arthropod  brain.  We  cannot  hope  to  identify  more  than  that 

41 


42  EVOLUTION    OF   THE   NERVOUS    SYSTEM   IN    SEGMENTED   ANIMALS. 

since  those  that  follow  and  which  make  up  the  greater  part  of  the  spinal  cord  were 
acquired  after  the  evolution  of  vertebrates  from  arachnids  had  taken  place. 

In  other  words,  the  ancestors  of  vertebrates  were  animals  provided  with  a 
comparatively  small  number  of  neuromeres,  21  ±,  most  of  which  had  already 
been  consolidated  into  a  complex  brain  of  the  vertebrate  type.  One  of  the  im- 
portant events  in  the  early  evolution  of  the  new  or  vertebrate  type  was  the  rapid 
increase  in  the  number  of  metameres  by  the  regular  process  of  apical  growth. 
The  new  metameres  formed  a  new  trunk  or  body,  while  nearly  the  whole  of  the 
old  arachnid  trunk  (head,  thorax,  and  abdomen,  14-16  metameres)  was  still 
further  consolidated  to  form  the  head  of  the  new  type.  The  whole  process  thus 


FIG.  35. — Diagrams  to  explain  the  probable  relations  between  the  structure  of  a  trochosphere  and  the  early 
embryonic  stages  of  a  primitive  arthropod;  A,  Trochosphere  in  mercator  projection,  seen  from  the  neural,  or  sub- 
umbrella  surface;  C,  same  from  the  side  seen  as  a  solid  object;  B,  early  stage  of  an  arthropod  embryo,  seen  in 
mercator  projection;  D,  same  seen  as  a  solid  object,  from  the  side.  In  A  and  C,  the  circumoral  area,  with  its 
system  of  radial  and  circular  nerves,  forms  a  part  of  the  sub-umbrella  of  the  trochosphere.  In  B  and  D  this  area 
is  supposed  to  be  infolded,  giving  rise  to  the  proximal  portion  of  the  stomodaeum,  from  which  the  system  of  stomo- 
dseal  nerves  and  ganglia  arise.  The  ancestral  coelenterate  body,  according  to  this  interpretation,  is  represented  in 
the  arthropod  embryo  by  the  procephalic  lobes  and  stomodaeum;  the  arthropod  trunk,  with  its  lateral  and 
median  nerve  cords,  is  a  new  formation,  arising  as  a  local  outgrowth  from  the  ancestral  coelenterate  body,  or  from 
the  procephalic  lobes  of  the  arthropod  embryo.  On  the  aboral  surface  of  the  trochosphere  is  the  area  of  yolk  de- 
posit and  the  "closing  in"  point,  a  pauperitic,  degenerative  region  that  is  called  the  cephalic  navel. 

presents  a  striking  analogy  to  the  way  in  which  the  primitive  body  of  segmented 
animals  was  formed  as  a  new  outgrowth  from  the  body  of  its  ccelenterate  ancestor, 
which  then  became  the  head  of  its  descendant.  (Fig.  35.) 


II.  THE  STOMOD^EAL  NERVES. 

We  recognize  two  distinct  systems  of  nerves  in  segmented  animals.  One 
belongs  to  the  stomodaeum,  and  probably  represents  the  remnants  of  the  cir- 
cular and  radial  sub-umbrellar  nerves  of  a  ccelenterate-like  ancestor;  the  other 
consists  of  longitudinal  and  transverse  nerves  that  developed  in  the  tentacle-like 
out-growth  that  gave  rise  to  the  body  of  the  new  animal.  (Fig.  35.) 

The  stomodaeum  is  looked  upon  as  representing,  in  part,  the  infolded  sub- 
umbrella.  When  invaginated,  it  carried  with  it  the  primitive  system  of  circum- 
oral nerves,  which  then  arise  as  circular  and  longitudinal  nerves  from  the  walls 
of  the  stomodaeum.  The  outermost  circular  nerve  (prototroch  nerve  ( ?)),  is  repre- 


THE    BRAIN.  43 

sented  by  the  supra-stomodaeal  commissure  with  its  anterior  median,  and  two 
lateral,  ganglia.  These  nerves  and  ganglia  are  without  doubt  very  ancient 
structures,  and  their  position  and  mode  of  development  clearly  indicate  that  they 
belong  to  a  different  system  of  nerves  from  those  in  the  remaining  part  of  the  head 
or  trunk. 

There  is  probably  a  distinct  post-cesophageal  ganglion  and  commissure 
belonging  to  this  system,  although  I  have  not  succeeded  in  locating  it,  or  in  dis- 
tinguishing it  from  the  more  anterior  post-oral  commissures.  The  supra-stomo- 
daeal commissure  always  sends  nerves  to  the  labrum,  or  rostrum,  which  receives 
nerves  from  this  source  only.  The  innervation  of  this  pair  of  appendages,  their 
median  position  in  front  of  the  mouth,  and  between  the  right  and  left  halves  of 
the  forebrain,  distinguish  them  from  all  others,  and  indicate  their  probable  origin 
from  tentacle-like  organs  of  some  very  remote  ancestor. 

Originally  the  stomodaeal  neives  appear  to  have  been  intimately  connected 
with  the  two  median  longitudinal  nerves  of  the  trunk,  i.e.,  with  the  median  cardiac, 
on  the  haemal  side,  and  the  median  sympathetic  on  the  neural.  Both  these  con- 
nections are  lost  in  the  adults  of  the  higher  arachnids,  i.e.,  in  Limulus,  although 
in  the  scorpion  the  connection  with  the  cardiacs  seems  to  be  retained. 

The  dividing  line  between  the  coelenterate  nervous  system  of  the  primitive 
head  and  that  belonging  to  the  bilateral  outgrowth  from  it,  cannot  be  accurately 
determined,  and  indeed  there  is  no  reason  to  suppose  the  two  were  ever  distinct 
systems,  the  post-oral  nerves  being  merely  extensions  of  the  older  ones  in 
the  head. 

III.  THE  FRAME-WORK  or  THE  NERVOUS  SYSTEM. 

The  nervous  system  of  segmented  animals  may  be  reduced  to  a  system  of 
longitudinal  and  transverse  strands  or  cords. 

Longitudinal  Cords. — In  the  arachnids,  eight  longitudinal  nerve  cords  may 
be  recognized:  a  median  haemal  one,  from  which  arises  the  cardiac  ganglion;  a 
median  neural  one,  or  middle-cord  (Mittelstrang  of  Hatschek),  from  which  arises 
the  so-called  median  sympathetic  nerve;  a  pair  of  ventral  cords,  which  give  rise 
to  the  main  axial  nervous  system,  or  neuron  (brain  and  spinal  cord) ;  the  paired 
pericardials;  and  the  lateral  sympathetics. 

The  Median  Nerve,  "  Median  Sympathetic,"  or  "Middle-Cord,"  of  arthropods 
appears  to  have  extended  backward,  from  the  posterior  part  of  the  cesophageal 
region,  or  of  the  circumoral  nerve  ring,  the  whole  length  of  the  body.  I  have  not 
been  able  to  determine  the  peripheral  distribution  of  its  fibers.  The  main  nerve 
and  its  sheath  undergo  many  modifications.  In  the  higher  arthropods  and  verte- 
brates, the  nerve  itself  atrophies  and  ceases  to  form  a  functional  part  of  the  nervous 
system.  It  serves,  however,  as  a  center  for  the  development  of  voluminous, 
resistent  envelopes  from  which  is  evolved  the  notochord.  The  history  of  the  middle 
chord  will  therefore  be  described  in  the  chapter  on  the  evolution  of  the  notochord 


44 


EVOLUTION    OF   THE   NERVOUS    SYSTEM    IN    SEGMENTED   ANIMALS. 


Transverse  Cords. — Numerous  transverse,  or  circular  bands  intersect  the 
longitudinal  ones,  and  lay  the  foundations  for  the  transverse  commissures,  and 
for  the  segmental  peripheral  nerves.  The  latter  usually  lead  by  smaller  branches 
into  a  subdermal  plexus,  from  which  the  nerve  ends  are  distributed  to  their  re- 
spective terminals. 

The  ventral  cords  and  the  middle  cord  are  confined  to  that  surface  of  the 
embryo  that  is  the  first  to  develop.  Their  position  djaring  the  early  stages  is  the 
same  in  all  segmented  animals,  and  their  presence  definitely  locates  the  primitive 
oral,  or  neural  surface  of  the  bodv. 


-^'•' 

-  -^  >f( ....:-."'  ^    i^p* 

f   4    -      ;r/v    A 

^»$ 

Ai^;  c    r^i!^ 


•  ^•:^-^  Bt.?R?liirrrf 


Ut.^ 


FIG.  36.-Bram  and  nerve  cord  of  a  young  Limulus  in  the  second  larval  stage.    A,  Jfemal  surface;  B  neural  surface 
FIG.  37.-Sectic.ns  of  same.    A,  Through  the  optic  ganglia  and  olfactory  organs;  B,  through  the  middle  o    the 
hemispheres  and   the  posterior  part  of  the  forebrain;  D,  through  the  cheliceral  ganglia;  E,  through  The  suprasto 
modeal  commissure  and  the  lateral  stomoda?al  ganglia. 

The  longitudinal  cords  serve  to  conduct  nervous  impulses  in  a  longitudinal 
direction;  in  them  are  located  the  great  majority  of  the  nerve  cells.  The  trans- 
verse bands  serve  to  conduct  nervous  impulses  in  a  centripetal  or  centrifugal 
direction.  The  comparatively  few  nerve  cells  that  belong  in  them,  as  a  rule,  lie 
near  their  central  or  peripheral  terminals. 


THK    SPECIALIZATION    OF    THE    NERVE    CORDS.  45 

The  Process  of  Specialization. — The  axial  or  central  nervous  system  under- 
goes progressive  evolution,  or  specialization,  in  a  transverse  and  in  a  longitudinal 
direction. 

The  first  process  consists  in  the  segregation  of  similar  nerve  fibers  and  cells 
into  concentric,  overlying  longitudinal  zones  or  tracts,  the  most  notable  example 
of  this  being  the  assembling  of  motor  elements  toward  the  haemal  surface,  and 
of  sensory  ones  toward  the  neural  surface  of  the  cords. 

The  second  is  the  transverse  division  of  the  cords  into  blocks,  or  neuromeres, 
which  then,  singly  or  in  groups,  become  the  centers  of  some  particular  function. 
The  linear  specialization  of  the  neuron  is  due  to  the  gradual  elimination  of  the 
heart,  digestive  and  locomotor  organs,  from  the  anterior  body  metameres,  and  to 
the  increased  size  of  the  sensory  and  ingestive  organs.  These  changes  lead  to  a 
great  reduction  in  the  number  and  volume  of  the  motor  nerve  elements  in  the 
anterior  metameres,  and  to  the  location  of  functional  centers  in  the  neuromeres 
according  to  a  definite  order,  which  follows  that  established  in  the  corresponding 
groups  of  metameres.  See  page  209.  This  order,  which  is  initiated  at  a  very 
early  period  in  the  history  of  segmented  animals,  is  as  follows:  olfactory;  coordinat- 
ing; visual;  ingestive  (i.e.,  masticatory,  swallowing,  and  gustatory);  auditory; 
locomotory;  respiratory  (cardiac  and  branchial);  digestive,  and  urogenital. 
This  process  of  cephalization  progresses  in  a  cephalo-caudal  direction,  the  func- 
tional centers  becoming  more  and  more  sharply  localized  in  the  direction  and 
order  named  above. 

IV.  THE  DIFFERENTIATION  OF  PERIPHERAL  NERVES. 

The  primary  system  of  transverse  nerves  forms  the  foundation  of  the  peri- 
pheral nervous  system.  The  evolution  of  these  nerves  consists  mainly  in  the 
resolution  of  the  primary  network  into  special  nerve  bundles  composed  of  fibers 
having  similar  central  and  peripheral  terminals. 

The  principal  stages  of  the  process  appear  to  be  as  follows :  i .  Each  neuromere 
is  at  first  connected  with  several  pairs  (four?)  of  transverse  nerves,  all  of  which 
may  contain  both  motor  and  sensory  elements.  2.  The  number  of  nerves  for 
each  neuromere  is  ultimately  reduced  to  two  main  pairs,  an  anterior  and  a  posterior. 
3.  The  roots  of  the  anterior  nerves  gradually  shift  toward  the  neural  surface  of 
the  cord;  the  posterior  ones  retain  a  more  haemal  position.  The  two  series  of 
nerves  thus  formed,  are  called  the  neural  and  the  haemal  nerves.  4.  The  neural 
nerves  develop  ganglia  on  their  proximal  ends,  and  in  those  regions  of  the  body 
where  appendages  are  developed,  supply  only  the  appendages.  The  haemal 
nerves  are  without  ganglia  and  supply  the  remaining  parts  of  the  metamere.  5. 
In  the  cephalothoracic,  or  head  region,  the  neural  and  haemal  nerves  remain 
separate  (vertebrates  and  arthropods),  while  in  the  more  posterior  regions  they 
may  unite,  for  a  longer  or  shorter  distance,  forming  single  nerves  with  two  sets  of 
roots,  ganglionated  neural  roots,  and  non-ganglionated  haemal  roots.  6.  Both 
neural  and  haemal  nerve  roots  contain  motor  and  sensory  elements,  but  at  an 


46  EVOLUTION    OF   THE   NERVOUS   SYSTEM  IN   SEGMENTED    ANIMALS. 

early  period  in  the  evolution  of  arthropods  the  sensory  elements  become  more  and 
more  predominant  in  the  neural  nerves,  and  the  motor  elements  in  the  haemal  ones, 
this  condition  being  most  strongly  marked  at  the  anterior  end,  and  diminishing 
gradually  in  a  caudal  direction. 

Factors  that  Modify  the  Arrangement  of  Peripheral  Nerves.— The  more 
important  factors  that  modify  the  primitive  segmental  arrangement  of  peripheral 
nerves  are  as  follows:  a.  the  location,  isolation,  and  size  of  the  peripheral  terminals; 
b.  the  elimination  of  other  terminals;  c.  the  organic  union  of  similar  terminals 
belonging  to  different  metameres;  d.  the  relative  age  of  the  metamere  in  which  they 
belong. 


FIG.  38. — Brain  of  a  young  Limulus,  about  three 
inches  long;  neural  surface. 


FIG.  39. — Same;  haemal  surface. 


a.  The  segregation  of  like  nerve  fibers  into  peripheral  nerves,  or  into  nerve 
tracts  in  the  central  nervous  system,  is  determined  by  the  time  and  place  of  origin 
of  the  peripheral  terminals. 

Wherever  there  are  highly  specialized  organs,  morphologically  isolated,  the 
associated  nerve  fibers  and  nerve  cells  show  a  similar  isolation  or  segregation,  the 
growth  of  each  correlated  part  keeping  pace,  in  the  main,  with  the  growth  of 
the  other.  The  primary  sensory  organs  are  superficial  in  position  and  lie  in  the 
ectoderm,  close  to  the  lateral  margins  of  the  neuron.  The  motor  ones  are  deeper, 
more  lateral  or  haemal  in  position.  The  corresponding  nerves  have,  in  the  main, 
similar  relative  positions,  and  these  factors  have  controlled  from  the  outset  the 


THE   ARRANGEMENT    OF    PERIPHERAL    NERVES.  47 

segregation  of  motor  components  in  a  haemal  direction,  and  the  sensory  ones  in  a 
neural  direction,  both  as  regards  their  location  in  the  peripheral  nerves  and  in 
the  central  nervous  system.  With  the  invagination  of  the  nerve  cords,  these  con- 
ditions were  still  further  exaggerated  by  the  union  of  the  neural  crests  in  the  median 
dorsal  line,  and  by  the  position  of  the  mesoblastic  somites.  (Fig.  137.) 

The  wide  separation  of  the  neural  and  haemal  nerves,  as  for  example  in  the 
thorax  of  Limulus  and  the  scorpion,  is  due  on  the  one  hand  to  the  location  and  spe- 
cialization of  the  appendages,  coxal  sense  organs  and  ganglia,  and  on  the  other  to  the 
location  of  the  more  peripheral  trunk  muscles  and  sense  organs.  It  no  doubt 
had  its  origin  at  a  very  early  period  in  the  evolution  of  metameres. 

b.  Elimination. — In  the  arthropods  there  is  a  progressive  elimination  from 
the  anterior  metameres  of  the  motor,  nutritive,  cardiac,  and  respiratory  organs, 
leaving  little  but  the  leg  and  jaw  muscles,  and  the  primary  sense  organs,  such  as 
the  eyes,  olfactory,  gustatory,  auditory,  and  tactile  organs.     The  nerve  elements 
associated  with  those  organs  disappear  with  them.     Those  that  remain  increase 
in  volume  and  independence  with  their  corresponding  peripheral  terminals,  while 
their  central  terminals  tend  to  completely  monopolize  their  appropriate  neuro- 
meres.     In  this  way  the  primitive  character  of  the  segmental  nerves  may  be  lost  or 
greatly  modified.     This  is  the  case  in  the  procephalon,  where  the  only  peripheral 
nerves  that  remain  belong  to  the  eyes  and  olfactory  organs,  all  other  peripheral 
elements  having  been  eliminated,  if  they  ever  existed  there. 

c.  Union. — Where  organs  belonging  to  different  metameres  perform  the  same 
function  their  nerves  tend  to  unite,  forming  a  common  bundle,  or  nerve,  or  tract. 
Such  compound  nerves,  consisting  of  the  united  branches  of  separate  segmental 
nerves,  may  themselves  simulate  independent  segmental  nerves,  and  greatly  dis- 
guise the  original  segmental  arrangement.     Examples  of  this  mode  of  segregation 
are  seen  in  the  segmental  cardiacs,  the  hypobranchial,  the  intestinal  (Figs.  57,  58), 
and  to  a  lesser  degree,  the  gustatory  nerves  of  Limulus. 

c.  Historic  Factor. — If  we  attempt  to  homologize  the  nerves  in  one  part  of 
the  head  with  those  in  another,  or  with  those  in  the  trunk,  we  meet  with  insuper- 
able difficulties  because,  as  we  have  seen,  each  group  of  metameres  has  a  history 
of  its  own  that  is  different  from  that  of  all  the  others,  and  this  history  is  reflected 
in  the  structure  of  its  nerves  and  neuromeres.  The  attempt  to  homologize  the 
structures  in  the  head  with  those  in  the  trunk  or  tail,  except  in  the  most  general 
way,  is  an  illogical  and  hopeless  undertaking,  for  the  caudal  metameres  belong 
to  later  generations  that  came  into  existence  under  new  conditions  and  were 
provided  with  different  organs  from  those  in  the  old.  Except  for  a  small  number 
of  the  most  anterior  ones,  the  trunk  and  caudal  metameres  of  vertebrates  did  not 
exist  in  the  arthropods.  They  arose  with  the  vertebrate  stock  and  never  developed 
any  organs  comparable  with  the  cephalic  appendages,  jaws,  gill  arches,  or  visual 
organs.  Hence  it  is  clear  that  there  can  be  no  exact  homology  between  the  head 
metameres  of  an  arachnid  or  a  vertebrate  and  a  trunk  metamere  of  the  same 
animal.  For  that  reason,  therefore,  we  may  not  consider  the  cranial  nerves,  or 


48 


EVOLUTION    OF    THE    NERVOUS    SYSTEM   IN    SEGMENTED   ANIMALS. 


cranial  neuromeres,  or  cephalic  sense  organs,  as  modifications  of  those  in  the 
trunk,  or  vice  versa,  without  conveying  an  entirely  false  impression  of  their  real 
history  and  meaning. 


In  the  lower  arthropods,  the  peripheral  nerves  are  generally  arranged  through- 
out the  whole  body,  in  typical  segmental  fashion.  In  the  higher  arachnids,  due 
to  the  operation  of  the  above  described  factors,  this  clear  cut  metamerism  declines, 
or  is  greatly  obscured.  The  broad  distinction  between  cranial  and  spinal  nerves 
becomes  clearly  established,  and  the  extensive  elimination  of  motor  elements,  as 
well  as  the  local  segregation  of  sensory  and  motor  components  of  different  nerves 


...  p.e 


st.c. 


A  B 

FIG.  40. — Models  of  the  brain  of  a  young  scorpion,  just  hatched.     A,  Haemal  surface;  B,  neural  surface. 

into  compound  nerves  having  a  similar  function  and  distribution,  has  given  to  the 
entire  system  the  same  structure  and  general  arrangement  of  parts  seen  in  the 
vertebrates. 

In  Limulus,  for  example,  this  process  of  specialization  has  produced  the 
highly  characteristic  olfactory,  pineal  eye,  and  lateral  eye  nerves,  as  well  as  the 
compound  system  of  gustatory,  branchio-thoracic  (hypoglossal),  cardiac,  and 
intestinal  nerves.  These  nerves  are  already  so  complex  and  highly  modified 
that  the  original  segmental  arrangement  is  now  exceedingly  difficult  or  impossible 
accurately  to  determine. 

The  same  conditions,  but  in  a  still  more  exaggerated  form,  are  seen  in  verte- 
brates, and  in  part  justifies  the  revolt  of  certain  American  neurologists  against  the 
apparently  hopeless  task  of  determining  the  segmental  value  of  vertebrate  cranial 
nerves  and  their  relation  to  the  dorsal  or  ventral  roots  of  spinal  nerves.  They 
have  laid  great  stress  on  the  analysis  of  nerves  into  their  functional  components; 
but  in  perfecting  a  highly  artificial  system,  they  have  neglected  the  deeper  mor- 
phological problems  involved  in  their  more  primitive  segmental  arrangement.  It 
is  clear  that  both  the  old  and  the  new  method  must  be  retained.  But  neither 


THE  EVOLUTION  OF  NEUROMERES.  49 

method  alone  applied  to  the  vertebrates  can  ever  give  us  a  true  picture  of  their 
ancestral  condition.  That  can  only  be  obtained  from  the  arthropods  where  the 
highly  specialized  condition  seen  in  the  vertebrates  has  its  origin. 

V.  NEUROMERES  AND  METAMERISM. 

Metamerism  of  the  body  and  the  subdivision  of  nerve  cords  into  blocks  or 
neuromeres  are  characters  that  were  probably  slowly  evolved  in  bilateral  animals; 
not  inherited,  even  in  a  rudimentary  form,  from  ccelenterate  ancestors. 

The  evolution  of  neuromeres  probably  began  in  the  trochozoa.  They  are 
well  developed  in  the  annelids  and  in  the  arthropods,  especially  in  the  ab- 
dominal regions.  In  the  higher  arthropods,  the  clear-cut  distinction  between 
adjacent  neuromeres  of  the  head  is  greatly  obscured  by  their  fusion  into  larger 
groups,  and  by  the  segregation  of  their  constituents  into  new  groups,  according 


FIG.  41. — Model  of  the  forebrain  region  of  an  embryo  scorpion,  stage  G,  Fig.  18. 

lo  their  function.  In  vertebrates,  the  post-cephalic  part  of  the  neuron,  which 
has  been  more  recently  acquired,  and  which  is  not  represented  in  arthropods, 
is  never  divided  into  distinct  neuromeres,  and  probably  never  was  so  divided. 

Even  in  the  arthropods  and  annelids,  it  is  doubtful  whether  there  is  any  such 
thing  as  a  neuromere,  complete  in  itself  and  devoted  to  a  single  body  joint  or 
metamere.  There  are  certainly  none  in  Limulus,  or  in  the  scorpion,  and  the 
lower  down  we  go,  as  for  example  into  the  phyllopods,  the  less  sharply  denned  the 
neuromeres  become;  that  is,  the  ganglionic  masses  are  more  diffuse,  and  the  pe- 
ripheral nerves  more  numerous,  and  not  so  strictly  segmental  in  their  origin  or  dis- 
tribution (Branchipus). 

In  Limulus  and  scorpion,  where  there  appears  to  be  such  an  exact  and  exclu- 
sive association  of  the  body  segment  with  its  neuromere  and  nerves,  there  is  no 
such  exclusive  association  in  fact,  because  many  motor  neuromeres  and  the  cen- 
tral ends  of  many  sensory  fibers  are  located  in  some  neuromere  anterior  to  the  one 
where  the  nerve  fibers  leave  the  cord  to  reach  their  peripheral  terminals.  That 


50  EVOLUTION   OF   THE   NERVOUS    SYSTEM   IN    SEGMENTED  ANIMALS. 

is,  the  central  nerve  terminals  and  the  centrally  located  nerve  cells,  in  many  cases 
lie  in  different  metameres  from  the  peripheral  organs  with  which  they  are  asso- 
ciated. (Figs.  59,  60.) 

This  condition  appears  to  prevail  in  the  most  primitive  arthropod  neuro- 
meres,  hence  that  complete  functional  and  morphological  correspondence,  sup- 
posed to  occur  between  a  body  joint  and  a  nerve  cord  joint,  does  not  exist.  Meta- 
merism has  developed  to  a  different  degree  in  the  two  systems  and  affects  them 
in  quite  a  different  manner.  The  morphological  segmentation  of  the  nerve 
axis  does  not  coincide  with  that  of  the  body,  and  the  functional  segmentation  of  the 
nerve  cord  does  not  coincide  with  its  morphological  segmentation,  for  both  motor 
nerve  cells  and  sensory  dendrites  are  frequently  located  in  a  neuromere  in  front 
of  the  metamere  in  which  the  corresponding  nerve  fibers  leave  the  cord,  and  in 
which  they  have  their  peripheral  terminals. 

A  partial  explanation  of  the  lack  of  correspondence  between  functional  and 
morphological  metamerism  of  the  nerve  cord  is  afforded  by  what  takes  place  in 
embryo  scorpions.  Here  each  neuromere  is  composed  of  two  distinct  segments, 
and  as  the  space  between  the  abdominal  ones  increases,  the  anterior  segment  of 
one  neuromere  unites  with  the  posterior  segment  of  the  one  in  front  of  it,  thus 
completely  changing  the  original  grouping  of  the  half  neuromeres.  It  is  not 
clear  whether  this  takes  place  in  Limulus  or  in  the  other  arthropods  I  have 
studied,  but  it  probably  does,  otherwise  it  is  hard  to  understand  how  the  cell 
bodies  of  the  motor  neurones  are  located  in  the  neuromeres  in  front  of  the  one 
from  which  the  corresponding  motor  nerves  leave  the  cord. 

It  is  therefore  clear  that  Loeb's  attempt  to  prove  that  each  abdominal  neuro- 
mere in  Limulus  is  a  complete  reflex  center  for  its  corresponding  gill,  is  based  on  a 
misconception  of  the  structure  of  the  nerve  cord.  His  interpretations  of  his  ex- 
periments are  incorrect  because,  as  we  shall  show  later,  they  are  based  on  a 
misunderstanding  of  the  structure  of  a  neuromere  and  the  distribution  of  its  nerves. 

VI.  THE  PRIMITIVE  SENSE  BUDS. 

The  main  nerve  trunks  in  the  arthropods  represent  bands  of  metamorphosed 
sense  organs,  and  they  coincide  with  the  lines  along  which  such  sense  organs 
were  distributed  in  the  remote  ancestral  forms.  The  transformation  of  these 
primitive  sense  organs  into  nerve  cells  constitutes  an  important  step  in  the  evolu- 
tion of  the  central  nervous  system.  Many  details  in  this  process  are  still  retained 
in  the  embryos  of  arachnids. 

In  the  scorpion,  the  entire  brain  and  cord  is  an  aggregate  of  innumerable, 
closely  packed  sense  buds  which,  under  a  low  power,  produce  a  mottled,  or  pitted 
appearance  that  is  very  characteristic.  (Figs.  15  and  16.)  Under  a  higher 
power,  and  in  sections,  each  bud  appears  pear-shaped,  with  a  goblet-shaped  cavity 
opening  to  the  exterior  at  one  end,  and  leading  into  a  narrow  vertical  canal  at  the 
other.  They  consist  of  typical  sensory  cells,  having  the  same  shape,  arrangement, 


THE    PRIMITIVE    SENSE    BUDS. 


51 


and  rod-like  ends  as  those  in  the  segmental  sense  organs  on  the  outer  margins  of 
the  coxae.     (Fig.  74,  E.) 

The  primitive  sense  buds  appear  as  soon  as  the  six  thoracic  appendages  are 
outlined  (stage  B),  and  are  at  first  uniformly  distributed  over  the  entire  cord  and 
cephalic  lobes,  with  the  sole  exception  of  the  olfactory  lobes.  (Fig.  15.)  At 
a  later  stage,  E,  those  on  the  lateral  margin  of  the  cord  are  distinctly  larger  than 
the  rest,  forming  two  dark  bands.  From  the  buds  on  the  posterior  lateral  margin 
of  each  neuromere,  arise  the  ganglion  cells  at  the  roots  of  the  post-thoracic  nerves 
(spinal  ganglia).  The  buds  on  the  smaller,  or  originally  posterior  segment  of 
the  neuromere  give  rise  to  the  cluster  of  motor  nerve  cells  which  are  found  near 
the  anterior  nerve  roots. 


FIG.  42.— Brain  of  adult  scorpion,  from  the  side. 

As  development  proceeds,  the  central  cavity  of  the  bud  closes,  the  sensory 
cells  lose  their  cylindrical  form,  and  their  hair-like,  or  rod-like  outer  ends  disap- 
pear; finally  each  bud  forms  a  small  cluster  of  ganglion  cells. 

In  the  late  embryonic  stages  of  the  scorpion,  the  metamorphosed  sense  buds 
form  long  conical  masses  of  cells  with  the  proliferating  apices  directed  inward. 
Their  appearance  is  then  much  like  the  cell  clusters  formed  by  neuroblasts. 
(Figs.  227-228.) 

Cell  division  in  the  sense  buds  diminishes  after  their  metamorphosis,  the 
ganglion  cells  reaching  an  approximately  fixed  number  at  an  early  embryonic 
period.  This,  however,  does  not  apply  to  the  minute  cells  in  the  hemispheres, 
in  the  olfactory  lobes,  or  in  the  pedal  ganglia  of  Limulus,  for  these  cells  appear 
to  increase  in  number  steadily,  at  least  as  long  as  the  animal  continues  to  grow 
in  size. 

In  Limulus  the  cells  descended  from  a  given  sense  bud,  during  the  late  larval 
periods,  form  well  defined  clusters  of  pear-shaped  ganglion  cells,  with  a  special 
neuroglia  investment.  Each  cell  of  the  same  cluster  appears  to  project  its  fibers 
along  the  same  path,  to  the  same  terminals.  (Figs.  61-64.) 

In  Limulus  it  has  not  been  possible  to  identify  each  nerve-cell  cluster  with 


52  EVOLUTION    OF    THE    NERVOUS    SYSTEM    IN    SEGMENTED    ANIMALS. 

the  antecedent  sense  buds,  for  the  latter  are  best  seen  in  the  embryos  of  scorpions, 
while  my  most  detailed  work  on  the  cord  has  been  done  on  Limulus.  But  the 
conditions  in  the  two  animals  are  so  similar  that  there  can  be  no  reasonable  doubt 
that  an  arthropod  neuromere  consists  of  distinct  clusters  of  nerve  cells,  each  sur- 
rounded by  a  special  sheath  of  neuroglia,  each  projecting  its  fibers  along  the  same 
paths  to  the  same  terminals,  and  each  directly  descended  from  one  or  more 
embryonic  sense  buds. 

In  the  early  stages  of  the  neuron  in  vertebrates,  as  in  the  late  stages  in  the 
scorpion,  the  nerve  cells  are  often  arranged  in  parallel  vertical  rows,  which  may 
be  interpreted  in  the  same  manner  as  in  arthropods,  that  is,  as  the  ontogenetic 
remnants  of  ancestral  sense  buds. 

There  are  many  familiar  instances  where  nerve  cells  arise  from  the  same 
points  in  the  ectoderm  as  the  sensory  ones.  It  is  highly  probable,  in  such  cases, 
that  the  nerve  cells  are  ultimate  phases  in  the  specialization  of  sense  cells.  For 
example,  in  Acilius,  a  few  cells  of  large  size  leave  the  embryonic  retina  at  a  com- 
paratively late  stage;  they  finally  join  the  optic  ganglion  and  become  giant  nerve 
cells,  having  such  a  peculiar  form  and  location  that  they  may  be  readily  recognized 
through  life  (Patten) .  Ganglion  cells  may  also  arise  from  the  gustatory  epithelium 
in  Limulus,  or  from  the  epithelium  of  lateral  line  organs  in  vertebrates.  But  in 
none  of  these  cases  has  it  been  clearly  shown,  to  my  knowledge,  that  a  functional 
and  structurally  complete  sensory  cell  is  bodily  metamorphosed  into  a  ganglion 
cell.  However,  just  such  a  metamorphosis  as  this  does  take  place  in  Limulus  and 
Branchipus,  where  the  large  rod-bearing  visual  cells  are  converted  into  true  gang- 
lion cells,  which  still  retain  indications  of  their  primitive  grouping  into  ommatidia 
and  remnants  of  the  visual  rods.  (p.  162  and  Fig.  109,  A.) 

The  transformation  of  well  developed  sense  buds  into  ganglion  cells,  as  just 
described  for  the  neuron  of  arachnids,  is  not,  therefore,  without  precedent. 

In  most  arthropods,  the  primitive  sense  buds,  while  undoubtedly  present  in 
some  form,  are  not  as  well  developed  as  they  are  in  scorpions.  Hence  certain 
authors'  have  failed  to  recognize  their  real  character,  and  have  interpreted  them 
as  neuroblasts,  or  even  as  nutritive  folds,  or  as  folds  produced  by  growth  pressure. 
Such  interpretations  are  untenable.  It  is  true  that  the  sense  buds  may  be  repre- 
sented by  small  conical  groups  of  cells,  or  nuclei,  arising  from  the  proliferation  of 
a  single  deep-lying  cell,  or  nucleus,  or  "neuroblast."  But  the  formation  of  these 
neuroblasts  is  to  be  regarded  as  an  abbreviated  method  of  repeating  the  sense  bud 
stage  so  clearly  seen  in  the  scorpion. 

Even  these  neuroblasts  may  be  omitted,  or  their  appearance  postponed  to  a 
relatively  late  embryonic  period;  the  entire  cord  then  has  its  origin  in  a  few 
terminal  neuroblasts  (or  telo-neuroblasts),  as  in  Cymothoa. 


CHAPTER  IV. 

THE  SUBDIVISIONS  OF  THE  BRAIN. 

I.  THE  PROSENCEPHALON,  OR  FOREBRAIN. 

The  fore  brain  of  arthropods  is  that  part  of  the  neuron  that  usually  lies  in 
front  of  the  stomodaeum.  In  the  embryos  it  is  the  anterior  expansion  of  the 
medullary  plate  called  the  procephalic  lobes.  As  nearly  all  traces  of  mesoderm 
and  appendages  have  disappeared  from  this  region,  there  is  but  little  evidence 
accessible  to  indicate  the  presence  there  of  metameres.  In  many  arthropods 
the  lobes  are  divided  into  three  main  divisions,  with  no  recognizable  separation, 
at  any  time,  between  them  and  the  postoral  sections  of  the  nerve  cords;  hence  we 


Kt: 

FIG.  43. — Sagittal  section  of  a  young  scorpion. 

may,  for  the  present,  regard  the  main  divisions  as  greatly  reduced  metameres, 
and  the  central  portions  as  neuromeres. 

********* 

Acilius. — The  structure  of  the  procephalic  lobes  is  best  seen  in  the  embryos 
of  those  insects  which  lead  an  active  larval  existence,  as  for  example  in  Acilius. 
(Fig.  14.)  Here  they  are  divided  transversely  into  three  similar  parts,  which 
probably  represent  all  there  is  left  of  three  procephalic  metameres.  Each  meta- 
mere  is  also  divided  into  three  parts:  a.  a  median  one,  representing  a  forebrain 
neuromere,  corresponding  to  the  postoral  neuromeres;  b.  a  middle  part,  repre- 
senting a  segment  of  the  optic  ganglion;  and  c.  a  lateral  one,  forming  a  segment  of 
the  optic  plate,  each  plate  containing  two  ocelli.  Between  each  segment  of  the 
optic  ganglion  and  the  optic  plate  is  a  deep  infolding,  w.1"3,  which  later  closes, 
covering  up  the  optic  ganglia,  but  leaving  the  ocelli  and  neuromeres  in  their 
original  position. 

53 


54 


THE    SUBDIVISIONS    OF    THE    BRAIN. 


A  pair  of  small  appendages,  ro,  lie  near  the  first  metamere.  Later,  they  fuse 
to  form  the  labrum.  The  second  metamere  has  no  appendages.  The  third  one 
is  closely  associated  with  the  antennae. 

From  the  upper  or  neural  surface  of  the  first  ( ?)  and  second  forebrain  neuro- 
meres,  are  developed  dense  masses  of  small  cells,  with  deeply  stained  nuclei,  that 
give  rise  to  the  characteristic  mushroom  bodies  of  insects.  They  may  be  recog- 
nized in  apparently  all  classes  of  arthropods,  attaining  enormous  size  in  the  ants, 
bees,  wasps  and  spiders,  and  reaching  extraordinary  dimensions  in  Limulus. 
In  structure  and  function  (Limulus),  they  are  true  coordinating  centers,  and 
are  to  be  regarded  as  the  earliest  stages  in  the  phyllogenetic  development  of  the 
cerebral  hemispheres  of  vertebrates. 


FIG.  44. — Sagittal  section  of  a  primitive  vertebrate  embryo,  showing  the  relation  of  its  principal  organs  to  those  in 

the  arachnids;  schematic. 

In  the  arachnids,  the  cephalic  lobes  differ  from  those  of  Acilius :  a.  in  the  in- 
distinct segmentation  of  the  optic  plate,  b.  in  the  relatively  late  appearance  of  the 
segmental  sense  organs  (median,  and  lateral,  eyes,  and  olfactory  organs),  and  c. 
in  the  peculiar  character  of  the  first  metamere.  Other  differences  appear  later, 
as  we  shall  presently  indicate. 

In  the  scorpion  (Fig.  15),  which  may  be  taken  as  the  type,  the  first  metamere 
is  never  divided  into  neuromere,  optic  ganglion,  and  optic  plate,  but  forms  at  the 
outset  a  deeply  grooved  transverse  band,  61. o.  The  walls  of  the  infolding  contain 
minute,  deeply  stained  nuclei,  that  make  it  very  conspicuous,  both  in  sections  and 
surface  views.  The  band  marks  the  primitive  anterior  end  of  the  brain  and  is  the 
anlage  of  the  olfactory  lobes.  The  infolding  deepens,  at  first  more  rapidly  at 
either  end,  and  ultimately  carries  the  whole  lobe  below  the  surface,  and  back- 
ward, underneath  the  brain.  Here  it  forms  a  hollow,  bilobed  transverse  band, 
conspicuous  in  all  subsequent  stages,  when  the  brain  is  viewed  from  the  haemal 
side,  but  almost  entirely  concealed  below  the  hemispheres  when  seen  from  the 
neural  side.  (Figs.  40-42,  43>  46,  47-) 


THE    PROSENCEPHALON. 


55 


olf.v 


oil. -nm. 


R: 


The  cerebral  hemispheres  arise  as  mushroom-like  expansions  of  the 
second  neuromere.  In  Limulus,  they  are  very  conspicuous  in  the  early  stages,  and 
ultimately  grow  to  an  enormous  size.  They  consist  of  dense  masses  of  minute  cells, 
with  deeply  stained  nuclei,  unlike  any  others  in  the  nervous  system.  (Figs.  37  and  38.) 

As  these  cells  multiply,  the  hemispheres  project  above  the  surface  of  the 
brain  and  then  mushroom,  forming  large,  overhanging  lobes.  We  may  dis- 
tinguish anterior,  lateral,  and  posterior  lobes,  the  latter  being  much  the  largest. 
In  addition,  there  is  a  large  lobe  on  the  median  face  of  each  hemisphere.  (Figs. 
47,  B,  48  and  49,  g.c.)  The  hemispheres,  throughout  life,  are  connected  with  the 
neural  surface  of  the  second  neuromere  by  a  thick,  vertical  stalk,  or  peduncle, 
composed  of  nerve  fibers. 

As  the  hemispheres  increase  in  volume,  the 
posterior  lobe  completely  overlaps  the  third  neu- 
romere, and  the  lateral  and  anterior  lobes  partly 
envelop  the  haemal  surface.  In  the  adult,  the 
hemispheres  are  irregularly  convoluted,  and 
their  median  faces  are  flattened  against  each 
other  so  that  they  form  a  large  spherical  mass 
that  has  a  striking  resemblance,  in  external  form, 
to  the  hemispheres  of  vertebrates.  (Figs.  38-48.) 

In  the  scorpion,  the  hemispheres  are  much 
smaller  than  in  Limulus  and  are  crowded  farther 
forward  by  the  optic  ganglia,  which  have  almost 
united  in  the  median  line  behind  them.  Later 
the  whole  prosencephalon  is  bent  toward  the 
haemal  surface,  through  an  angle  of  something 
more  than  90°.  (Fig.  47,  A.)  When  the  fore- 
brain  flexure  is  completed,  about  the  time  of 
hatching,  the  hemispheres  lie  on  the  anterior 
haemal  surface  of  the  procephalon.  (Figs.  42 
and  43,  c.h.  or  h.)  This  flexure  is  very  marked 
in  all  arachnids,  so  far  as  known,  except  in 
Limulus. 

The  third  neuromere  undergoes  very  little  change.  It  may  be  recognized  for 
a  considerable  period  as  a  separate  neuromere  whose  neural  surface  is  covered 
with  tufts  of  large  ganglion  cells.  It  is  gradually  incorporated  into  the  thick 
mass  of  tissue  that  constitutes  the  body  of  the  forebrain  commissures,  and  upon 
which  the  hemispheres  rest  (basal  ganglia).  (Fig.  46.) 

The  history  of  the  procephalic  sense  organs  and  their  nerves  and  ganglia 
will  be  considered  under  their  appropriate  heads. 

II.  THE  DIENCEPHALON. 

The  diencephalon  in  arthropods  consists  of  a  variable  number  of  neuromeres 
surrounding  the  mouth.  The  first  neuromere  following  the  procephalic  lobes 
(antennal  neuromere  of  insects,  cheliceral  neuromere  of  arachnids)  may  be  re- 


FIG.  45. — Diagram  of  the  arachnid 
brain,  showing  the  number  and  grouping 
of  the  neuromeres,  the  ventricles,  the  vagus 
lobes,  and  the  longitudinal  gustatory 
tracts  and  their  relation  to  the  stomodaeal 
ganglia. 


.56 


THE    SUBDIVISIONS    OF    THE    BRAIN. 


garded  as  the  initial  neuromere  of  this  subdivision  of  the  brain.  Its  large  size 
and  its  special  relations,  on  one  side  with  the  hemispheres,  and  on  the  other  with 
the  stomodaeum  and  the  gustatory  organs,  lend  to  this  neuromere  and  those  im- 


l) 


ol 


FIG.  46. — Semi-diagrammatic  sagittal  sections,  showing  the  relations  of  the  piincipal  nerve  centers  in  the 
brains  of  arachnids  and  vertebrates.  A,  Embryo  scorpion  (stage  B,  Fig.  15) ;  B,  embryo  scorpion  (stage  G,  Fig.  18); 
C,  hypothetical  intermediate  condition,  based  on  the  conditions  in  both  scorpion  and  Limulus.  The  embryonic 
palium  and  the  anterior  neuropore  have  been  carried  over  into  the  adult,  and  the  lateral  eye  ganglia  have  been  pro- 
jected onto  the  neural  surface,  otherwise  the  typical  arachnid  conditions  remain  essentially  unmodified.  D  shows 
the  probable  position  and  relation  of  these  parts  in  a  vertebrate.  The  ventricles  are  indicated  in  Roman  numerals, 
the  neuromeres  in  Arabic  numerals. 

mediately  associated  with  it  in  the  circumoral  region,  a  special  distinction  that 
justifies  their  elevation  to  the  rank  of  a  distinct  brain  region. 

*  *  *  ****** 

In  the  scorpion    (Figs.  15,  1 6,  43),  by  the  time  the  embryo  hatches,  the  fore- 
brain  is  bent  through  something  more  than  90°,  onto  the  anterior  hasmal  surface 


THE    DIENCEPHALON. 


57 


of  the  egg.  The  angle  of  this  bend  lies  behind  the  cheliceral  neuromere,  which, 
therefore,  faces  forward,  connecting  the  forebrain,  now  on  the  haemal  surface  of  the 
egg,  with  the  thoracic  neuromeres  on  the  neural  surface.  (Figs.  43-46.) 

In  practically  all  adult  arachnids,  the  chelicerae  move  forward  to  the  very  an- 
terior end  of  the  head  and  lie  close  together  in  front  of  the  rostrum  and  stomo- 
daeum,  instead  of  behind  them,  as  in  the  earlier  stages.  The  result  is  that  the 
cheliceral  nerves,  instead  of  arising  from  the  sides  of  the  brain,  like  all  the  other 


,,pec. 


pa.ey.r-- 


vg.n. 


ol.l.-' 


st.n. 


ol.co. 


FIG.  47. — Sagittal  sections  of  brain  models.  A,  young  scorpion;  B,  Limulus;  C,  a  hypothetical  brain,  combining 
the  principal  characters  of  the  brain  of  Limulus  and  scorpion,  and  with  the  parts  in  the  position  they  are  supposed 
to  occupy  in  a  primitive  vertebrate. 

nerves  to  the   appendages,  arise   from  the  median,  neural  surface,  and  point 
cephalad  and  neurad.     (Fig.  40.) 

In  Limulus,  the  cheliceral  neuromere  is  less  conspicuous  in  the  older  stages 
because  it  is  partly  covered  by  the  posterior,  lobes  of  the  hemispheres,  which  grow 
back  over  it.  (Figs.  37,  38,  47  and  48.)  As  the  cheliceral  neuromere  moves 
forward,  it  unites  so  intimately  with  the  third  neuromere  of  the  forebrain  that  it 
is  difficult  to  distinguish  the  boundaries  between  them.  Both  neuromeres  help 


THE    SUBDIVISIONS    OF    THE    BRAIN. 


form  the  basal  ganglia  that  lie  anderneath  the  lobes  of  the  hemispheres,  and 
which  may  be  said  to  form  the  floor  of  the  prosencoel.     (Figs.  57,  58.) 

********* 

Minute  Structure. — The  minute  structure  of  this  region  has  been  worked 

out  in  some  detail  in  Limulus. 
Two  great  masses  of  neurones  that 
probably  belong  to  the  third  pro- 
cephalic  neuromere  are  found  on 
the  median,  neural  surface  of  the 
brain,  underneath  the  posterior 
lobes  of  the  cerebral  hemispheres 
(Fig.  49,  eh.,  H.c.)  Their  neurites 
extend  caudad  and  outward,  and 
then  cephalad,  forming  a  large 
part  of  the  posterior  cerebral  ped- 
uncle. On  reaching  the  base  of 
the  hemispheres,  they  spread  out 
into  great  fan-shaped  masses  that 
penetrate  into  the  cortex  of  every 
lobe  and  convolution,  except  the 
median  or  gustatory  one.  (Fig.  48 
and  49,  G.c.)  They  run  parallel 
with  similar  fibers  arising  from  the 
two  clusters  of  association  cells, 
H.a.s.  lying  above  the  gustatory 
lobes.  They  terminate  in  minute, 
spherical  masses  of  neuropile  that 
form  an  indistinct,  sub-cortical 
layer  in  each  lobe,  and  near  which 
the  neurites  of  the  cortical,  granule 
cells  terminate.  Some  of  the  fibers 
appear  to  terminate  between  the 
cortical  cells.  (Fig.  50.)  Numerous 
branches  from  these  neurites  ramify 
in  the  forebrain  commissure,  c.o., 
and  in  the  cheliceral  lobes,  ch.l. 

The  cheliceral  lobes  (Fig.  49, 
ch.l.)  are  large  spherical  masses  of 
neuropile  lying  on  the  anterior 
lateral  margin  of  the  cheliceral 
neuromeres.  Their  lateral  surface  is  covered  with  small  cells,  whose  neurites 
together  with  many  others,  ramify  in  their  interior.  The  most  conspicuous  ones 
are  those  belonging  to  the  two  sets  of  cerebral  association  cells,  ch,  H.c.  and  H.as, 
and  the  terminal  dendrites  of  the  main  gustatory  tract,  G.tr2. 


FIG.  48.- 


-Median  surface  of  a  model  of  the  forebrain  of  a 
young  Limulus  about  four  inches  long. 


THE    DIENCEPHALON.  59 

The  middle  portion  of  the  cheliceral  neuromere  forms  the  posterior  portion  of 
the  great  mass  of  commissural  fibers  and  neuropile  upon  which  the  hemispheres 
rest.  (Fig.  48,  b  and  c.)  One  may  recognize  in  it:  coarse  fibers  of  the  lateral  cell 
clusters,  C05;  fibers  from  the  large,  central  cells  of  the  olfactory  lobes  (Figs. 
48  and  5i,o/.c1.);  a  dark,  central  mass  of  neuropile,  b.  in  which  innumerable  neu- 
rites,  apparently  from  all  parts  of  the  brain,  terminate;  and  a  thin  layer  of  com- 
missural fibers  extending  from  one  crus  to  the  other,  c. 

The  more  anterior  portion  of  the  commissural  mass  (Fig.  48),  represents  the 

cort. 

.c.im 
G.ctf 

^'frymit 

H.as* 


FIG.  49. — Diagonal  section  of  the  forebrain  of  a  young  Limulus  (about  four  inches  long),  methylene-blue  prepara- 
tion stained  with  carmine.     Camera  outlines. 

commissural  bundles  of  the  second  and  third  forebrain  neuromeres,  a  and  d\ 
and  the  several  olfactory,  ol.cl~4,  and  optic  commissures,  op.g.4  . 

********* 
The  Stomodaeal  Ganglia  and  The  Supra  stomodaeal  Commissure.— The 
cheliceral  neuromere  is  always  intimately  associated  with  the  system  of  stomodaeal 
nerves  and  ganglia.  The  lateral  stomodaeal  ganglia  lie  on  the  median  side  of  the 
nerve  cords,  st.  g.  The  stomodaeal  commissure,  st.c.,  which  always  crosses  in  front 
of,  or  over,  the  stomodaeum,  forms  one  of  the  most  conspicuous  and  constant 
landmarks  of  the  arthropod  brain. 

In  the  scorpion,  the  anlage  to  the  lateral  stomodaeal  ganglion  may  be  faintly 
seen  from  the  surface,  on  the  anterior  median  face  of  each  half  of  the  cheliceral 
neuromere.  (Figs.  14  and  15,  st.g.)  The  same  anlage  may  be  seen  in  sections, 
in  Blatta,  Acilius,  Buthus  and  Limulus,  as  a  thickening  or  evagination  of  the  side 


60  THE    SUBDIVISIONS    OF    THE    BRAIN. 

walls  of  the  stomodaeum.  (Fig.  53,  c.)  The  evaginated  part  separates  from  the 
stomodaeum  and,  uniting  with  the  adjacent  neuromere  (cheliceral  or  antennal), 
forms  the  lateral  stomodaeal  ganglion.  In  insects,  a  frontal,  or  median  stomodaeal 
ganglion  arises  in  a  similar  manner  from  the  anterior  median  wall  of  the  stomo- 
daeum. (Fig.  14.) 

Nerves  extend  backward  from  the  stomodaeal  commissure  into  the  labrum, 
which  never  receives  nerves  from  any  other  source.  From  the  deep,  or  haemal, 
end  of  each  lateral  ganglion,  a  nerve  extends  along  the  side  walls  of  the  oesophagus, 
connected  by  several  transverse  bands  with  a  median  nerve  arising  from  the  frontal 
ganglion.  (Fig.  35.) 

In  arachnids,  the  median  stomodaeal  nerve  seen  in  the  insects  is  absent,  and 
there  are  no  traces  of  ganglion  cells  in  the  commissure. 

The  stomodaeal  nerves  and  ganglia  represent  a  distinct  system  of  nerves  that 
cannot  be  compared  with  any  others.  That  it  is  a  very  ancient  system  is  shown  by 
its  vigorous  growth  at  an  early  ontogenetic  period.  The  ganglia,  nerves,  and 
commissure,  form  a  special  system  controlling  the  peristaltic  actions  of  the  stomo- 
daeum in  swallowing,  grinding,  or  sucking  food.  These  reflexes  may  possibly  be 
directly  stimulated  through  the  sensory  cells  in  the  inner  lining  of  the  stomodaeum 
or  in  the  lips;  but,  in  Limulus  at  least,  an  essential  condition  appears  to  be  an  initial 
stimulation  of  the  gustatory  organs  in  the  jaws,  or  of  the  olfactory  organ.  From 
the  gustatory  organs,  important  nerve  tracts  converge  toward  a  common  center 
in  the  cheliceral  neuromere,  and  toward  the  lateral  stomodeal  ganglion.  (Fig. 
114.) 

Comparison  of  the  Diencephalon  of  Arachnids  and  Vertebrates.— 
When  the  mouth  of  the  arachnids  was  shut  off  from  the  exterior  by  the  backward 
overgrowth  of  the  rostrum  and  of  the  optic  lobes,  and  by  the  closing  up  of  the  cerebral 
vesicle,  the  stomodaeum  and  the  adjacent  ectoderm  remained  in  the  vertebrates  as 
the  epithelial  lining  of  the  third  ventricle  and  adjoining  chambers;  and  the  opening 
through  the  floor  of  the  brain,  which  served  as  the  passageway  for  the  old  oesopha- 
gus, remained  as  the  infundibulum.  The  inner  end  of  the  stomodaeum,  that  pro- 
trudes through  the  infundibulum,  became  the  sacci  vasculosi;  the  lateral  stomo- 
daeal ganglia,  the  lobi  inferiori;  and  the  stomodaeal  commissure,  the  anlage  of  the 
cerebellum.  (Figs.  3,  46.) 

The  median  haemal  portion  of  the  cheliceral  neuromere,  which  is  the  princi- 
pal center  for  the  olfactory,  gustatory,  and  stomodaeal  impulses,  corresponds  with 
the  hypothalmic  region,  while  the  cheliceral  lobes  and  the  cerebral  association 
cells,  ch.H.c,  mark  the  beginning  of  the  thalamus.  On  the  roof  of  the  ventricle, 
the  median  ocellus  persists  as  the  parietal  eye.  (Fig.  47,  c.)  Let  us  examine 
these  comparisons  more  carefully. 

********* 

The  Stomodaeum.— In  existing  arachnids,  the  roof  of  the  diencephalic  region 
consists  of  the  epithelium  that  was  produced  by  the  backward  migration  of  the 
rostrum  and  the  mouth.  (Figs.  3,  46,  and  47.)  Owing  partly  to  the  manner  in 


THE    DIENCEPHALON    OF   ARACHNIDS   AND    VERTEBRATES.  6 1 

which  the  entire  brain  has  slipped  forward,  underneath  the  skin,  arid  in  part  to 
this  backward  growth  of  the  rostrum,  the  stomodaeum  becomes  divided  into  two 
sections,  an  inner  one  passing  through  a  narrow  opening  between  the  crura  to  the 
enteron;  the  other  extending  backward,  over  its  outer  surface,  to  the  mouth. 
The  outer  section  is  dilated  in  most  arachnids  to  form  a  large  chamber  or  sucking 
stomach.  It  is  merely  a  matter  of  terminology  whether  we  call  the  mouth  the 
opening  of  the  original  infolding,  leading  directly  through  the  brain,  or  the  opening 
which  lies  much  farther  back  beneath  the  overhanging  rostrum.  In  the  verte- 
brates, both  the  original  infolding  and  its  secondary  extension  may  be  recognized. 

As  we  have  shown  elsewhere,  the  closing  of  the  primitive  oesophagus  was  due 
to  several  factors,  among  which  were:  the  crowding  together  of  the  cranial  neuro- 
meres;  the  increasing  size  of  the  palial  fold;  the  backward  growth  of  the  rostrum 
and  optic  ganglia  along  the  anterior  median  line;  and  the  deepening  of  the  median, 
neural  groove  by  the  precocious  thickening  of  the  lateral  cords.  These  condi- 
tions lead  to  the  infolding  of  the  entire  neuron  at  an  early  embryonic  period,  and 
to  its  complete  separation  from  the  overlying  ectoderm.  Thus,  not  only  were  the 
eyes  enclosed  within  the  brain  chamber,  but  the  passage  way  for  the  stomodaeum 
first  became  greatly  constricted,  and  then  the  opening  into  it  was  covered  over  by 
the  neural  crests  and  optic  ganglia,  thus  forever  closing  the  entrance  to  the  enteron 
from  that  direction. 

The  several  processes  seen  in  the  arachnids,  in  vertebrate  embryos,  are  blended 
and  abbreviated  into  a  simple  marginal  overgrowth  and  an  axial  depression 
of  the  medullary  plate.  The  chamber  thus  formed  over  the  cheliceral  neuromere 
then  becomes  the  third  ventricle;  the  epithelium  of  the  extra-neural  part  of  the 
stomodaeum  merging  with,  and  forming  a  part  of,  the  epithelial  lining  of  the  ad- 
jacent cavities.  The  primitive  stomodaeal  infolding  may  still  be  seen  in  the 
amphibia,  as  a  minute  pit  in  the  middle  of  the  procephalic  lobes,  near  their  anterior 
margin.  (Fig.  46.)  This  pit  lies  in  the  position  of  the  future  infundibulum  and 
appears  to  deepen,  giving  rise  to  it.  The  epithelium  forming  the  floor  of  the 
depression  is  continuous  with  the  epithelial  layer  that  covers  the  inner  surface  of 
the  adjacent  brain  cavities,  and  represents  the  deeper  end  of  the  stomodaeum,  now 
converted  into  the  membranous  saccus  vasculosus. 

From  the  posterior  lateral  walls  of  the  infundibulum,  two  rounded  ganglionic 
lobes  are  evaginated,  the  lobi  inferiori.  They  correspond  with  the  lateral  stomo- 
daeal ganglia  of  the  arachnids.  Like  them,  they  have  direct  nervous  connections 
only  with  the  adjacent  epithelial  sac  (stomodaeum),  although  the  nerve  centers 
themselves  are  of  considerable  size. 

According  to  Johnston,  the  epithelial  wall  of  the  saccus  contains  large  spindle- 
shaped,  sensory  cells,  bearing  a  tuft  of  cilia  which  projects  into  the  cavity  of  the 
tube  (ventricle).  From  them  arise  nerve  fibers  that  help  form  a  nerve  plexus 
over  the  outer  surface  of  the  sac.  The  afferent  and  efferent  fibers  form  two 
lateral  symmetrical  tracts  which  run  through  the  corpus  mammalare  to  the  ventral 
part  of  the  thalamus. 

********* 


62 


THE    SUBDIVISIONS    OF    THE    BRAIN. 


The  Optic  Ganglia.— The  palial  overgrowth  carried  the  united  parietal  eyes 
over  the  region  of  the  old  stomodaeum,  thus  helping  to  form  the  roof  of  the  third 
ventricle,  and  giving  rise  to  the  ganglion  habenulae  and  its  commissural  strands. 

The  lateral  eye  ganglia  also  united  to  form  a  part  of  the  brain  roof,  but  were 
crowded  still  farther  backward,  beyond  the  tween-brain  neuromeres,  carrying  with 
them  the  stomodseal  commissure.  The  latter  then  became  the  anlage  of  the  cere- 
bellum, and  the  optic  ganglion  became  the  tectum  opticum.  (Figs.  3  and  46.) 

The  optic  tracts  extend  diagonally  backward  and  upward  from  the  optic 
chiasma  to  the  optic  ganglia,  and  help  to  form  the  external  lateral  walls  of  the 
diencephalon;  the  primitive  cerebellar  tracts  extend  diagonally  downward  and 
forward,  over  the  inner  or  ventricular  surface  of  the  diencephalon  to  the  lobi 
inferiores.  Thus  the  location  and  direction  of  these  important  fiber  tracts  still 

tell  the  history  of  the  parts  in  which  they  term- 
corc  inate.  (Fig.  46,  D.) 

C.COT. 

The  Cheliceral  Neuromere.— The  an- 
terior neural  portion  of  the  cheliceral  neuro- 
mere is  probably  in  part  comparable  with  the 
thalamus  division  of  the  diencephalon.  It  con- 
tains the  great  masses  of  association  cells  going 
to  the  hemispheres  and  cheliceral  lobes  (Fig. 
49,  ch.l.)  and  also  the  cheliceral  nerves  and 
ganglia.  (Fig.  49,  ch.g.)  The  cheliceral  nerves 
unlike  all  the  other  cranial  nerves,  arise  from 
the  median  neural  surface  of  the  neuromere. 
of  a  young  Limuius.  Camera,  Zeiss  obj.  16  They  are  probably  crowded  against  the  cere- 

mm.,  oc.  18.      Golgi  preparation.  in  •  i          ,  i  i  j 

bellar    commissure  by  the  enlargement    and 

backward  migration  of  the  optic  lobes.  (Figs.  47,  57  and  58.)  They  are  represented 
in  vertebrates  apparently  by  the  fourth  nerves,  which  seem  to  arise  from  the  roof 
of  the  brain,  between  the  optic  lobes  and  the  cerebellum,  although  their  roots 
have  their  origin  far  forward,  on  the  floor  of  the  midbrain  region.  The  excep- 
tional location  and  direction  of  these  nerves  in  vertebrates,  therefore,  is  in  harmony 
with  their  exceptional  location  and  direction  in  arachnids;  and  the  extraordinary 
resemblance  between  them  affords  collateral  evidence  in  confirmation  of  the  ex- 
planation just  given  for  the  origin  of  the  tectum  opticum  and  the  cerebellum. 
Otherwise  these  peculiarities  of  the  fourth  nerve  are  inexplicable. 

*******  ** 

It  will  be  observed  that  the  whole  floor  of  the  vertebrate  brain  consists  of 
more  or  less  modified  neuromeres.  Those  structures  which  now  form  the  roof 
of  the  brain,  such  as  the  paliurn,  ganglion  habenulae,  optic  lobes,  and  cere- 
bellum, are  not  in  any  way  comparable  with  neuromeres;  they  have  no  segmental 
value,  and  they  now  have  no  genetic  relations  with  the  neuromeres  over  which 


FIG.  50. — Portion  of  the  cerebral  cortex 


THE   DIENCEPHALON    OF  ARACHNIDS  AND    VERTEBRATES.  63 

they  happen  to  lie.  The  tectum  opticum,  for  example,  really  belongs  to  the  third 
forebrain  neuromere,  in  front  of  the  diencephalon,  and  except  as  a  matter  of 
convenience,  cannot  be  classified  as  part  of  the  mesencephalic  neuromeres. 

(See  p.  157.) 

Summary. — Thus  we  have  in  the  diencephalon  of  vertebrates  a  remarkable 
combination  of  special  characters;  viz.  a  sharp  cranial  flexure;  a  funnel-like  depres- 


FIG.  51. — Forebrain  of  a  young  Limulus,  haemal  surface.     A.   A  single  optic  fibre,  showing  the  arrangement  of 
its  principal  branches.      Methylene-blue  preparation.     Camera  outlines. 

sion  in  the  floor,  with  voluminous  nerve  centers  (lobi  inferiori)  on  its  side  wall; 
the  presence  of  important  nerve  tracts  arising  from  the  cerebellum,  the  olfactory 
and  gustatory  organs,  and  that  converge  toward  a  common  center  in  the  in- 
fundibular region;  the  presence  of  a  membranous  sac  that  contains  the  remnants 
of  sensory  cells,  and  a  special  set  of  neuro-muscular  reflexes.  Each  of  these 
characters  is  without  parallel  elsewhere  in  the  brain  of  vertebrates.  They  indi- 
cate that  this  particular  region  either  has  some  very  unusual  part  to  play  in  cere- 


54  THE    SUBDIVISIONS    OF    THE    BRAIN. 

bral  activities  now,  or  did  have  some  such  function  in  the  past;  but  what  that 
function  was,  or  is,  or  what  is  the  history  and  the  meaning  of  these  parts,  neither 
vertebrate  anatomy  or  physiology  gives  us  the  slightest  clue. 

But  when  we  compare  these  conditions  with  those  in  the  arachnids,  their 
meaning  is  sufficiently  clear.  The  infundibulum  is  the  passageway  for  the  old 
stomodaeum,  and  the  latter  is  the  saccus  vasculosus.  The  lobi  inferiori  are  the 
lateral  stomodaeal  ganglia;  the  nerve  plexus  of  the  saccus,  the  stomodaeal  nerves; 
the  tween-brain  flexure  is  the  one  which  occurs  in  arachnids  between  the  supra- 
and  infra-stomodaeal  ganglia;  the  remarkable  centralization  of  fiber  tracts  in  the  in- 
fundibular region  is  the  retention  of  the  ancestral  condition  seen  in  arachnids, 
where  fiber  tracts  from  the  olfactory  and  gustatory  organs,  and  from  the  stomodaeal 
commissure,  converge  toward  the  swallowing  centers,  or  the  ganglia  that  control 
the  neuro-muscular  apparatus  of  the  stomodaeum. 

In  both  vertebrates  and  arachnids,  there  is :  a.  but  one  passage,  or  opening, 


op.tr.- -£ 

FIG.  52. — Semi-diagrammatic,  longitudinal  section  of  the  lateral  eye  ganglion  of  a  young   Limulus. 

in  the  floor  of  the  brain,  and  in  both  cases  that  passage  is  of  similar  relative 
dimensions;  b.  it  lies  just  behind  the  hemispheres  and  the  lateral  eye  nerve  roots;  and 
c.  just  in  front  of  the  anterior  end  of  the  notochord;  and  d.  in  front  of  the  first 
pair  of  somatic  motor  nerves;  e.  in  both  cases,  the  opening  lies  approximately 
between  the  right  and  left  halves  of  the  fourth  and  fifth  neuromeres;  /.  in  both 
cases  evaginations  from  the  median  face  of  the  half  neuromeres  form  special 
ganglia  (stomodaeal  ganglia,  lobi  inferiores),  quite  unlike  any  other  cranial  ganglia; 
g.  in  both  cases,  these  ganglia  have  similar  associations  with  the  stomodaeal  com- 
missure (cerebellum) ,  with  the  epithelial  tube  lying  in  the  opening  through  the  brain 
floor,  and  with  the  olfactory  and  gustatory  organs;  h.  and  although  of  considerable 
size,  the  ganglia  have  no  direct  nerve  connections  with  any  organs  external  to 
the  brain. 


The  diencephalon,  therefore,  of  arachnids  and  vertebrates,  may  be  defined 
as  one,  two,  or  more,  neuromeres  surrounding  the  primitive  stomodaeum,  and 
uniting  the  primitive  forebrain  (supra-oesphageal  ganglion)  with  the  midbrain, 
or  thoracic  neuromeres.  It  lies  in  the  angle  of  the  oldest,  and  most  striking 


THE    MESENCEPHALON.  65 

cranial  flexure,  and  marks  the  anterior  termination  of  the  notochord.  The 
chamber  lying  between  and  above  these  neuromeres  (the  third  ventricle)  was 
for  a  certain  period  an  extension  of  the  stomodaeum,  or  an  antechamber  to  that  por- 
tion that  actually  perforates  the  brain  floor.  The  final  closing  of  the  entrance  of 
this  antechamber  took  place  probably  at  a  point  just  back  of  the  cerebellum 
(rhomboidal  fissure).  (Figs.  3,  46,  58.) 

III.  THE  MESENCEPHALON. 

In  typical  arachnids  there  are  six  thoracic  neuromeres.  The  first  one  (two  or 
three)  (cheliceral  neuromere)  has  already  been  described  as  the  tween-brain,  the 
remaining  ones  constitute  the  midbrain.  They  differ  from  all  other  neuromeres 
in  their  great  breadth,  and  in  the  enormous  size  of  their  ganglia  and  gustatory 
nerves. 


C 


FIG.  53. — Longitudinal,  horizontal  sections  of  the  head  of  an  embryo  scorpion,  showing  origin  of  the  lateral 
stomodaeal  ganglion.  A,  At  the  level  of  the  suprastomodaeal  commissure;  B,  at  a  lower  level,  showing  the  inner 
end  of  the  lateral  stomodaeal  ganglion;  C,  still  deeper  level,  showing  its  origin  from  the  sides  of  the  stomodaeum. 
Camera  outlines. 

In  the  scorpion,  they  rapidly  increase  in  size  with  the  growth  of  the  append- 
ages, forming  a  compact  group  in  which  the  original  segmentation  is  clearly  in- 
dicated by  the  arrangement  of  the  nerves  and  cross  commissures.  They  are 
never  so  completely  fused  in  the  adult  as  to  lose  their  identity,  differing  in  this 
respect  from  those  in  the  forebrain  in  front  of  them,  and  in  the  vagus  region 
behind. 

In  the  adult  scorpion,  the  cross  commissures  are  very  short,  and  the  thick 
cords,  or  crura,  are  but  slightly  separated,  leaving  a  very  small  opening  for  the 
passage  of  the  oesophagus.  (Figs.  40-43.) 

In  Limulus,  this  opening  is  larger,  the  anterior  ends  of  the  crura  are  widely 
separated,  and  the  elongated  anterior  commissures  are  bent  into  wide  loops  by  the 
backward  movements  of  the  cesophagus.  (Figs.  38-39.)  These  conditions,  and 
the  absence  of  a  tween-brain  flexure,  give  the  brain  of  Limulus  a  different  out- 
ward appearance  from  that  of  the  scorpion,  although  structurally  they  are  very 
much  alike. 

The  great  size  of  the  neuromeres  and  the  divergence  of  the  crura  make  this 
the  broadest  and  most  voluminous  part  of  the  brain,  giving  it  a  rhomboidal  out- 
line, when  seen  from  above. 
5 


66 


THE    SUBDIVISIONS    OF    THE    BRAIN. 


S.A3. 


Mesencoele.— The  mesencoele  is  formed  in  part  by  a  marginal,  epithelial 
overgrowth,  and  in  part  by  a  deep  median  depression  or  infolding,  between  the 
two  cords.  The  epithelial  overgrowth  is  formed  along  the  whole  margin,  as  a 
thin  overarching  shelf  that  projects  about  a  third  way  over  each  cord.  (Figs. 
136,  227  and  231.)  Later,  the  cords  settle  bodily  below  the  surface,  and  as  there 
is  no  delamination  of  a  superficial  epithelium,  a  wide  opening  is  left  that  is 
gradually  closed  in  by  the  union  of  the  thin  edges  of  the  two  overgrowths. 

As  the  two  cords  in  this  brain  region  remain  horizontal,  the  potential  mesen- 
ccele  is  broad  and  shallow,  except  in  the  middle  line  where  it  extends  to  the  bottom 
of  the  deep  median  fissure. 

In  the  more  posterior  parts  of  the  brain,  in  the  vagus  and  branchial  regions, 

similar  overgrowths  occur.  But  the  cords  are 
much  narrower  here  and  their  margins  are  brought 
together,  like  the  closed  covers  of  a  book,  by  the 
deep  median  infolding,  so  that  the  chamber  is  con- 
verted into  a  deep,  narrow  fissure.  The  bottom 
of  this  fissure  is  converted  into  a  "canalis  cen- 
tralis,"  when  the  neural  commissures  grow  across 
the  fissure,  just  above  the  floor.  (Figs.  55,  69.) 
*  *  *  *  *  * 

Comparison  with  Vertebrates. — The  mid- 
brain  neuromeres  of  arachnids  are  represented  in 
vertebrates  by  the  group  of  conspicuous  neuromeres 
forming  the  floor  of  the  brain,  from  the  infundi- 
bulum  to  the  vagus  region.  The  region  is  charac- 
terized by:  a.  its  great  width;  b.  its  enormous 
FIG.  54.—Vagus  and  branchial  craniai  ganglia,  widely  separated  from  the  crura 

neuromeres     of    an     embryo    scorpion  &        &  J 

about  ready  to  hatch.  Camera  outline,  and  associated  with  the  oral  and  hyoid  arches ;  c.  by 

the  segmentally  arranged  gustatory  organs;  and  d. 

by  the  unusual  distinctness  of  the  neuromeres  in  the  early  embryonic  stages,  in 
marked  contrast  with  the  regions  just  in  front  and  behind. 

In  vertebrates  confusion  has  arisen  from  the  failure  to  distinguish  between 
the  true  neuromeres  on  the  floor  of  the  brain  and  the  various  structures  that  have 
been  forced  out  of  their  original  positions  onto  the  roof  of  the  brain.  The  cerebel- 
lum and  the  optic  lobes,  as  we  have  indicated  above,  are  of  very  unequal  value, 
and  in  no  wise  comparable  with  neuromeres.  Their  position  in  vertebrates  is  a 
purely  secondary  one,  a  long  way  caudad  to  their  original  position  and  connec- 
tions. The  optic  lobes,  that  in  vertebrates  form  the  roof  to  the  midbrain,  clearly 
belong  to  the  procephalic  neuromeres,  while  the  primitive  cerebellar  "neuromere" 
is  a  special  commissure  primarily  associated  solely  with  the  diencephalon.  The 
area  covered  by  these  structures  therefore,  varies  greatly,  and  has  no  constant 
relation  to  the  underlying  neuromeres. 

If  we  remove  the  cerebellum  and  optic  lobes  from  the  brain  of  a  vertebrate 


THE    MESENCEPHALON.  67 

embryo  (frog),  it  will  be  seen  that  the  underlying  neuromeres  form  a  natural 
group,  that  corresponds  approximately  with  the  posterior  four  or  five  thoracic 
neuromeres  of  arachnids.  It  is  impossible,  at  present,  to  accurately  fix  either  the 
anterior  or  the  posterior  boundaries  of  this  group,  for  we  do  not  know  how  many, 
if  any,  have  been  added  to  the  cheliceral  neuromere  to  form  the  diencephalon,  or 
whether  or  no  the  last,  or  sixth,  thoracic  neuromere  has  fused  with  the  first  vagus. 
There  should  be,  according  to  our  provisional  interpretation,  five  neuromeres, 
from  the  fifth  to  the  ninth  inclusive,  in  this  region;  and  that  number  corresponds 
pretty  nearly  with  the  estimated  number  of  neuromeres,  and  with  the  number 
of  head  cavities,  segmental  nerves,  and  visceral  arches  that  are  known  to  occur 
there.  (Fig.  58.) 

IV.  THE  METENCEPHALON,  OR  VAGUS  NEUROMERES,  AND  V,  THE  BRANCHIEN- 

CEPHALON,    OR    BRANCHIAL   NEUROMERES. 

The  Metencephalon. — In  many  arthropods,  the  transitional  region  between 
one  group  of  metameres  and  the  next  is  often  marked  by  an  abrupt  change 
in  the  size  of  the  pertinent  organs,  and  by  their  union  in  the  middle  line  and 


FIG.  55. — Section  through  the  first  vagus  neuromere  of  Limulus,  showing  the  neural  and  haemal  commissures,  central 

canal,  and  lemmatochord. 

ultimate  disappearance;  at  the  same  time,  in  the  next  following  segments,  the  same 
kind  of  organs  may  be  greatly  enlarged.  These  conditions  are  similar  to  those 
seen  at  the  point  of  incomplete  fissure  in  annelids. 

The  most  striking  cleavage  zone  of  this  character  in  the  arthropods  is  the 
vagus  region.  It  lies  between  the  thorax  and  abdomen,  and  is  one  of  the  most 
conspicuous  transitional  zones  in  the  whole  body.  It  is  present  in  many  insects, 
Crustacea,  trilobites,  etc.,  but  its  character  is  best  known  in  the  arachnids. 

In  Limulus,  there  are  two  vagus  metameres,  the  chilarial  and  opercular, 
whose  neuromeres  form  an  important  part  of  the  brain.  The  external  boundaries 
of  the  two  metameres  are  at  first  similar  to  those  of  the  other  abdominal  segments. 
(Figs.  141,  142.)  But  ultimately,  the  only  remaining  external  traces  of  the  chil- 
arial segment  are  the  chilarial  appendages  and  the  narrow  ridge  on  the  posterior 


68 


THE    SUBDIVISIONS    OF    THE    BRAIN. 


margin  of  the  thoracic  shield.  (Fig.  152,  ch.pl.)  The  tergite  of  the  opercular 
metamere  is  a  narrow  wing  plate,  still  clearly  outlined  on  the  lateral  margins,  but 
in  the  middle  it  is  completely  fused  with  the  abdominal  shield  op.pl.  The  hinge 
of  the  cranial  buckler  comes  between  the  tergites  of  the  chilarial  and  the  opercular 
metameres.  (Fig.  155.) 

The  chilarial  and  opercular  neuromeres  are  completely  fused  with  each  other 
and  with  the  posterior  end  of  the  midbrain;  and  in  the  adult  only  the  commis- 
sures and  the  distribution  of  the  corresponding  nerves  afford  a  clue  to  their 
identity.  (Figs.  65,  66,  and  70.) 

In  the  scorpion  the  first  four  abdominal  metameres  belong  to  the  vagus 
group.  At  an  early  period,  there  is  a  pair  of  rudimentary  appendages  on  each  of 
these  four  metameres.  (Figs.  15  and  16.)  In  young  scorpions,  the  first  two  pairs 


op.tt. 


FIG.  56. — Cross-section  of  the  mesencephalon  of  a  young  Limulus  brain.  On  the  right,  the  section  passes 
through  the  fourth  pedal  ganglion,  with  its  gustatory,  entocoxal,  and  pedal  nerves,  and  the  ascending  roots  of  the 
fifth  and  sixth  gustatory  nerve  roots.  On  the  left,  the  section  passes  through  the  root  of  the  fourth  haemal  nerves, 
showing  its  relation  to  the  commissures  and  to  the  great  longitudinal  tracts.  The  figure  is  constructed  from 
methylene  blue  and  von  Rath's  preparations.  The  capital  letters  indicate  the  same  neurones  as  in  Figs.  65  and  66. 

form  minute  papillae  near  the  unpaired  genital  opening.     The  third  pair  form  the 
pectines,  and  the  fourth,  in  part,  the  first  pair  of  lung  books. 

The  four  neuromeres  become  fused  into  a  dense  triangular  mass,  crowded 
forward  beneath  the  midbrain.  Its  posterior  end  is  elevated,  producing  a  pro- 
nounced hind-brain  flexure.  (Figs.  40,  43,  54.)  The  vagus  neuromeres  are  rela- 
tively narrow,  and  as  the  corresponding  appendages  are  crowded  toward  the 
middle  line,  their  pedal  nerves  arise  in  a  characteristic  manner  from  the  neural 
surface,  not  from  the  sides. 


The  Branchiencephalon. — In  the  arachnids,  the  anterior  vagus  meta- 
meres are  characterized  by  their  highly  modified  tactile  or  sensory  appendages, 
the  posterior  ones  only  being  respiratory.  In  the  transitional  forms  between 
vertebrates  and  arachnids,  respiratory  segments  were  doubtless  added  to  the 


THE    METENCEPHALON   AND    THE    BARNCHIENCEPHALON. 


69 


head,  from    time   to   time,  thus  leading  to  the   union  of  the   entire  group  of 
branchial  neuromeres  with  the  posterior  portion  of  the  brain. 

We  therefore  look  on  the  hindbrain  of  vertebrates  as  composed 'of  two  main 
parts  successively  added  to  the  midbrain,  namely :  a.  the  vagus  section,  consisting 
of  from  two  to  four  fused  neuromeres,  associated  with  appendages  that  have  under- 


Olfactory 

olfactory  organ 

Coordination 

hemispheres 

Visual 

parietal  eye 
lateral  eye 

Swallowing 

stomodaeum 
infundibulum 

Chewing 

and 

Gustatory 

leg-jaws 
oral-arches 

Locomotor 
Auditory 
Equilibrium 
Vagus 

tactile 
lateral  line 

Respiratory 

cardiac 
branchial 
hypobranchial 
sympathetic 

Digestive 

excretory 
genital 


FIG.  57. — Semi-diagrammatic  figure  of  an  arachnid  brain  and  nerve  cord.  It  represents,  in  the  main,  the 
conditions  in  Limulus.  The  parietal  eye  and  the  vagus  lobes  and  nerves,  however,  have  the  location  and  arrange- 
ment characteristic  of  the  scorpions. 

FIG.  58. — Here  the  same  parts  are  shown  in  the  position  they  are  supposed  to  have  in  vertebrates.  The  prin- 
cipal changes  are  in  the  position  of  the  optic  ganglia,  which  have  moved  backward  and  upward  to  form  the 
optic  lobes;  the  pedal  ganglia  have  become  the  cranial  ganglia  of  the  fifth,  seventh,  eighth,  ninth,  and  tenth  nerves; 
the  coxal  gustatory  organs  lay  the  foundations  for  the  principal  lines  of  canal  organs.  The  branchial  neuromeres 
have  fused  with  the  brain,  and  the  components  of  the  several  branchial  and  vagus  nerves  going  to  the  same  kind  of 
organ  have  united,  forming  compound  nerves  distributed  respectively  to  the  respiratory  organs,  sense  organs, 
heart,  intestine,  and  the  great  branchio-thoracic,  or  hypo-branchial  muscles.  The  similiarity  in  the  linear  distribu- 
tion of  the  principal  functions,  or  the  nerve  centers  controlling  them,  is  shown  by  the  inscriptions  between  the 
figures. 


gone  extensive  reduction  and  modification,  the  most  important  remnants  forming 
organs  of  a  tactile  or  gustatory  nature;  this  characteristic  group  of  neuromeres 
is  largely  sensory,  and  now  forms  the  hindbrain,  or  metencephalon  of  arachnids; 
and  b.  a  second  group  of  neuromeres  belonging  to  the  branchial  segments,  and 


70  THE   SUBDIVISIONS    OF   THE   BRAIN. 

which  were  mainly  associated  with  the  gills,  heart,  and  viscera;  they  united  with 
the  brain  at  a  later  period. 

The  combined  vagal  and  branchial  neuromeres  of  arachnids  form  the  hind- 
brain,  or  medulla,  of  vertebrates.  (Figs.  57-58.) 

In  arachnids,  all  the  branchial  neuromeres  do  not  form  a  part  of  the  brain, 
except  possibly  in  certain  specialized  land  forms;  but  they  already  show  the  begin- 
ning of  that  functional  segregation  of  sensory,  branchial,  cardiac,  and  motor 
nerve  fibers  that  is  so  characteristic  of  the  hindbrain  neuromeres  of  vertebrates. 
(See  cardiac  and  hypobranchial  nerves). 

In  arachnids  (scorpion  and  Limulus)  there  are  in  all  seven  vagus  and  branchial 
neuromeres,  which  is  close  to  the  estimated  number  in  this  region  in  vertebrates. 
However,  this  number  probably  varies  in  vertebrates,  as  it  does  in  arachnids,  and 
only  an  approximate  numerical  agreement  and  serial  location  is  offered  as  having 
significance.  The  segregation  of  the  vagal  and  branchial  nerves  (in  both  verte- 
brates and  arachnids)  into  the  groups  mentioned  above,  is  of  much  greater  sig- 
nificance than  the  agreement  in  the  number  of  neuromeres. 


CHAPTER  V. 

MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF   ARACHNIDS. 

METHODS. 

In  order  to  better  understand  the  minute  structure  of  the  brain,  it  is  essential 
to  first  determine  the  structure  of  one  of  the  isolated  neuromeres  of  the  spinal 
cord.  I  have  used  for  this  purpose  one  of  the  branchial  neuromeres  of  Limulus. 
But  even  here  the  structure  is  so  exceedingly  complex  that  it  is  possible  to  work 
out  or  to  represent  but  a  very  small  part  of  it  in  detail.  Our  observations  on  the 
structure  of  an  invertebrate  neuromere  will  be  of  special  interest,  because  they 
are  based  on  the  gross  anatomy  and  embryology,  upon  the  minute  structure  as 
obtained  by  several  methods  of  analysis,  and  upon  the  physiology. 

For  the  development  of  the  primitive  sense  buds  and  the  early  embryonic 
stages  of  the  cord,  I  have  used  the  scorpion.  For  the  distribution  of  the  fibers 
and  cells,  I  have  used  the  methylene  blue  "intra  vitam"  method  on  Limuli  from 
2  to  6  inches  long;  the  brain  and  cord  being  mounted  whole,  or  sectioned.  The 
adult  nervous  system  has  been  tested  for  its  physiological  reactions,  and  the  dis- 
tribution of  the  peripheral  nerves  has  been  followed  with  care.  Its  minute 
structure  was  worked  out  from  sections  prepared  by  several  methods,  the  most 
satisfactory  ones  being  the  usual  Golgi  method,  and  von  Rath's  picro-osmic- 
platinum  chloride  mixture,  followed  by  methyl  alcohol. 

I.  THE  BRANCHIAL  NEUROMERES  IN  LIMULUS. 

Development. — In  Limulus  and  in  the  scorpion,  during  the  late  embryonic 
stages,  a  thin  epithelial  overgrowth  forms  on  the  lateral  margins  of  the  cords,  and  a 
deep  infolding  appears  in  the  median  line  between  them.  The  lining  to  the  me- 
dian infolding  is  in  part  converted  into  the  epithelium  of  the  canalis  centralis, 
in  part  into  the  inner  neurilemma,  or  neuroglia,  and  into  the  lemmatochord. 

The  lateral  cords  themselves  sink  bodily  below  the  surface,  without  separat- 
ing off  a  surface  epithelium,  and  are  covered  by  an  ectodermic  layer  formed  from 
the  marginal  overgrowth  united  to  the  local  (interganglionic)  remnants  of  the 
middle  cord.  (Figs.  227,231.)  As  in  the  arthropods  generally,  they  lie  in  a 
horizontal  plane,  separated  by  the  median  infolding  that  forms  the  middle  cord. 
In  the  vertebrates,  similar  conditions  prevail,  except  that  the  median  margins  of 
the  lateral  cords  are  more  deeply  infolded  than  the  lateral  ones,  thus  bringing  the 
neurogenic  surfaces  of  the  cords  face  to  face.  Thus  the  two  lateral  cords  of  the 


72        MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

arachnids  form  the  lateral  walls  of  the  neural  tube  of  vertebrates,  the  middle  cord 
canal  (canalis  centralis)  being  at  the  inner  surface,  the  neural  crests  on  the  outer 
one,  and  the  nerve  fiber  layers  on  the  lateral  wall  of  the  tube.  (Figs.  134-137.) 


-bth 


Commissures. — In  arachnids,  two  main  sets  of  transverse  commissural 
fibers  are  formed  in  each  neuromere,  the  neural  and  the  haemal  commissures. 

The  haemal  commissures  are  the  first  to 
arise.  They  make  their  appearance  on  the 
haemal  side  of  the  epithelium  of  the  median 
groove  as  two  separate  bundles,  an  anterior 
and  a  posterior  one.  (Fig.  64,  a.h.co.  and 
p.h.co.)  The  haemal  commissures  are  im- 
portant features  in  all  arthropod  neuromeres, 
and  represent  in  part  the  remnants  of  the 
transverse  fiber  tracts  that  extend  round  the 
body  and  unite  the  longitudinal  cords.  Seen 
in  sagittal  sections  of  the  adult  cord,  they 
appear  as  two  large  irregular  bundles, 
separated  by  a  narrow  space  through  which 
the  neuroglia  of  the  median  canal  and  me- 
dian fissure  is  continuous  with  that  of  the 
lemmatochord. 

At  a  much  later  period  a  new  set  of  com- 
missures appears  above  the  floor  of  the  median 
groove,  thus  converting  that  part  of  the  groove 
into  a  canal;  a  large  anterior  one,  an.n.co.,  a 
small  middle  one  m.n.co.,  and  a  small  but 
sharply  defined  posterior  one,  p.n.co. 

As  the  longitudinal  commissures  approach 
the  neuromere,  they  divide  into  distinct  neu- 
ral and  haemal  tracts  h.tr.  and  n.tr.  In  the 
main,  the  fibers  in  each  tract  either  terminate 
in  neuropile  masses  situated  on  the  corres- 
ponding side  of  the  neuromere,  or  run  straight 
through  it,  the  neural  tracts  passing  above  the 
neural  commissures,  and  the  haemal  tracts  below  the  haemal  commissures. 

The  peripheral  nerves  consist  of  two  main  pairs.  (Figs.  59,  60.)  The 
anterior  pair  is  the  more  complex.  Owing  to  the  central  relation  of  its  fibers, 
its  position  on  leaving  the  cord,  and  its  relation  to  the  great  muscle  masses,  it  may 
be  spoken  of  as  a  haemal  nerve.  It  is  comparable  with  the  motor  or  ventral  root 
of  a  vertebrate  spinal  nerve.  It  is  a  mixed,  non-ganglionated  nerve,  and  contains 
general  cutaneous,  somatic  motor,  cardiac,  and  visceral,  or  intestinal  elements. 


FIG.  59.— The  branchial  and  abdominal 

neuromeres    of  a  young  Limulus  three  inches 

long.   The  intestinal  plexus,  i.  pi,  longitudinal 


thoracic  nerves,  b.  th,  are  shown. 


THE    CELL    CLUSTERS    OF    THE    BRANCHIAL    NEUROMERES. 


73 


n.tr. 


The  posterior  nerve  is  more  voluminous,  but  less  complex  than  the  anterior 
one.  It  arises  from  a  large  conical  "  ganglion,"  consisting  of  neuropile  and 
ganglion  cells,  situated  on  the  posterior  neural  surface  of  the  cord.  (Fig.  63,  C.) 
It  supplies  the  gill  muscles  and  the  sense  organs  of  the  corresponding  branchial 
appendage.  The  majority  of  the  elements  are  sensory,  and  are  confined  in  the 
main  to  the  neural  side  of  the  nerve.  It  is  therefore  a  neural  ganglionated  nerve, 
comparable  with  the  posterior,  or  ganglionated 
root  of  a  vertebrate  cranial,  or  spinal  nerve. 

Cell  Clusters. — The  nerve  cells  in  the 
neuromere  are  arranged  in  clusters  that  are 
remarkably  constant  in  their  size,  location,  and 
relation  to  fiber  tracts.  Each  cluster  probably 
represents  the  remnants  of  one  or  more  primitive 
sense  buds. 

In  exceptional  cases,  the  methylene  blue  fails 
to  affect  the  cells  and  fibers,  but  stains  rather 
sharply  the  neuroglia,  thus  giving  excellent  pic- 
tures of  the  nerve  cell  clusters  and  their  sheaths. 
(Fig.  6 1.)  The  clusters  are  mainly  confined  to 
the  neural  surface  and  lateral  margins  of  the 
neuromere.  The  more  important  ones  are  as 
follows : 

a.  A  cluster  of  large  cells  in   two  or  more 
groups,  on  both  neural  and  haemal  sides  of  the 
anterior  lateral  margin.      (Fig.    61,  A.)     Their 
axones  cross  to  the  opposite  side,  forming  part  of 
the    anterior   haemal   commissure,   entering   the 
haemal  nerve  as  its  third  root.     (Figs.  61  h.r.3  and 
62,  a.)     Before  crossing,  each  axone  gives  off  a 
large  collateral,  a',  that  extends  backward  into 
the  longitudinal  haemal  tracts. 

b.  A  very  large  cluster  of  medium  size  cells 
(Fig.  61,  B.),  whose  axones,  b.  converge  into  a 
large  bundle,  directed  vertically.     On  reaching 
the  haemal  surface  of  the  cord,  each  fiber  divides, 

one  branch  entering  the  main  longitudinal  haemal  tracts  and  extending  backward, 
as  a  coarse  unbranching  fiber,  to  the  more  posterior  neuromeres.  (Fig.  62,  b.); 
the  other  branch,  b',  crosses  to  the  opposite  side,  in  the  posterior  part  of  the 
anterior  haemal  commissure,  behind  the  fibers  forming  the  third  root 
of  the  haemal  nerve.  They  are  probably  association  elements.  (Figs.  61,  62 
and  64.) 

c.  Numerous  clusters  of   minute  cells,  on  the  neural  surface  of  the  pedal 
ganglion.     Their  axones  terminate  in  the  large  mass  of  interwoven  fiber  bundles 


h. 


n.fi. 


FIG.  60. — Two  branchial  neuromeres, 
seen  from  the  neural  surfaces,  showing  the 
location  of  the  principal  masses  of  neu- 
ropile, fiber  bundles,  and  neurones. 


74  MINUTE    STRUCTURE    OF    THE    BRAIN  AND    CORD    OF   ARCHNIDS. 

that  constitute  the  exceedingly  complex  core  of  the  ganglion.     Probably  sensory. 
(Figs.  62,  C.  and  63  C.) 

d.  A  cluster  of  about  thirty  very  large  motor  cells  on  the  posterior  median 
part  of  the  neural  surface.  Their  axones  form  a  compact  bundle  which  extends 
vertically  and  then  crosses  to  the  opposite  side,  the  two  crossing  bundles  constitut- 
ing a  large  part  of  the  posterior  haemal  commissure.  (Figs.  60,  61,  62  and  64, 
D  and  d.) 


h  r 


-.        i 

.   .Ji  r 


L.   h.ti 


FIG.  61. — One  of  the  anterior  branchial  neuromeres  of  a  young  Limulus.  On  the  right,  the  principal  cell  clusters 
of  the  neural  surface  are  shown;  on  the  left,  the  principal  nerve  roots,  fiber  tracts,  and  neuropile  centers.  The 
capital  letters  indicate  the  cell  clusters,  and  the  small  letters,  the  corresponding  fibers;  Hr.  l~t>,  the  five  roots  of  the 
haemal  nerves.  Composite  figure,  based  on  methylene  blue  and  von  Rath's  preparations. 

The  crossed  fibers  extend  backward  along  the  haemo-lateral  side  of  the  cord 
to  the  next  posterior  neuromere,  forming  the  fifth  root  to  the  haemal  nerve,  h.  r5. 
They  are  very  conspicuous  in  sections  on  account  of  the  large  size  and  pro- 
nounced coloring  of  the  axis  cylinders  and  their  sheaths.  On  approaching  the 
next  following  neuromere,  the  bundle  becomes  more  compact  and  gradually 
moves  toward  the  outer  margin  of  the  cord,  where  it  turns  sharply  forward  and 


THE    CELL    CLUSTERS    OF    THE    BRANCHIAL   NEUROMERES. 


75 


outward  into  the  haemal  nerve.  It  there  divides  into  two  bundles;  one  Lab,  con- 
stitutes the  nerve  supplying  the  haemo-neural  and  longitudinal  abdominal  muscles; 
the  other,  extending  onward  into  the  main  trunk,  forms  the  branch  that  supplies 
the  branchio-thoracic  muscles  (hypoglossal  elements). 

e.  A  group  of  large  cells,  lying  on  the  posterior  haemal  side  of  the   pedal 
ganglion.  (Figs.  60,  61,  63,  and  64,  E  and  e.)     Their  axones  are  directed  diago- 


hr  . 


FIG.  62. — Anterior  branchial  neuromereof  a  young  Limulus  seen  from  the  haemal  surface.  On  the  right,  the 
superficial,  longitudinal,  haemal  tracts  are  shown;  on  the  left,  the  underlying  haemal  cross  commissures,  and  their 
relation  to  the  principal  groups  of  neurones.  Outside  the  main  figure,  on  the  left,  is  a  cross-section  of  a  neural 
nerve,  N.  N',  and  a  haemal  nerve,  H.  N,  On  the  right,  the  minute  structure  of  some  of  the  nerve  tubes  in 
the  haemal  nerve,  m.  t,  and  in  the  neural  nerve,  s.  t,  is  shown. 

nally  forward,  inward  and  upward;  they  then  cross  over  to  the  opposite  side,  and 
return  again  to  the  haemal  surface  where  they  extend  forward  as  fine  unbranched 
fibers  on  the  lateral  margins  of  the  longitudinal,  haemal  tracts. 

The  crossing  bundles  constitute  apparently  the  whole  of  the  posterior  neural 
commissure.  (Fig.  64,  p.n.co.).  Before  and  after  crossing,  the  axones  give  off 
numerous  dentrites  which  ramify  in  the  neuropile  core  of  the  pedal  ganglia  (Fig. 
63,  E.)  Probably  association  fibers. 

/.  A  medium  sized  cluster  on  the  anterior  lateral  margin  of  the  pedal  ganglion. 


76       MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

(Figs.  60,  61,  63,  64,  F.)  Their  axones  form  a  rather  loose  bundle  (ill  defined  in 
sections)  extending  downward,  inward,  and  forward  along  the  neural  portion 
of  the  cord,  to  the  next  anterior  neuromeres.  Before  and  after  crossing,  the  large 
irregular  fibers  give  off  numerous  collaterals  which  ramify  diffusely  in  the  central 
fibrous  portions  of  the  neuromere.  (Fig.  63,  -F.)  Termination  unknown;  probably 
association  fibers. 

g.  On  the  anterior  lateral  part  of  the  neural  surface,  small  clusters  not 
clearly  defined,  that  send  axones  diagonally  inward  and  forward.  (Figs.  61  and 
63,  G.)  The  scattering  axones  appear  to  cross  in  both  the  anterior  neural  and 
the  anterior  haemal  commissures.  Some  appear  to  send  collaterals  backward  to 
the  next  posterior  neuromere.  Termination  unknown. 

h.  Along  the  lateral  haemal  margin,  on  either  side  of  the  pedal  ganglion, 
are  several  groups  of  cells  that  are  usually  very  conspicuous.  (Fig.  62,  H1"''3.) 
They  differ  from  the  other  neurones  in  that  each  cell  gives  rise  to  a  large  number 
of  dendrites  and  axones.  The  dendrites  are  minute  and  their  innumerable 
branches  fill  the  core  of  the  pedal  ganglion,  often  giving  it  a  dark  blue,  finely 
granular  appearance.  The  axones  are  large,  irregularly  branching  fibers  ex- 
tending outward,  as  bundles  of  parallel  fibers,  onto  the  haemal  surface  of  the  pedal 
nerve.  These  neurones  are  the  only  ones  of  their  kind  and  are  characteristic  of 
the  pedal  nerves,  both  in  the  thorax  and  in  the  abdomen. 

The  axone  bundles  from  the  several  clusters,  H*~3,  converge  toward  the 
haemal  side  of  the  pedal  nerve  where  they  form  a  distinct  bundle,  readily  recog- 
nizable in  sections.  (Fig.  68,  h.)  They  are  the  motor  nerves  that  supply  the  gill 
muscles. 

i.  Two  large  groups  of  neurites  on  the  anterior  haemo-lateral  margin,  sending 
great  bundles  of  fibers  forward  and  inward  into  the  anterior  portion  of  the  an- 
terior haemal  commissure.  (Fig.  62,  /  and  64,2.) 

j.  A  large  group  of  cells  on  the  posterior  haemal  margin,  projecting  their 
fibers  forward  and  then  across  to  the  opposite  side,  in  the  anterior  portion  of  the 
posterior  haemal  commissure.  (Figs.  62,  and  68,  J,  von  Rath's  preparations.)  The 
cells  and  fibers  of  this  group  have  not  been  identified  by  the  methylene  blue 
process. 

NERVE-ROOTS. 

The  Neural  Roots. — The  neural  or  branchial  nerve  arises  from  a  large 
ganglion  on  the  posterior  neuro-lateral  surface  of  the  neuromere,  and  extends 
upward  (neurally)  and  outward  to  the  gill.  In  cross  sections  (von  Rath's 
preparations)  near  the  neuromere,  it  consists  of  two  portions,  the  larger  one 
formed  of  a  coarse  polygonal  meshwork  of  neuroglia,  each  mesh  crowded  with 
black  dots,  representing  the  cut  ends  of  innumerable  nerve  fibers.  (Fig.  62,  s.t.) 
These  sensory  fibers  constitute  about  three-quarters  of  the  entire  nerve.  Most 
of  them  terminate  in  very  fine  dendrites,  in  the  large  oval  mass  of  neuropile  that 


BRANCHIAL    NERVE    ROOTS. 


77 


constitutes  the  posterior  lateral  portion  of  the  core  of  the  pedal  ganglion.  (Fig.  63). 
A  small  group  of  fibers  extends  beyond  the  main  core  into  the  median  neural 
region  of  the  neuromere.  (Figs.  60  and  61,  n.p.)  This  neuropile  center  is  very 
dense  and  stains  with  great  intensity  in  von  Rath's  preparations. 

The  haemal  fascicle  consists  of  small  nerve  tubes  with  sharply  defined  axis 
cylinders,  separated  by  a  wide,  clear  space  from  the  outer  sheath.  They  are 
motor  fibers  arising  from  the  peculiar  haemal  neuromeres,  H1'3.  (Fig.  62,  m.t.) 
They  supply  the  branchial  muscles. 


h.n. 


FIG.  63. — Anterior  branchial  neuromere  of  a  young  Limulus,  showing  the  course  of  the  principal  neurones. 

surface;  methylene-blue  preparation. 


Neural 


The  oval  mass  of  neuropile  in  the  ganglion  of  the  branchial  nerve  consists  of 
an  extraordinarily  complex  system  of  interwoven  bundles  of  very  fine  fibrils.  In 
this  neuropile  the  following  fibers  terminate:  i.  the  dendrites  of  about  three- 
quarters  of  all  the  fibers  of  the  branchial  nerve:  these  fibers  are  sensory.  2. 
the  collaterals  of  the  motor  neurones,  H1'3.  3.  the  collaterals  of  the  E  neurones 
whose  axones  cross  in  the  posterior  neural  commissure,  and  4.  the  dendrites  of  the 
minute  C  neurones  that  constitute  the  principal  cellular  covering  of  the  ganglion. 


78       MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

The  Haemal  Nerve  Roots. — The  haemal  nerves  arise  from  the  anterior  haemal 
margin  of  the  neuromere  and  extend  upward  and  outward.  In  the  more  pos- 
terior segments,  their  apparent  point  of  attachment  shifts  forwards  to  a  point 
midway  between  the  two  neuromeres,  but  without  changing  the  root  terminals. 
(Fig.  59.)  They  contain  the  following  fascicles: 

a.  A  bundle  of  large  motor  fibers  arising  from  the  D  neurones  on  the  neural 
surface  of  the  opposite  side,  in  the  next  anterior  neuromere.     (Figs.  61,62.   h.r5.) 
It  runs  along  the  haemo-lateral  margin  of  the  cord,  becoming  more  and  more  dis- 
tinct as  it  approaches  the  next  following  neuromere.     There  it  bends  outward 
onto  the  anterior  haemal  surface  of  the  haemal  nerve,  dividing  into  two  fascicles. 
One  forms  the  small,  purely  motor  nerve  supplying  the  haemo-neural  and  the 
longitudinal  abdominal  muscles,  Lab.',  the  other  passes  into  the  main  part  of  the 
nerve,  and  separates  farther  on,  as  the  branch  that  supplies  the  branchio-thoracic 
muscles.  (Fig.  59,  b.th.)  (hypoglossal  elements.) 

b.  A  large  fascicle  of  pale  fibers  (Figs.  61  and  62,  H.r'),  that  extends  along 
the  anterior  neural  margin  of  the  nerve.     On  entering  the  cord,  it  runs  diagonally 
forward,  inward,  and  upward,  terminating  in  an  elongated  mass  of  neuropile, 
on  the  median,  neural  surface  of  the  next  anterior  neuromere.     Its  peripheral 
termination  is  unknown.     Probably  cardiac. 

c.  A  large  central  fascicle,  H.r2,  terminates  in  a  conspicuous,  isolated  mass  of 
neuropile  of  the  same  side,  near  the  anterior  median  region  of  the  same  neuromere. 
Some  of  the  fibers  pass  through  the  neuropile,  neurad,  and  cephalad,  joining  the 
median,  longitudinal,  neural  tracts.     Probably  general  cutaneous  fibers. 

d.  This  fascicle,  H.r3,  is  not  easily  followed  in  sections,  but  its  fibers  are 
frequently  seen  in  methylene  blue  preparations.     They  spring  from  the  large 
neurones  A,  on  the  opposite  side  of  the  neuromere.     Their  collaterals  are  shown 
at  a7.   (Figs.  61,  62.) 

e.  This  fascicle  extends  backward  toward  the  neuropile  center  of  the  bran- 
chial nerve,  H.r*.    (Fig.  61.) 

/.  The  intestinal  fascicle  is  a  small  bundle  of  fine  fibers,  int.  In  the  anterior 
segments,  it  leaves  the  cord  with  the  haemal  nerve;  in  the  more  posterior  ones,  it 
arises  separately.  (Fig.  59,  J1'14.)  In  the  first  ganglion  (Fig.  61,  int.),  it  runs 
along  the  posterior  side  of  the  haemal  nerve  and  then  turns  sharply  forward  over 
the  neural  surface  of  the  sensory  root,  H.r1,  to  a  small,  ill  defined  group  of  cells 
lying  in  group  A. 

We  find,  therefore,  in  the  haemal  nerves,  the  following  roots  or  fascicles :  two 
sensory  roots  terminating  in  neuropile  on  the  neural  surface  of  the  cord,  on  the 
same  side,  one  in  the  same  neuromere,  H.r2,  the  other  in  the  one  next  in  front 
of  it,  H.r1.  (Fig.  60.)  Two  roots,  ending  in  cell  groups  on  the  opposite  side  of  the 
cord,  one  group,  D,  on  the  posterior  neural  side  of  the  next  anterior  neuromere,  the 
other,  on  the  anterior  neural  surface  of  the  same  neuromere,^.  A  fifth  root, 
H.r4,  extends  caudad,  disappearing  in  the  neuropile  near  the  base  of  the  neural 
nerve.  The  intestinal  branch  should  perhaps  be  counted  as  a  separate  nerve. 


COMMISSURES    OF    THE    BRANCHIAL    NEUROMERES. 


79 


It  is  only  in  the  second  neuromere  that  it  is  anatomically  a  branch  of  the  haemal 
nerve;  in  the  more  posterior  neuromeres  it  arises  from  the  side  of  the  neuromere 
and  entirely  separate  from  the  haemal  nerve  roots. 

Commissures. — The  transverse  commissures  of  the  cord  may  be  divided 
into  two  sets:  a.  the  primary,  or  haemal  commissures,  passing  underneath  the 
epithelium  of  the  embryonic  median  groove,  and  representing  the  primitive  nerve 
tracts  uniting  the  right  and  left  cords;  and  b.  the  secondary,  or  neural  commissures, 
crossing  the  neural  fissure  above  the  floor  of  the  median  groove.  The  neural 
commissures,  phyllogenetically  and  ontogenetically  form  much  later  than  the 
haemal,  and  only  after  the  median  groove  becomes  deep  enough  to  bring  the 
superior  median  margin  of  the  two  cords  into  contact. 

The  anterior  and  posterior  haemal  commissures  are  separated  by  an  opening 
in  the  floor  of  the  neural  canal  through  which  the  neuroglia  passes  to  the  underlying 
lemmatochord  (Figs.  64  and  68.) 

The  anterior  haemal  commissure  consists  of  several  indistinct  bundles.     So 


an.n.co.  m.n.co.    p.n.co. 


m.n.co.         p.n.co. 

F        .*~~-" 
-•••"  i 


n.tr. 


a.h.co. 


p.h.co. 


FIG.  64. — Sagittal  section  of  two  anterior  branchial  neuromeres,  showing  the  relation  of  the  principal  neurones 

and  fiber  tracts  to  the  cross  commissures. 

far  as  could  be  determined,  the  anterior  portion  consists  of  fibers  from  neurones 
I;  the  middle  portion  from  neurones  A;  and  the  posterior  portion  from  neurones 
B  (Fig.  64). 

The  posterior  haemal  commissure  contains  fibers  from  group  J  (anterior 
bundle)  and  from  group  D  (posterior  bundle). 

The  haemal  commissures  therefore  contain,  among  fibers  of  undetermined 
character,  crossed  motor  axones  and  various  collaterals. 

The  neural  commissures  are  three  in  number;  an  anterior,  a  middle  and 
a  posterior  one.  (Fig.  64.)  The  sources  of  the  fibers  in  the  anterior  neural 
commissure  could  not  be  certainly  determined.  Those  of  the  middle  commissure 
m.n.co.,  are  derived  from  neurones,  F,  and  those  of  the  posterior  commissure  from 
neurones  E.  No  other  fibers  could  be  located  in  these  commissures.  The  neural 
commissures  appear  to  be  largely  composed  of  association  fibers. 

The  Neuropile  Centers.— The  neuropile  centers  are  dense  masses  of  inter- 
woven terminal  dendrites.  They  appear  in  von  Rath's  preparations  as  dense 
black  masses  of  fine  fibrils,  and  in  methylene  blue,  as  masses  of  fine  blue  dots  or 
lines,  according  to  the  character  of  the  stain. 


80       MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

The  principal  centers  are  :  a.  a  large  oval  center  at  the  root  of  the  pedal  nerve, 
forming  the  medullary  core  to  the  pedal  ganglion.  (Fig.  60.)  A  prolongation  of 
it  extends  cephalad  and  mesad,  forming  a  compact,  oblong  mass,  near  the  center 
of  the  neural  surface  of  the  neuromeres;  n.p.  b.  A  center  for  the  cephalic  root 
of  the  haemal  nerve,  Hr',  extending  along  the  median  neural  surface  of  each 
ganglion,  c.  A  center  for  the  middle  root  of  the  haemal  nerve,  Hf 2,  on  the  ante- 
rior median  face  of  the  neuromere.  d.  The  haemal  tracts;  large,  spindle-shaped 
tracts,  one  on  either  side  of  the  median  line,  on  the  haemal  surface.  (Figs.  62,  67, 
68,  Lh.tr.)  (e)  Four  small  masses,  on  the  haemal  surface,  between  the  anterior 
and  posterior  haemal  commissures  and  the  longitudinal  haemal  tracts.  For  longi- 
tudinal tracts  in  sections,  see  Figs.  67  and  68. 

II.  THE  CEPHALIC  NEUROMERES. 

We  are  now  in  a  position  to  describe  the  arrangement  of  cells  and  fibers  in 
the  cephalic  neuromeres. 

The  brain  neuromeres,  in  the  main,  closely  resemble  those  of  the  cord. 
The  principal  differences  in  form  are  due  to  their  linear  union  and  to  the  lateral 
divergence  of  the  crura.  The  histological  differences  are  due  mainly  to  the 
absence  of  motor  neurones  such  as  the  hypobranchial,  intestinal,  and  cardiacs,  to 
the  greater  size  and  isolation  of  the  pedal  ganglia,  and  to  the  presence  of  the 
large  gustatory  nerves. 

Cell  Clusters. — The  nerve  cells  are  arranged  in  clusters,  of  varying  sizes, 
that  have  special  neurilemma  sheaths,  as  in  the  cord,  but  they  are  so  crowded 
together  that  it  is  difficult  to  determine  the  exact  arrangement.  They  cover  the 
neural  surface  and  lateral  margin  of  the  crura,  leaving  the  commissures,  part  of 
the  gustatory  tracts,  and  the  haemal  surface  exposed.  (Figs.  65  and  66.) 

The  Commissures. — Each  neuromere  has  several  bundles  of  cross  commis- 
sures that  have  terminal  relations  similar  to  those  described  for  the  branchial  neu- 
romeres. Owing  to  the  divergence  of  the  crura  they  form  long,  backwardly  directed 
loops  in  which  the  commissural  fascicles  are  difficult  to  identify,  except  where 
they  approach  the  crura. 

In  very  young  crabs,  sagittal  sections  show  that  the  commissures  of  each 
neuromere  are  surrounded  by  distinct  membranes.  There  are  two  groups  of 
commissures  for  each  neuromere,  corresponding  to  the  neural  and  haemal  commis- 
sures of  the  cord,  and  no  doubt  containing  similar  components.  In  the  adult, 
the  median  portion  of  the  anterior  thoracic  commissures  form  compact  bundles 
with  a  common  neurilemma  sheath;  near  the  crus,  the  several  fascicles  separate 
to  their  respective  terminals.  (Fig.  56.)  The  more  posterior  thoracic  commis- 
sures, and  those  in  the  hindbrain,  are  shorter,  and  the  neural  and  haemal  fascicles 
are  widely  separated,  leaving  a  space  between  them,  which  represents  the  begin- 
ning of  the  fourth  ventricle.  (Figs.  46,  47  and  55.) 

The  neural  commissures. — I  have  not  been  able  to  work  out  the  relation 


THE    CEPHALIC    NEUROMERES.  8 1 

of  all  the  commissural  elements  in  detail,  although  some  of  them  stand  out  very 
clearly.  For  example,  in  methylene  blue,  the  posterior  neural  commissures  are 
often  very  conspicuous.  They  extend  diagonally  forward  (anterior  ones)  and 
outward  on  to  the  neural  surface  of  the  crura,  over  the  great  gustatory  tracts,  to 
the  cell  clusters  E,  on  the  posterior  haemo-lateral  margin  of  its  neuromere.  (Fig. 
65,  E,  p.n.co.) 

A  bundle  of  these  fibers,  on  the  anterior  side  of  the  second  thoracic  neuro- 
mere, indicates  that  the  cheliceral  neuromere,  in  spite  of  its  distinctly  pre-oral 
position,  has  its  commissure  behind  the  oesophagus.  In  most  cases,  one  may 
recognize  two  sets  of  these  neural  fibers  to  each  neuromere,  an  anterior  one, 
arising  from  the  neurones  £,  and  a  posterior  fascicle,  ending  in  a  separate  mass  of 
neuropile  on  the  lateral  half  of  each  crus. 

The  haemal  commissures  contain  several  sets  of  fibers.  The  ones  most 
clearly  seen  in  methylene  blue  preparations  are  rather  large  fibers  which  arise  from 
neurones  A  (Fig.  66)  and  after  crossing,  enter  the  roots  of  the  haemal  nerves. 

Between  the  nerve  roots  and  the  commissure,  are  two  sets  of  longitudinal 
fibers  extending  outward  and  backward  along  the  haemal  surface  of  each  crus, 
one  on  the  median  side,  one  on  the  lateral.  (Fig.  66,  left.)  Most  of  the  longi- 
tudinal fibers  come  from  the  opposite  side  through  the  haemal  commissure;  some 
of  the  lateral  ones  probably  come  from  neurites,  B,  on  the  neural  surface  of  the 
same  side,  corresponding  with  group  B  of  the  cord. 

The  Haemal  Nerve  Roots. — It  will  be  recalled  that  the  cranial  haemal 
nerves  supply  the  integument  of  the  cephalothorax.  The  branches,  which 
in  the  abdominal  nerves  run  to  the  great  longitudinal  muscles,  to  the  branchio- 
thoracic  muscles,  to  the  heart  and  to  the  intestines,  are  absent  from  the  six  pairs 
of  thoracic,  haemal  nerves.  Hence  they  have  but  a  single  root,  mainly,  if  not  wholly 
sensory,  and  representing  root  two,  H.r2,  of  the  branchial  neuromeres. 

In  most  preparations,  the  haemal  roots  appear  to  extend  only  part  way  through 
the  crus,  terminating  abruptly  in  the  main  longitudinal  tracts  on  the  median  side 
(Fig.  66,  right  side).  They  appear  to  end  there  in  a  mass  of  neuropile,  like  that 
of  the  second  root  of  the  abdominal  nerves,  H.r2.  In  other  preparations,  many 
fibers  are  seen  to  enter  the  haemal  commissure  and  terminate  in  the  A  neurones 
of  the  opposite  side,  which  no  doubt  correspond  to  the  A  neurones  of  the  abdomen. 

No  trace  of  any  other  roots  could  be  found.  From  this  observation  we  may 
infer  that  roots  two  and  three,  H.r2,  and  H.r3,  of  the  abdominal  haemal  nerves 
are  sensory  general  cutaneous;  that  root  one,  H.r1,  contains  the  cardiac  elements; 
and  that  the  neurones,  D,  are  distributed  to  the  branchio-thoracic  muscles. 

The  Neural  Nerve  Roots  and  the  Cranial  Ganglia. — Owing  to  the  greater 
size  and  specialization  of  their  terminal  organs,  the  neural,  or  pedal  nerves  of  the 
head  are  much  larger  and  more  complex  than  those  of  the  branchial  region,  but 
in  the  minute  structure  of  their  ganglia  and  nerve  roots  they  are  much  alike. 

Ganglia. — In  the  adult  Limulus  (Fig.  218),  each  cranial  ganglion  forms  a 
large  oval  mass  of  neuropile,  projecting  a  considerable  distance  from  the  sides 


82       MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

r—  ol.m.n. 


FIG.  65. — Brain  of  young  Limulus  about  three  inches  long,  seen  from  the  neural  surface.  The  cerebral  hemi- 
sphere, on  the  left,  is  shown  in  optical  section,  at  the  level  of  the  giant  association  neurones,  H.  as,  and  on  the  right, 
at  a  deeper  level,  showing  the  principal  cerebral  lobes  and  the  gustatory  tracts,  G.  ctr3.  and  g.  tr.-  Compare  with 
Fig.  48.  On  the  right,  behind  the  hemispheres,  the  superficial  arrangement  of  the  clusters  of  nerve  cells,  and 
the  cranial  nerves  are  shown.  On  the  left  are  seen  the  gustatory  nerve  roots,  the  great  longitudinal  gustatory 
tracts,  the  principal  neurones  and  their  relation  to  the  neural  commissures,  and  the  principal  neuropile  masses 
in  the  pedal  and  stomodaeal  ganglia;  methylene-blue  preparation. 


THE    CEPHALIC    NEUROMERES. 


op.tr. .— 


h.ri 


p.n 


FIG.  66. — Same  from  the  hasmal  surface,  showing  on  the  left  the  more  superficial  neurones,  fiber  tracts, 
nerve  roots,  and  commissural  fibers;  on  the  right,  the  deeper  ones.  Note  the  great  extension  backward  of  the 
fibers  arising  from  the  optic  nerve,  op.  tt,  and  from  the  giant  nerve  cells  of  the  optic  ganglia,  op.  g4,  and  olfactory 
lobes,  ol.  cl  and  ol.  c3. 


84       MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

of  the  brain.     In  the  young  (Figs.  35,  36,  38,  39),  they  are  separated  from  the 
brain  by  long  narrow  stalks,  or  tracts,  devoid  of  neurones  or  neuropile. 

Most  of  the  peripheral  fibers  terminate  in  the  neuropile  core,  forming  many 
minute  irregular  centers  (Fig.  65).  There  is  a  special  neuropile  center  on  the 
anterior  neuro-median  side  of  each  ganglion,  formed  by  the  terminals  of  a  small 
bundle  of  rather  distinct  fibers.  This  center  in  some  cases  appears  like  a  dark 
granular  blotch,  in  others  like  a  beautifully  distinct,  anastomosing  network  (Fig. 

65,  tt>).     Their  peripheral  relations  were  not  determined. 

The  nerve  cells  of  the  ganglia  are  very  small  and  numerous.  They  are  con- 
fined in  the  main  to  the  haemal  surface,  sending  short,  vertical  axones  into  the 
neuropile,  where  they  divide,  one  branch  passing  outward  into  the  nerve, .the 
other  forming  branches  in  the  neuropile,  which  extend  toward  the  crus.  (Fig. 

66,  C). 

The  Motor  Neurones. — On  the  haemal  surface  of  the  ganglia,  there  are  two 
very  conspicuous  bundles  of  coarse  fibers,  coming  from  large  neurones  on  either 
side  of  the  ganglion.  A  third  bundle  joins  them,  coming  from  cells  on  the  neural 
side  of  the  ganglion.  They  form  the  small  anterior  and  posterior  ento-coxal 
nerves  supply  that  the  coxal  muscles.  (Fig.  66,  a.en.cx  and  p.en.cx.) 

These  neurones  agree  with  the  H  neurones  of  the  branchial  ganglia,  in  that 
each  one  sends  a  considerable  number  of  axones  into  the  nerve  trunk. 

The  Gustatory  Nerves  and  Tracts. — The  most  striking  features  of  the  mid- 
brain  neuromeres  are  the  large  nerve  roots  supplying  the  taste  organs  in  the  coxal 
spurs  (Fig.  65,  g.n.r.2~6).  They  extend  along  the  neural  face  of  the  pedal  ganglia, 
sweeping  diagonally  inward  and  forward,  the  inner  end  of  each  root  overlapping 
the  next  posterior  one.  The  united  bundles  form  an  immense  longitudinal  tract 
along  the  median  neural  margin  of  each  crus,  between  the  neural  and  haemal 
commissures. 

The  more  posterior  roots  are  the  largest,  that  of  the  sixth  appendage  largest 
of  all.  The  deep,  inner  ends  of  the  fifth  and  sixth  roots  form  large  oval  neuropile 
enlargements  on  the  posterior  median  face  of  each  crus. 

Toward  the  anterior  end,  one  may  recognize  two  subdivisions  to  the  main 
tract.  Near  the  stomodaeal  ganglion  both  divisions  turn  outward  and  downward, 
then  upward,  forming  a  sharp  semi-circular  turn  round  the  crus,  giving  the  latter, 
in  cross-sections,  a  very  unusual  spiral  structure.  The  larger  bundle  apparently 
terminates  in  the  great  cheliceral  lobe  (Figs.  65  and  1 14,  ch.l) ;  the  smaller  one  g.tr 
forms  a  slight  dilatation,  consisting  of  very  dense  nodular  neuropile,  on  the  lateral 
margin  of  this  lobe,  and  then  passes  straight  forward,  along  the  neural  surface 
of  the  cheliceral  neuromere  to  the  median  cerebral  lobe,  G.ctr*. 

The  very  large  fasciculus  of  the  sixth  nerve  comes,  not  from  the  coxal  spurs, 
which  here  form  crushing  mandibles  devoid  of  gustatory  spines,  but  from  the 
large  spatulate  organ  on  the  outer  margin  of  the  coxa  (flabellum),  the  function  of 
which  is  to  test  the  composition  of  the  water  passing  to  the  gill  chamber. 


THE    CEPHALIC    NEUROMERES.  85 

A  small,  posterior  fasciculus,  coming  from  the  chilaria,  joins  the  main  gusta- 
tory tract.  (Fig.  65,  chela. n.) 

In  methylene  blue,  the  gustatory  tracts  have  a  very  characteristic  appearance, 
as  each  fascicle  contains  a  great  many  parallel  bundles  of  extremely  minute  fibers 
that  look  like  rows  of  dots. 

In  sections  (von  Rath's  method),  they  may  be  recognized  by  their  dense  black 
masses  of  fine  parallel  fibers  (except  in  the  nodes,  where  they  are  twisted  and  inter- 
woven) and  by  the  small  quantity  of  neuroglia  contained  in  them.  (Fig.  56, 
g.n.r^.) 

In  the  scorpion,  a  similar  tract  may  be  recognized.  (Fig.  69.)  But  here  the 
most  conspicuous  portion  is  the  immense  neuropile  bodies  on  the  roots  of  the  first 
three  vagus  nerves.  These  nerves  supply  the  genital  papillae  and  the  pectines, 
and  the  immense  size  of  these  vagal  lobes  is  due  to  the  great  development  of 
sensory  (tactile)  organs  in  the  pectines. 

Similar  lobes  are  seen  in  Limulus,  but  lying  farther  forward,  and  associated 
with  the  immense,  flabellar  nerve  (gustatory)  belonging  to  the  sixth  pair  of  legs. 

III.  LONGITUDINAL  TRACTS. 

There  are  several  well  defined  longitudinal  tracts  in  Limulus  that  may  be 
traced  the  entire  length  of  the  brain  and  cord,  but  their  relations  to  the  various 
centers  and  to  the  motor  and  sensory  terminals  is  exceedingly  difficult  to  deter- 
mine. In  the  main,  the  sensory  elements  run  on  the  neural  surface  of  the  cords, 
and  the  motor  ones  on  the  haemal  surface. 

We  may  distinguish  the  following  tracts: 

The  Longitudinal  Haemal  Tracts  of  the  Brain  and  Cord.— The  haemal 
tracts  are  great  sheets  of  longitudinal  fibers  covering  the  haemal  surface  of  the  brain 
and  cord.  They  can  be  seen  in  von  Rath's  preparations,  along  the  haemal  surface 
of  the  neuromeres,  haemal  to  the  transverse  commissures  (Figs.  55,  56,  67,  68,  l.h. 
tr).  They  leave  the  anterior  and  the  posterior  ends  of  the  neuromere  in  the  nearly 
isolated  haemal  sections  of  the  longitudinal  connectives  (Fig.  64).  Midway 
between  the  ganglia  they  cannot  be  distinguished  from  the  other  fibers  of  the  cord. 

In  methylene  blue  preparations,  these  fibers  of  the  cord  may  be  followed  at  least 
from  one  ganglion  through  the  next  without  branching,  and  in  some  cases  through 
several  ganglia.  In  the  brain,  individual  fibers  may  be  followed  the  whole  length 
of  the  crus.  (Fig.  66.) 

In  the  brain  many  of  these  fibers  terminate  in  small  clusters  of  dendrites, 
scattered  over  the  crura,  just  below  the  haemal  surface  (Fig.  66,  left  side) ;  in  the 
branchial  neuromeres  they  are  seen  on  the  haemo-lateral  surface  just  neurad  of  the 
longitudinal  fibers.  (Fig.  62,  right  side.) 

The  fibers  of  the  haemal  tracts  are  derived  from  several  sources.  One  impor- 
tant source  is  the  large  cluster  of  B  neurones  on  the  neural  surface  of  each  bran- 
chial neuromere.  Their  fibers,  after  reaching  the  haemal  surface,  divide,  one  cross- 


86       MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

ing  to  the  opposite  side  in  the  anterior  haemal  commissures,  the  other  turning 
backward  into  the  haemal  tracts.     (Figs.  61  and  62.) 

In  the  cerebral  neuromeres,  the  haemal  tracts  contain  similar  fibers,  derived 
from  similar  cells.  The  latter  may  be  seen  in  small  clusters  between  the  roots 
of  the  ganglia,  on  the  neural  surface  of  the  crura.  Their  fibers  pass  vertically 
through  the  crus,  joining  the  longitudinal  tracts  and  the  cross  commissures.  (Figs. 
56,  65,  66,  B,  b,  and  b.') 

The  lateral  margin  of  the  haemal  tracts  of  the  crura  receives  conspicuous 
fibers  that  cross  in  the  haemal  commissures  with  the  roots  of  the  haemal  nerve. 
Near  the  lateral  margin  of  the  crura  they  turn  backward  and  join  the  lateral 
margin  of  the  haemal  tracts  (Fig.  66,  left  side).  A  similar,  but  less  conspicuous,  set 
of  fibers  forms  on  the  median  side  of  the  tract. 

The  lateral  margin  of  the  haemal  tracts  also  receives  a  considerable  number  of 
large  fibers  from  the  third  and  fourth  lobes  of  the  optic  ganglion,  op.g3'4,  and 
from  the  crossed  and  uncrossed  fibers  of  the  large  lateral  neurones  of  the  olfactory 
lobes,  olx1 '. 

The  median  margin  receives  fibers  from  the  large  central  cells  of  the  olfactory 
lobes,  0/.c.3,  and  from  the  giant  association  neurones  in  the  median  lobe  of  the 
hemispheres.  (Fig.  49,  H .  as.  tr.) 

A  remarkable  band  of  fine  fibers  comes  from  the  optic  nerve,  passing  through, 
or  over,  the  lateral  margin  of  the  first  two  optic  lobes,  along  the  lateral  margin  of 
the  optic  stalk,  through  the  tween-brain,  and  along  the  median  haemal  surface  of 
the  crura  to  the  beginning  of  the  cord.  (Fig.  66,  op.tr.)  A  similar  band  of  fine 
fibers  extends  along  the  entire  lateral  margin  of  each  haemal  tract  of  the  cord. 
The  two  bands  are  united  by  a  narrow  commissure  extending  across  the  anterior 
margin  of  each  neuromere.  (Fig.  62,  op.tr.  ?)  It  is  not  clear  whether  these  fine 
fibered  bands  of  the  cord  are  continuations  of  the  optic  bands  in  the  brain  or  not. 

The  great  majority  of  the  longitudinal  fibers  of  the  crura  that  are  directed 
forward  appear  to  terminate  on  the  haemal  surface  of  the  forebrain  commissures. 

The  Longitudinal  Neural  Tracts.— These  tracts  lie  close  to  the  median 
line,  on  the  neural  surface  of  the  cord.  They  receive  all  the  fibers  of  the  first 
root  of  the  haemal  nerves,  many  fibers  from  the  nucleus  at  the  root  of  the  pedal 
ganglia,  and  large  unbranched  fibers  whose  origin  is  unknown,  that  pass  through 
the  cord  over  several  neuromeres. 

In  the  crura  the  neural  roots  of  the  haemal  nerves  appear  to  be  absent,  and 
the  other  constituents  of  the  neural  tracts  could  not  be  certainly  identified. 
But  we  may  recognize  the  following  tracts  which  may  or  may  not  be  modifications 
of  those  already  described. 

The  lateral  or  pedal  ganglion  tracts  are  large  and  exceedingly  complex, 
consisting  of  a  confused  mass  of  interlacing  fiber  bundles  which  form  the  lateral 
margins  of  the  crura;  most  of  their  fibers  come  in  roughly  parallel  bundles  from 
the  roots  of  the  pedal  ganglia  (Fig.  56,  l,tr.,  right  side).  On  reaching  the  tract, 
the  fiber  bundles  take  on  a  longitudinal  trend.  The  tracts  are  greatly  enlarged 


THE    NERVE    FIBER    TRACTS.  87 

opposite  the  pedal  ganglia,  and  between  them  they  are  reduced  to  narrow  bands. 
(Fig.  56.  l.tr,  left  side).  At  the  anterior  end  of  the  crura,  they  appear  to  pass 
outside  (neural)  the  gustatory  tracts,  onto  the  posterior  neural  surface  of  the 
tween-brain  commissure.  (Fig.  49,  l.tr.) 

The  general  cutaneous  tracts  are  two  large,  continuous  columns  of 
neuropile  on  the  median  side  of  each  crus.  They  lie  between  the  gustatory  and 
haemal  tracts,  and  extend  from  the  tween-brain  to  the  last  vagus  neuromere, 
(Fig.  56  G.c.tr.) 

They  usually  have  a  slightly  different  color  and  appearance  from  the  lateral 
ones,  from  which  they  are  separated  by  numerous  bundles  of  vertical  fibers.  The 
latter  are  arranged  with  considerable  regularity,  the  more  important  sources  being 
neurones  B,  sending  axones  haemally,  and  neurones  E,  sending  them  neurally. 
(Figs.  56  and  65,  B  and  E.)  The  principal  constituents  of  the  tracts  are  the 
crossed  and  uncrossed  terminals  of  the  cutaneous  nerves,  h.n.  The  majority 
of  the  uncrossed  fibers  and  the  optic  fascicles,  op.tr,  extend  lengthwise  of  the 
tracts. 

Comparison  of  the  Fiber  Tracts  of  the  Arachnid  and  Vertebrate  Brain.— 
A  comparison  of  the  fiber  tracts  in  Limulus  with  those  in  the  vertebrates  presents 
great  difficulties.  These  difficulties  are  partly  due  to  our  imperfect  knowledge 
of  the  brain  of  arachnids  and  of  the  lower  vertebrates,  and  partly  to  the  fact  that 
the  latter  is  generally  studied  for  the  purpose  of  explaining  the  structure  of  the 
higher  types  of  brain,  not  for  comparison  with  an  invertebrate  brain,  that  its 
own  structure  might  be  better  understood.  When  we  know  more  about  the 
brain  of  arthropods,  and  less  emphasis  is  laid  on  the  artificial  system  of  classify- 
ing cranial  nerves,  now  in  vogue  among  American  neurologists,  many  points  are 
likely  to  be  cleared  up  which  are  now  obscure. 

Nevertheless  the  facts,  so  far  as  we  understand  them,  indicate  that  the  arachnid 
and  vertebrate  brain  are  in  essential  agreement  in  the  distribution  and  relations 
of  their  main  fiber  tracts.  The  agreements  to  which  we  would  call  attention  are : 

a.  In  both  classes,  important  longitudinal  tracts  containing  the  principal 
motor  fibers  extend  along  the  haemal  surface  of  the  brain  and  cord. 

b.  In  both  classes,  conspicuous  sensory  tracts  lie  near  the  median  neural 
surface,  coming  from  segmentally  arranged  taste  organs  and  from  other  sense 
organs,  i.e.,  tactile,  temperature,  or  auditory,  having  a  less  precisely  determined 
function. 

c.  In  these  sensory  columns,  there  are  local  enlargements,  or  lobes,  corre- 
sponding with  special  local  functions;  i.e.,  in  Limulus,  the  flabellar  lobes  of  the 
eighth  and  ninth  neuromeres;  in  the  scorpion,  the  pectinal  lobes  of  the  tenth, 
eleventh,  and  twelfth  vagus  neuromeres;  'in  vertebrates,  the  auditory  and  the 
vagal  lobes  of  their  respective  neuromeres. 

d.  In  both  classes,  the  numerous  gustatory  fascicles  and  those  from  the 
vagus  neuromeres  form  a  very  conspicuous  median  neural  tract,  extending  the 
whole  length  of  the  brain  floor.     It  terminates  in  a  special  center,  in  the  dienceph- 


88       MIUNTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 


FIG.    68. 

FIGS.  67  and  68. — A  series  of  sections  of  the  first  branchial  neuromere  of  an  adult  Limulus,  showing  the  location 
of  the  principal  cell  clusters,  commissures,  fiber  tracts,  lemmatochord,  and  central  canal.  The  numbers  66  to  138 
indicate  the  serial  numbers  of  the  sections. 


THE    NERVE    FIBER    TRACTS.  89 

alon,  which  is  in  turn  connected  by  special  tracts  with  the  olfactory  lobes,  hem- 
ispheres, and  cerebellum. 

e.  In  both  classes,  the  nerve  roots  are  arranged  in  two  distinct  series,  neural 
and  haemal;  each  series  may  contain  both  motor  and  sensory  elements. 

In  Limulus,  the  haemal  roots  enter  the  brain  toward  the  haemal  surface  and 
extend  horizontally,  through  the  crus  and  the  haemal  commissures,  to  the  main 
nucleus  or  cell  cluster  on  the  other  side  of  the  median  line.  But  many  fibers 
end  in  dendrites  on  the  same  side  the  nerve  enters. 

In  vertebrates,  a  similar  condition  prevails  in  the  ventral  or  haemal  nerves, 
for  according  to  Johnston  "  It  is  a  noticeable  peculiarity  in  the  origin  of  the  nerve 
(i.e.,  the  third  nerve  of  vertebrates)  that  a  large  part  of  the  fibers  arise  from  the 


FIG.  69. — Four  cross-sections  of  the  brain  in  the  vagus  region  of  an  adult  scorpion,  showing  the  enormous  vagal 
lobes,  the  central  canal,  and  the  cephalic  portion  of  the  middle  cord,  or  lemmatochord. 

nucleus  of  one  side,  and  cross  to  enter  the  root  of  the  opposite  side.  The  same 
arrangement  is  found  in  the  roots  of  other  ventral  nerves,  but  to  a  much  less 
degree." 

On  the  other  hand,  the  neural  nerves  are  associated  with  special  ganglia,  which 
arise  independently  of  the  brain,  and  which  are  attached  to  its  neuro-lateral 
margin.  In  Limulus,  these  fibers  end,  or  originate,  on  the  same  side  the  nerve 
enters  the  brain;  very  few,  if  any,  fibers  of  a  neural  nerve  arise  from  cells  located 
on  the  opposite  side  of  the  median  line. 

/.  In  both  classes,  the  floor  of  the  brain  is  divided  lengthwise  into  two  main 
columns,  a  median  and  a  lateral  one,  by  an  important  series  of  vertical  fibers; 
arcuate  fibers,  vertebrates,  B  and  £,  fibers,  arachnids.  The  prolongations  of 
these  fibers  run  lengthwise  in  the  haemal  (and  neural  ?)  tracts  and  crosswise  in 
the  commissures. 

g.  In  Limulus,  there  is  in  the  vagus  region  an  important  decussation  of  im- 
pulses coming  from  the  trunk  (See  section  on  Physiology,  p.  191) ;  and  there  is  also 


90       MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

there  a  special  condensation  of  commissural  bundles,  that  have  been  crowded 
together  from  before  backward,  in  order  to  leave  a  large  opening  for  the  end  of 
the  oesophagus.  (Figs.  57  and  58.)  In  the  vertebrates,  there  is  a  similar  back- 
ward dislocation  of  commissural  bundles,  forming  a  special  group  just  back  of  the 
choroid plexus  of  the  fourth  ventricle, "commissurainfirma."  Johnston  says  (p.  287) 
that  "Behind  the  choroid  plexus  the  c.  infirma  contains  the  visceral,  sensory 
elements  proper  to  the  segments  of  the  VII,  IX  and  X  nerves.  It  is  probable  that  the 
course  of  the  root  fibers  of  these  nerves  within  the  brain  has  been  influenced  by  the 
crowding  backward  of  their  decussation  and  median  nucleus  by  the  choroid 
plexus."  The  choroid  plexus  could  hardly  have  the  power  to  dislocate  the 
cerebral  framework.  The  dislocation  was  probably  brought  about,  as  in  Limulus, 
by  the  backward  migration  of  the  outer  end  of  the  old  stomodaeum,  and  when 
that  closed,  the  choroid  plexus  grew  over  it,  leaving  the  permanently  distorted 
commissures  to  testify  to  the  event. 

h.  In  both  classes,  there  is  a  remarkable  ganglionated  commissure  extending 
over  the  neural  surface  of  the  brain,  the  stomodaeal  commissure  of  arthropods 
and  the  cerebellum  of  vertebrates.  Both  structures  represent  very  primitive 
commissural  tracts,  the  only  ones  which  develop,  primarily,  from  the  roof  of  the 
brain  chamber.  Both  commissures  may  be  ganglionated,  and  they  are  the  only 
ganglionated  transverse  commissures  in  the  primitive  brain.  Both  commissures 
develop  in  connection  with  the  fourth  neuromere.  Both  have  special  relations 
with  the  gustatory  centers  on  the  posterior  median  face  of  the  stomodaeal  opening 
(infundibulum). 

In  arachnids,  this  commissural  arch  lies  behind  the  cerebral  lobes  and  the 
optic  ganglia.  In  vertebrates,  it  is  crowded  backward  by  the  enlarged  optic 
lobes  so  that  part  of  its  fibers  are  directed  downward  and  forward,  toward  the 
posterior  wall  of  the  diencephalon.  (Figs.  3,  46,  57,  58.) 

IV.  COMMISSURES. 

Summary. — The  facts,  bearing  on  the  cross  commissures  of  arachnids, 
that  have  been  brought  out  in  the  preceding  pages,  may  be  summarized  as  follows: 

i.  The  right  and  left  cords  of  the  primitive  neuron  were  united  by  a  series 
of  transverse  commissures,  two  for  each  neuromere.  The  anterior  commissure 
is  primarily  related  to  the  anterior  segment  of  the  neuromere  and  to  its  peripheral 
nerve,  the  other  to  the  posterior  segment,  and  to  its  nerve.  The  commissures 
arise,  during  the  early  embryonic  periods,  as  fibrous,  non-cellular  bands,  extending 
across  the  floor  of  the  middle  groove.  They  are  separated  from  one  another  by 
deeper  infoldings  of  the  groove.  In  the  adult,  they  are  still  separate,  and  each 
contains  several  distinct  fiber  bundles,  differing  in  origin  and  in  histological 
characters. 

These  commissures  always  retain  their  primitive  position  on  the  floor  of  the 
neuron,  hence  they  are  called  the  haemal  commissures.  They  are  the  primary 


THE    COMMISSURES.  91 

communicating  paths  between  the  right  and  left  cords,  and  between  the  right  and 
left  peripheral  nerves. 

During  the  later  stages,  the  cords  increase  in  thickness,  the  median  in- 
folding forms  a  deep  fissure,  and  then  three  new  commissures  appear  in  each 
neuromere,  extending  across  the  fissure  above  the  old  ones.  These  secondary,  or 
neural  commissures,  consist  largely  of  association  fibers. 

With  the  formation  of  the  secondary  commissures  the  bottom  of  the  fissure, 
at  certain  points  in  each  neuromere,  is  converted  into  a  canal,  "  canalis  centralis" 
(Figs.  55,  56,  68,  69,  c.c.),  that  is  lined  in  the  earlier  stages  with  an  epithelium 
derived  from  a  part  of  the  original  median  infolding.  (Figs.  222,  224  and  231.) 
The  floor  of  the  canal  is  formed  in  part  by  the  haemal  commissures,  and  the  roof 
is  formed  in  part  by  the  neural  commissures.  When  the  neuromeres  are  widely 
separated,  there  are  of  course  wide  gaps  in  the  roof  and  floor  of  the  canal  between 
the  commissures  in  front  and  those  behind.  In  the  vagus  region,  owing  partly 
to  the  increased  thickness  of  the  crura,  the  canal  is  greatly  enlarged,  marking  the 
beginning  of  a  chamber  comparable  with  the  fourth  ventricle  or  metenccele. 
(Figs.  55  and  56).  Here  the  neuromeres  are  more  closely  united  than  elsewhere, 
and  their  neural  commissures,  together  with  some  of  those  belonging  to  the  more 
anterior  neuromeres  that  have  been  crowded  backward  into  this  territory  by  the 
oesophagus,  form  a  special  group  over  the  posterior  part  of  the  region  of  the 
fourth  ventricle.  (Fig.  47,  C.) 

In  the  scorpion  also,  the  immense  vagal  lobes  are  united  by  two  special  neural 
commissures.  (Fig.  47,  A  and  69.) 

The  combined  neural  commissures  of  the  vagal  neuromeres  of  the  arachnids, 
and  the  more  posterior  thoracic  ones,  consist  largely  of  somatic  sensory  associa- 
tion fibers;  they  probably  represent  the  beginning  of  the  commissura  infima  of 
vertebrates. 


In  the  arachnids  there  is  a  wide  gap  between  the  forebrain  and  midbrain, 
where  there  are  no  primitive  commissures.  This  opening,  or  infundibulum,  is  the 
passageway  for  the  old  oesophagus.  In  front  of  it,  the  character  of  the  commis- 
sures changes  greatly,  in  both  vertebrates  and  arachnids.  There  are  no  neural 
commissures,  and  the  haemal  ones  form  practically  a  single,  but  very  complex 
mass  of  fibers.  (Fig.  48.)  In  it  we  may  recognize  the  olfactory  commissure, 
representing  the  commissure  of  the  first  cerebral  neuromere,  and  lying,  morphologi- 
cally, at  the  anterior  end  of  the  nervous  system.  Owing,  however,  to  the  in- 
ward and  backward  migration  of  the  lobes,  it  lies,  in  the  adult  Limulus  and 
scorpion,  on  the  posterior  haemal  surface  of  the  forebrain.  Fig.  47  A  and  B. 
The  forebrain  commissure  also  contains  the  commissures  of  the  second  and  third 
neuromeres,  and  of  the  lateral  eyes;  but  these  commissures,  which  arise  at  an 
extremely  early  period,  are  not  separated  into  distinct  bundles.  (Fig.  48.) 

In  Apus,  the  ganglia  of  the  median  ocelli  unite  with  each  other  in  the  middle 


92       MINUTE  STRUCTURE  OF  THE  BRAIN  AND  CORD  OF  ARACHNIDS. 

line,  over  the  neural  surface  of  the  brain;  they  rest  on  the  latter  by  two  short 
stalks.  In  many  other  phyllopods  and  arachnids,  the  optic  ganglia  occupy  a 
similar  position,  in  that  they  lie  close  together,  over  the  neural  surface  of  the 
brain,  behind  the  hemispheres.  In  the  vertebrates,  they  permanently  occupy 
this  position,  and  have  become  united  by  secondary  commissures,  one  in  the 
habenulae,  the  other  in  the  optic  lobes.  (Fig.  308.) 

The  most  conspicuous  commissure  in  the  arthropods  and  one  of  the  first  to 
appear,  is  that  belonging  to  the  system  of  stomodaeal  nerves.  Its  ganglia,  one 
median  and  two  lateral,  arise  from  the  walls  of  the  stomodaeum.  It  is  the  only 
commissure  originally  provided  with  ganglion  cells,  and  the  only  one  formed 
primarily  across  the  neural  surface  of  the  brain.  It  has  especial  relations  to 
nutrition,  through  its  association  with  the  olfactory,  swallowing,  and  taste  centers. 
It  represents  the  primitive  cerebellar  commissure  of  vertebrates,  where  it  appears 
to  have  had  similar  relations. 

V.  THE  NEUROCCELIA. 

Summary. — The  transformation  of  the  paired  nerve  cords  of  invertebrates 
into  the  hollow  nerve  tube  of  vertebrates  is  affected  by  several  independent  factors. 
These  factors  make  their  appearance  as  active  forces  in  the  arachnids,  and  they 
have  already  established  there  the  salient  features  of  the  vertebrate  neuroccelia. 
These  factors  are  as  follows : 

1.  The  infolding  for  the  middle  cord  initiates  the  canalis  centralis  and  the 
more  posterior  parts  of  the  brain  cavities. 

2.  The  increasing  depth  of  the  median  groove,  and  the  increasing  thickness 
of  the  two  cords,  brings  the  median  edges  of  the  cords  together,  and  leads  to  the 
formation  of  the  neural  commissures,  which  form  the  rafters  over  the  median 
groove,  and  aid  in  its  conversion  into  a  canal. 

3.  The  formation  of  the  palial  overgrowth  for  the  forebrain,  and  the  mar- 
ginal overgrowths  in  the  hindbrain  region,  initiates  the  development  of  the  broad, 
membrane-roofed  ventricles  of  the  whole  brain. 

4.  The  stomodseal  infolding,  between  the  forebrain  and  midbrain,  establishes 
the  deep  and  narrow  third  ventricle  of  the  diencephalon. 

5.  The  deep  transverse  infolding  across  the  very  anterior  end  of  the  medul- 
lary plate  gives  rise  to  the  cavity  of  the  olfactory  lobes. 

6.  The  broad  chamber  formed  by  the  palial  overgrowth,  and  into  which  the 
hemispheres  project,  establishes  the  prosenccele,  or  the  first  and  second  ventricles. 

7.  The  median  and  the  lateral  eye  ganglia  unite  above  the  neural  surface 
of  the  brain,  one  forming  a  partial  roof  to  the  diencephalon,  and  the  other,  owing 
to  the  shape  of  the  ganglion,  forming  a  broad,  dome-like  covering  for  that  part  of 
the  brain  chamber  known  as  the  mesenccele. 

8.  The   stomodaeal  commissure,   forced  backward  by  the   enlarging  optic 
ganglia,  forms  the  first  stage  of  the  narrow  arch  (cerebellum)  over  the  future 
metenccele. 


THE    NEUROCCELIA   AND    THE    NEUROGLIA.  93 

9.  In  the  forebrain,  and  in  the  midbrain  regions,  the  medullary  cords,  in  both 
vertebrates    and    arachnids,  remain    practically  horizontal.     As  they  are  very 
broad  (the  diencephalon  alone  showing  a  marked  lateral  compression),  and  as 
the  overgrowth  is  almost  entirely  from  the  sides,  the  resulting  cavities  are  broad 
and  shallow.     The  roof  is  epithelial  in  character,  and  all  the  nervous  material, 
except  the  optic  ganglia  and  the  stomodaeal  commissures,  forms  the  floor  of  the 
chamber. 

10.  In  the  region  of  the  cord,  however,  the  infolding  combines  another  factor, 
especially  prominent  in  the  vertebrates,  in  that,  as  the  median  groove  deepens,  the 
two  cords  close  like  the  covers  of  a  book,  bringing  their  outer,  or  neurogenic  sur- 
faces, face  to  face,  converting  the  neural  groove  into  a  deep  lying  canalis  centralis, 
and  reducing  the  marginal  overgrowths  to  the  narrow  strip  of  epithelium  that 
roofs  over  the  posterior  fissure.     (Figs.  134,  137  and  231.) 


VI.  THE  NEUROGLIA. 

The  neuroglia  arises  from  the  epithelial  lining  of  the  middle  cord  groove  or 
canal.  Referring  to  the  early  stages  of  Acilius  (Figi  221),  it  will  be  seen  that  the 
medulla  is  invaded  by  numerous  small,  dark  nuclei,  that  spread  out  laterally  from 
the  middle  cord,  forming  a  uniform  envelope  about  the  medulla,  the  so-called  inner 
neurilemma.  Later  these  cells  multiply  and  invade  the  medulla  and  surround  the 
nerve  cells,  forming  a  coarse,  nucleated  reticulum  or  neuroglia. 

In  the  adult,  this  tissue  is  easily  recognized.  In  sections  of  the  brain  and  cord 
of  Limulus  stained  in  haemotoxylin,  Lyons  blue,  and  acid  fuchsin,  it  is  intense  red, 
and  the  nerve  fiber  masses  blue.  In  preparations  treated  by  von  Rath's  method, 
it  is  intense  black,  nerve  fibers  gray.  In  sections  of  the  adult  cord  (Figs.  67  and 
68),  the  neuroglia  network  may  be  seen  springing  in  root-like  processes  from  the 
thick  layer  lining  the  central  canal,  as  well  as  from  the  sides  of  the  neural  fissures 
and  the  surface  of  the  medulla. 


CHAPTER  VI. 

PERIPHERAL  NERVES  AND  GANGLIA. 

I.  COMPONENTS  OF  A  NEUROMERE. 

In  my  first  paper  "On  the  Origin  of  Vertebrates,"  1889,  it  was  maintained 
that  the  primitive  arthropod  neuromere  was  a  complex  structure,  consisting  of 
two  segments,  four  pairs  of  nerves,  and  a  segmented  middle  cord. 

While  I  have  seen  no  reason  to  change  my  view  as  to  the  composite  nature 
of  primitive  neuromeres,  I  do  not  now  regard  the  ancestral  arthropod  as  an  elon- 
gated worm-like  animal  of  many  like  metameres,  but  as  a  small-bodied  one  of 
about  three  imperfect  segments.  In  the  arachnid  and  crustacean  descendants 
of  this  stock,  the  evolution  of  neuromeres,  as  we  have  explained  elsewhere 
(Chap.  XIII)  was  a  gradual  process  that  advanced  with  the  successive  additions 
of  new  groups  of  unlike  metameres. 

II.  NERVES  OF  THE  DIENCEPHALON  AND  MESENCEPHALON.  * 
A.  Neural  Nerves. 

In  Limulus,  there  are  six  pairs  of  thoracic  neural  nerves.  (Figs.  36-39.) 
The  third  nerve  is  typical.  (Fig.  79.)  It  divides,  soon  after  leaving  the  brain, 
into  three  sets  of  nerves.  The  gustatory  nerves,  three  in  number,  are  ganglionated 
and  terminate  in  the  numerous  sensory  buds  of  the  mandibular  spines.  They  are 
absent,  or  very  minute,  in  the  sixth  pair  of  legs.  The  anterior  and  posterior 
entocoxal  nerves,  a.e.n.,  and  p.e.n.  are  motor  and  supply  the  tergocoxal  muscles; 
the  median  entocoxal  nerve,  m.e.n.,  supplies  the  sensory  knobs  of  the  coxopodite. 
The  main  pedal  nerve,  consisting  of  two  principal  branches,  supplies  the  muscles 
and  sense  organs  of  the  appendage. 

The  Flabellum. — There  are  a  few  minor  differences  between  the  six  pairs 
of  pedal  nerves;  the  most  important  is  an  enormous  enlargement  of  the 
median  entocoxal  nerve  of  the  sixth  leg  to  form  the  nerve  of  the  flabellum.  (Fig. 
80,  flab.) 

The  flabellum  is  a  large  spatulate  organ  attached  to  the  outer  side  of  the 
coxal  joint  of  the  sixth  leg.  It  is  first  seen  in  the  embryos  as  a  rounded  knob, 
lateral  to  the  sixth  leg,  and  quite  separate  from  it.  Hence  it  has  the  same  rela- 
tion to  the  outer  side  of  the  appendage  that  the  mandibular  placode  has  to  the 
inner.  (Figs.  141-148.)  There  are  indications  of  flabellar  placodes  on  the  other 

1For  nerves  of  the  prosencephalon  see  Chapters  VIII  to  X. 

94 


CRANIAL    GANGLIA.  95 

thoracic  segments,  but  they  quickly  disappear.  The  flabellar  placode  finally 
unites  with  the  base  of  the  sixth  leg  and  then  appears  to  be  a  part  of  it.  The  flabellum 
lies  in  the  mouth  of  the  channel  leading  to  the  gill  chamber,  and  practically  all 
the  water  going  to  the  gills,  either  from  the  front  or  from  the  sides,  must  pass  over 
its  anterior  surface.  This  surface  is  pigmented  and  very  richly  supplied  with 
sense  organs  and  nerves,  and  it  undoubtedly  serves  to  test  the  quality  of  water 
going  to  the  gill  chamber,  (see  p.  113.) 

The  flabellar  placodes  are  probably  represented  in  scorpion  by  the  lateral 
coxal  sense  organs.  (Figs.  15-16,  s.so.) 

The  Cranial  Ganglia. — The  base  of  each  pedal  nerve,  near  its  origin 
from  the  brain,  enlarges  to  form  an  immense  spindle-shaped  ganglion.  Similar 
ganglia  are  present  in  Branchipus,  scorpions,  and  spiders,  and  they  are  also  present 
on  the  pedal  nerves  of  many  other  arachnids  and  phyllopods. 

In  Limulus  they  arise,  at  an  early  period,  from  large  ectodermic  thickenings 
between  the  base  of  the  legs  and  the  corresponding  neuromere.  How  the  con- 
nection between  the  ganglion  and  neuromere  is  established  was  not  determined,  but 
it  is  certain  that  the  ganglion  is  not  an  outgrowth  of  the  nerve  cord.  The  body  of 
the  ganglion  separates  from  the  thickening,  but  retains  its  connection  with  the 
overlying  ectoderm  by  ganglionated  nerve  strands.  The  latter  become  the 
gustatory  nerves,  and  the  ectodermic  remnant  of  the  thickening  becomes  the 
mandibles  with  their  gustatory  spines.  The  thick  mass  of  slime  buds  on  the 
inner  face  of  the  mandibles  also  arises  from  these  thickenings. 

In  young  Limuli,  the  ganglia  are  relatively  large  masses  of  cells,  separated 
by  clear  fibrous  stalks  from  the  corresponding  neuromere.  (Figs.  36-39.)  In 
the  adult  (Figs.  70  and  218)  they  are  drawn  a  little  closer  to  the  brain,  but  are 
never  completely  merged  with  it. 

In  scorpion  embryos,  similar  ectodermic  thickenings  appear  at  the  base  of  the 
thoracic  appendages,  furnishing  the  anlage  for  the  coxal  ganglia.  (Fig.  74,  D.) 
The  thickenings  on  the  third  and  fourth  appendages  become  greatly  enlarged  to 
form  the  four  hypostomeal  spurs  that  lie  on  either  side  of  the  mouth  and  rostrum. 
(Figs.  15-16.)  The  median  face  of  these  spurs  is  highly  sensitive  (gustatory?) 
and  from  them  are  developed  enormous  ganglia  and  thick  masses  of  mucous  glands 
or  slime  buds. 

In  the  adult,  the  median  face  of  the  anterior  pair  of  spurs  is  deeply  grooved. 
The  two  grooves  lie  close  together  and  thus  form  a  thick  chitenous  tube,  lined  with 
sensory  hairs.  In  feeding,  the  scorpion  thrusts  the  spurs  into  its  prey  and  sucks 
the  blood  and  other  fluids  through  this  tube  into  the  mouth.  (Fig.  43,  mxl.) 

The  independent  origin  of  the  flabellar  and  coxal  spur  placodes  of  Limulus 
and  the  scorpion  suggests  their  homology  with  the  inner  and  outer  branches  of 
an  originally  triramous  appendage. 

The  coxal  placodes  represent  the  supra-branchial  placodes  of  vertebrates. 
Their  homology  is  indicated  by  their  similarity  in  position,  in  function,  in  devel- 
opment, and,  so  far  as  may  be  determined,  in  number.  In  both  cases,  the  pla- 


PERIPHERAL    NERVES   AND    GANGLIA. 


FIG.  70. — Nervous  system,  endocranium  and  endochondrites  of  an  adult  Limulus. 


THE   H^MAL    NERVES    OF    THE    DIENCEPHALON   AND    MESENCEPHALON.        97 

codes  split  into  a  special  group  of  gustatory  organs,  and  into  a  large  cranial  gang- 
lion. (Figs.  27-34.) 

The  Ganglia  of  the  Cord.—Limulus. — The  roots  of  the  neural  nerves 
arising  from  the  postcephalic  neuromeres  are  also  provided  with  ganglia,  but  they 
are  not  as  large  as  those  on  the  cranial  nerves.  They  merge  with  the  cord  at  an 
early  period,  and,  in  the  adult  form  the  large  swellings  on  the  roots  of  the  branchial 
nerves.  (Figs.  59-64.) 

Scorpion. — In  the  scorpion  the  anterior  pairs  of  nerves  are  much  larger  than 
the  posterior  ones,  and  spring  from  the  outer  or  neural  surface  of  the  cord.  In 
stage  H,  just  before  hatching,  the  root  of  the  anterior  nerve  contains  a  large  mass 
of  cells,  evidently  arising  independently  of  the  nerve  cords,  just  as  the  pedal 
ganglia  do  in  the  thorax.  (Fig.  73,  D.)  In  the  later  stages,  just  after  hatching, 
the  ganglion  is  drawn  toward  and  partly  merges  with  the  cord.  In  the  adult, 
ganglion  cells  are  scattered  for  some  distance  over  the  root  of  the  nerve.  (Fig. 

73.  C.) 

Meantime  the  two  haemal  nerves  move  forward,  and  unite  to  form  a  single 
one  with  two  roots,  which  in  turn  unite,  a  short  distance  from  the  cord,  with  the 
ganglionated  neural  nerve.  There  is  no  actual  mingling  of  fibers,  but  the  nerves 
run  together,  for  a  short  distance,  as  a  single  nerve.  (Fig.  72.) 

The  Haemal  Nerves. — Limulus. — A  single  pair  of  haemal  nerves  arise  from 
the  anterior  haemal  surface  of  each  thoracic  nueromere.  (Fig.  70,  h.n.)  They 
are  much  smaller  than  the  pedal  nerves,  without  ganglia,  contain  motor  and 
sensory  fibers  and  are  distributed  mainly  to  the  integument  and  other  tissues  of 
the  thoracic  shield.  The  sixth  pair  alone  sends  branches  to  the  heart  and  intes- 
tine. Near  the  outer  margin  of  the  entacoxite,  the  nerves  which  are  elsewhere 
round,  become  broad,  flat  bands;  the  parallel  bundles  of  nerve  fibers  become  inter- 
woven in  a  complicated  manner,  and  there  is  an  increased  number  of  neurilemma 
nuclei,  but  no  ganglion  cells.  Beyond  this  swelling,  the  nerve  divides  into  two 
main  branches,  n  and  h\  one  going  to  the  neural  surface  of  the  carapace,  and  the 
other  to  the  haemal.  After  several  subdivisions  (see  original  memoir) ,  the  end 
branches  of  all  these  nerves  form  a  continuous,  subdermal  plexus,  distributed 
over  the  whole  inner  surface  of  the  neural  and  haemal  integument,  supplying  the 
skin,  glands,  muscles  and  sensory  hairs. 

Lateral  Line  Nerve  of  Cheliceral  Neuromere. — All  these  thoracic  haemal 
nerves  are  essentially  alike,  except  the  first  one  or  that  of  the  chelicerttl  neuro- 
mere.  This  remarkable  nerve  (Fig.  70,  l.c.n.),  at  first  extends  forward,  and  then, 
bending  backward  in  a  broad  curve,  extends  the  whole  length  of  the  body.  It 
runs  close  to  the  neural  surface,  just  outside  the  bases  of  the  appendages,  and  does 
not  begin  to  branch  till  it  reaches  a  large  sclerite  behind  the  base  of  the  sixth  leg. 
The  main  nerve  continues  beyond  this  point  the  whole  length  of  the  branchial 
chamber,  sending  one  small  branch  toward  the  base  of  each  of  the  five  gills.  This 
is  a  purely  sensory  nerve  and  supplies  the  skin  lining  the  channel  along  which  the 
water  is  carried  to  the  gills.  It  is  very  remarkable  that  this  nerve  should  cross  the 
7 


98  PERIPHERAL    NERVES   AND    GANGLIA. 

territory  of  so  many  other  nerves  of  the  same  nature,  in  order  to  innervate  a 
region  so  far  removed  from  its  origin.  It  is  suggestive  of  the  lateral  line  nerve  of 
vertebrates,  but  its  origin  from  the  tween-brain  region  is  strongly  against  such 
an  interpretation.  It  resembles  the  large  nerve  in  ganoids  and  teleosts,  the 
ramus  lateralis  accessorius,  which  arises  well  forward  in  the  head  and  is  distributed 
to  the  taste  buds  of  the  head,  back,  tail,  and  fins. 

The  character  of  the  sensory  terminals  to  this  nerve  in  Limulus  is  unknown. 

III.  THE  NERVES  OF  THE  METENCEPHALON. 

We  have  already  shown  that  a  certain  number  of  abdominal  metameres  in 
arthropods  move  forward  and  unite  with  the  thorax,  and  that  there  is  a  great 
reduction  in  their  size  and  an  obliteration  of  their  external  boundaries.  The 
appendages  and  muscles  show  a  similar  reduction,  but  the  corresponding  nerves, 
neuromeres  and  heart  segments  are  but  little  changed.  In  fact,  the  nerves  and 
neuromeres  may  be  relatively  more  voluminous  or  extensive  than  elsewhere. 
These  metameres  constitute  the  vagus  zone  and  their  neuromeres  the  meten- 
cephalon.  Their  nerves  may  be  appropriately  called  vagus  nerves,  because,  as  in 
the  vertebrates,  they  extend  backward  into  regions  to  which  they  did  not  originally 
belong. 

LIMULUS. 

Neural  Nerves. — In  Limulus,  this  region  contains  two  metameres,  the  chi- 
larial  and  the  opercular.  The  tergites  of  these  metameres  are  still  visible  in  the 
adult,  the  chilarial  tergite  forming  a  narrow  band  on  the  posterior  margin  of  the 
thoracic  shield,  the  opercular  tergite,  two  wing-like  segments  on  the  anterior 
margin  of  the  branchial  shield.  The  hinge  joint  between  the  two  shields  lies 
between  these  two  metameres.  (Figs.  150-155.)  The  first  entapophysis  is 
formed  between  the  chilarial  tergites  and  the  true  thoracic  metameres.  (Fig.  193.) 

The  chilaria  are  without  question  true  appendages.  Their  early  develop- 
ment is  like  that  of  the  other  appendages,  and  they  have  separate  nerves,  muscles, 
mesoblastic  somites,  and  gill  bars.  The  chilarial  and  opercular  neuromeres 
have  all  the  typical  nerve  elements.  They  resemble  the  branchial  neuromeres 
more  than  the  thoracic,  although  in  the  adult  they  are  intimately  fused  with  the 
hindbrain  and  widely  separated  from  the  branchial  neuromeres.  Their  nerves 
pass  out  of  the  occipital  foramen  of  the  endocranium  together  with  the  spinal 
cord.  (Figs.  70-218.) 

The  chilarial  nerves  arise  close  together  from  the  posterior  neural  surface 
of  the  accessory  brain.  They  pass  out  of  the  endocranium  just  below  the  roof 
of  the  occipital  ring,  enter  the  chilaria  and  supply  their  muscles,  the  adjacent 
skin,  and  the  numerous  gustatory  spines  on  their  median  side.  (Fig.  81,  n.n7.) 

The  opercular  nerve  follows  the  same  course,  and  on  reaching  the  operculum 


THE  NERVES  OF  THE  METENCEPHALON. 


99 


h.n. 


FIG.  71. — Brain  and  nerve 
cord  of  a  new-born  scorpion, 
seen  from  the  haemal  surface. 


FIG.  72. — First  two  free  bran- 
chial neuromeres,  with  the  lemma- 
tochord,  spinal  ganglia,  and  spinal 
nerves  for  an  adult  scorpion. 


IOO 


PERIPHERAL    NERVES   AND    GANGLIA. 


divides  into  three  nerves  which  then  subdivide  into  motor  and  sensory  branches 
(for  details,  see  original  memoir,  1893). 

The  hcemal  nerves  (Fig.  70,  h.n7  and  h.n8),  pass  out  of  the  endocranium 
through  the  occipital  ring  and  are  distributed  to  the  sides  of  the  body,  between  the 
sixth  pair  of  legs  and  the  operculum.  For  the  distribution  of  the  intestinal  and 
cardiac  branches,  see  pp.  103,  200. 

Scorpion. 

In  the  scorpion,  the  vagus  region  consists  of  four  metameres,  two  genital, 
one  pectinal,  and  the  first  branchial,  as  I  demonstrated  in  my  first  paper  on  this 
subject,  1889. 

The  tergite  of  the  first  metamere  fuses  with  the  thorax  and  cannot  be  detected 


FIG.  73. — A,  Section  of  an  abdominal  neuromere  of  a  new-born  scorpion,  showing  the  ganglionated  "dorsal," 
or  neural,  nerve  root;  B;  same  through  the  non-ganglionated  haemal  nerve  root;  C,  section  of  the  second  free 
branchial  neuromere  of  an  adult  scorpion,  showing  both  neural  and  haemal  nerve  roots  and  the  neural  and  haemal 
transverse  commissures;  D,  third  branchial  neuromere  of  an  old  embryo  of  a  scorpion,  showing  the  large  ganglionic 
lobe  at  the  root  of  the  neural  nerve. 

in  the  adult.  The  other  three  tergites  remain  separate  throughout  life,  the  second 
or  genital,  and  the  third  or  pectinal,  being  much  narrower  than  the  fourth  (Fig.  17). 
The  fusion  of  these  metameres  is  more  strongly  marked  on  the  neural  than  on  the 
haemal  surface. 

During  the  early  embryonic  stages,  one  may  recognize  four  distinct  pairs  of 
rudimentary,  abdominal  appendages  (Figs.  15,  16).  During  the  later  stages, 
the  first  lung  book  appears  in  place  of  the  last  appendage.  The  third  pair  gives 
rise  to  the  pectines;  the  first  pair  disappear  altogether  by  the  time  of  hatching; 
while  the  second  pair  finally  unite  in  the  median  line  in  front  of  the  pectines  to 
form  the  genital  cushion,  or  tubercles. 

Before  they  unite,  about  stage  G  (Fig.  16,  B),  the  genital  openings  may  be 


THE  NERVES  OF  THE  METENCEPHALON. 


101 


recognized  on  the  median  margin  of  each  appendage.  About  the  time  of  hatching, 
the  genital  ducts  are  carried  forward  and  unite  to  form  an  unpaired  opening, 
between  the  remnants  of  the  first  pair  of  abdominal  appendages  and  the  second. 
During  stage  G,  the  brain  may  be  dissected  out,  and  the  arrangement  of  neuro- 
meres  and  some  of  the  nerves  observed  (Fig.  54).  The  first  two  neuromeres  are 
crowded  forward  and  are  overlapped  by  the  posterior  margin  of  the  hindbrain. 
This  produces  a  sharp  haemal  flexure  in  the  brain,  at  the  dividing  line  between  the 


FIG.  74. — D,  Section  through  the  basal  lobe  of  the  third  thoracic  appendage  of  an  embryo  scorpion;  E,  section  of  one 
of  the  segmental  sense  organs  on  the  outer  margin  of  the  thoracic  appendages.     See  Figs.  15  and  16. 

thoracic  and  vagus  region;  the  first  two  vagus  neuromeres  are  thus  partly  concealed, 
in  surface  views,  under  the  overlapping  hindbrain.  The  vagus  nerves,  during 
these  early  stages,  are  small  and  cannot  be  followed  with  certainty.  In  the  adult 
scorpion,  they  present  an  interesting  condition.  The  nerves  are  now  divided 
into  two  groups,  one  containing  all  the  neural  nerves,  the  other  all  the  haemal. 
(Fig.  42.)  This  is  largely  due  to  the  constriction  which  is  such  a  characteristic 


tg.prpl/r: 


ervt. 


FlG.  75- — Side  view  of  the  endocranium,  brain,  neural  arches,  and  associated  muscles  of  an  adult  Limulus;  semi- 
diagrammatic. 

feature  of  the  vagus  region  in  arthropods.  All  the  paired  organs  in  this 
vicinity,  such  as  genital  ducts  and  appendages,  are  drawn  toward  a  median 
position,  hence  in  the  adult  the  corresponding  nerves  take  their  origin  from  the 
neural  surface  of  the  cord,  near  the  middle  line. 

The  Neural  Nerves. — One  small  nerve,  supplying  the  sexual  ducts  and  papil- 
lae, probably  represents  the  nerve  of  the  second  pair  of  rudimentary  appendages 
(Fig.  42,  vl).  The  nerve  to  the  pectines  has  three  roots,  the  first  one  g.n.  forming 


!02  PERIPHERAL   NERVES   AND    GANGLIA. 

an  immense  bilobed  ganglion  (ganglion  nodosum)  composed  of  ganglion  cells  and 
concentric  lamminae  of  medullary  substance  (g.nd). 

It  is  united  with  its  mate  by  two  distinct  bridges  of  nerve- tissue  that  lie  some 
distance  above  the  surface  of  the  neuromeres  they  thus  form  an  imperfect  roof  to 
a  deep,  narrow  canal  between  the  two  ganglia  and  the  median  sides  of  the  under- 
lying neuromeres  (Fig.  40).  The  anterior  ends  of  the  ganglia  may  be  traced  in 
transverse  sections  a  long  distance  forward,  as  two  great  longitudinal  fasciculse, 
just  below  the  neural  surface  of  the  thoracic  neuromeres.  (Fig.  69,  v.g.l.  and 
g.t.r.) 

•  *  -The  ganglion .  pa  fthe  second  root  (ganglion  f usif orme)  is  smaller,  spindle- 
shaped/and^a'sTnear  'as1  Can  be  determined,  appears  to  belong  to  the  third  neuromere. 
The  thMfroOt!  is-  sri.iall,,  fibrous,  and  without  any  ganglionic  enlargement. 

The  Haemal  Nerves. — The  two  haemal  nerves  of  the  first  neuromere  remain 
separate,  as  in  a  typical  thoracic  neuromere.  In  each  of  the  three  following 
neuromeres,  they  unite  to  form  a  single  nerve,  each  with  a  double  root.  (Fig.  42, 
h.n.1'4.)  A  short  distance  from  the  brain  all  five  haemal  nerves  form  a  compact 
bundle  that  extends  backward  through  the  occipital  foramen  of  the  cartilaginous 
cranium.  (Figs.  71  and  217.)  The  nerves  to  the  third  and  fourth  neuromeres, 
h.n.3  and/z.w.4,  some  distance  from  the  brain,  fuse  to  form  a  single  nerve  supplying 
the  first  and  second  lung  books  and  the  ventral  surface  of  the  body  (Fig.  71). 
On  its  way  to  these  organs,  it  passes  over  the  ventral  surface  of  the  liver,  to  which 
it  possibly  gives  branches.  The  anterior  haemal  nerve  of  the  first  vagus  neuromere, 
h.n.1  runs  close  to  the  coxal  gland,  and  dividing  into  numerous  branches,  is  lost 
on  the  surface  of  a  thick,  peritoneum-like  membrane.  The  posterior  nerve, 
h.n. 2  extends  along  the  arthroideal  membrane,  supplying  numerous  sense  organs 
on  the  lateral  and  the  haemal  surface  of  the  abdomen.  The  fourth  vagus  nerve, 
h.  n.4  supplies  the  skin  and  the  longitudinal  muscles  on  the  ventral  surface  of  the 
abdomen. 

IV.  NERVES  OF  THE  BRANCHIENCEPHALON. 

The  branchial  neuromeres  differ  from  those  of  the  brain  in  that  they  remain 
separate  through  life.  Their  nerves  are  noteworthy  for  their  association  with  the 
respiratory  muscles,  the  heart,  and  the  intestine. 

Limulus. 

The  Branchial  Nerves. — In  Limulus,  the  branchial,  or  neural  nerves,  contain 
both  motor  and  sensory  fibers.  They  arise  from  large  ganglia  on  the  posterior 
neural  surface  of  the  neuromeres;  on  entering  the  gills  they  divide  into  three  branches. 
(Fig.  82.)  The  external  branch,  eb.n.  supplies  the  abductor  muscles  and  the  skin 
on  the  anterior  lateral  surface  of  the  appendage.  The  median  branch  g.n. 
supplies  the  adductor  muscles  and  the  gill  books.  The  internal  branch  i.bn. 
upplies  the  skin  and  muscles  in  the  terminal  portion  of  the  appendage. 


THE  NERVES  OF  THE  BRANCHIENCEPHALON. 


I03 


The  Haemal  Nerves  (Figs.  59  and  70),  arise  from  the  anterior  margin  of 
the  neuromere  and  extend  outward  over  the  neural  surface  of  the  abdominal 
muscles.  They  divide  into  five  principal  branches;  one  goes  to  the  enteron,  one 
to  the  longitudinal  abdominal  muscles,  one  to  the  branchio-thoracic  muscles, 
one  to  the  heart,  and  one  to  the  integument. 


ol.o 


rost. 


Cran. 


78 


carcLg. 


per.c. 


FIG.  76. — Side  view  of  the  brain,  endocranium,  alimentary  canal,  and  principal  vascular  channels  in  Limulus;  semi 

diagrammatic. 
FIG.   77. — Same,   showing   the  relation  of  the  compound  branchio-thoracic,  or  hypobranchial  nerve  to  the  haemal 

nerves  of  the  vagus  and  branchial-neuromeres.      Semi-diagrammatic. 

FIG.  78. — Same,  showing  the  relation  of  the  segmental  cardiac  nerves,  s.c.n.6-i3,  to  the  heart  and  to  the  vagus  and 

branchial  neuromeres.     Semi-diagrammatic. 

The  Enteric  Nerves. — In  Limulus,  the  enteric  nerves  are  intimately  associated 
with  the  nerves  to  the  longitudinal  abdominal  and  haemo-neural  muscles.  The 
enteric  nerves  form  a  plexus  which  covers  the  entire  mesenteron  and  the  plexus 
is  united  with  the  roots  of  all  the  haemal  nerves,  from  the  sixth  to  the  sixteenth,  by 
paired  rami  communicant es.  (Fig.  59  i7"14.)  Those  from  the  sixth  and  seventh 
neuromeres  pass  through  the  foramina  in  the  posterior  lateral  wall  of  the 
endocranium.  (Figs.  597~8  and  218.) 


!04  PERIPHERAL    NERVES   AND    GANGLIA. 

The  rami  from  the  sixth  thoracic,  the  chilarial,  and  opercular  neuromeres 
mingle  with  a  plexus  of  nerves  distributed  over  the  longitudinal  abdominal  muscles ; 
from  there  branches  pass  forward,  ramifying  over  the  surface  of  the  mesenteron 
as  far  as  the  stomodaeum. 

In  the  branchial  neuromeres,  the  nerves  supplying  the  longitudinal  abdominal 
muscles  and  the  intestine  are  separate.  The  former  arise  from  the  anterior  side 
of  the  haemal  nerve  root  (Fig.  60,  Lab.),  and  the  latter  from  the  posterior  side. 
In  the  more  posterior  neuromeres,  the  intestinal  branches  gradually  shift  their 
point  of  origin  from  the  root  of  the  haemal  nerve  to  the  median  margin  of  the  neu- 
romere.  In  the  branchial  segments,  the  intestinal  rami  send  a  small  branch  to 
the  corresponding  haemo-neural  muscle. 

The  enteric  nerves  appear  to  represent  the  initial  stages  of  the  sympathetic 


.   .car- 
Wr- 


FIG.  79. — Muscles  and  distribution  of  nerves  in  the  third  leg  of  Limulus,  from  the  anterior  side,  i-cox.,  Coxo- 
podite,  or  first  joint;  2-bas.,  basipodite,  or  second  joint;  3 -we.,  ischiopodite,  or  third  joint;  4-car.,  mer.,  fused  car- 
popodite  and  meropodite,  or  fourth  joint;  5-pro.,  propodite,  or  fifth  joint;  6-dac.,  dactylopodite,  or  sixth  joint;  apo., 
apodeme. 

MUSCLES:  3^  and  &,  Plastro-coxal  muscles  inserted  upon  anterior  side  of  entocoxite;  36  and  3^,  tergo-coxal 
muscles  inserted  upon  anterior  side  of  entocoxite;  e.2'f>,  extensors  of  second  to  sixth  joints;/.2 "6,  flexors  of  second 
to  sixth  joints; /.w,  flexor  of  inner  manible. 

NERVES:  a.e.n.,  Anterior  ento-coxal  nerve;  br.,  brain;  e.p.n.,  external  pedal  nerve;  h.,  haemal  branch  of  in 
tegumentary  nerve;  h.n.3,  haemal  nerve;  in.n.x,  integumentary  branch;  i.p.n.,  internal  pedal  nerve;  wi.e.n.,  median  en- 
coxal  nerve;  m.n.,  mandibular  nerves;  n.,  neural  branch  of  integumentary  nerve;  n.n.*,  neural  nerve;  p.e.n., 
posterior  ento-coxal  nerve. 

system  of  vertebrates.  It  is  a  noteworthy  fact  that  in  Limulus  the  anterior  cranial 
nerves  are  not  directly  united  with  the  enteric  plexus  by  segmental  communicating 
branches.  The  most  anteiior  connecting  branch  that  is  recognizable  belongs  to 
the  ninth  cranial  neuromere,  resembling,  in  this  respect,  the  well  known  condition 
in  vertebrates. 

The  Longitudinal  Abdominal  Muscles  and  Nerves. — The  longitudinal 
abdominal  muscles  arise  from  the  posterior  haemal  side  of  the  endocranium  and 
pass  backward,  giving  slips  to  each  pair  of  the  abdominal  entapophases  and  to 
the  abdominal  endochondrites.  (Fig.  75.) 

The  muscles  are  provided  with  a  rich  nerve  plexus,  extending  their  whole  length. 


THE   HYPO-BRANCHIAL    MUSCLES   AND    NERVES.  105 

(Figs.  57,59,  Lab.)  The  fibers  arise  from  small  branches  of  the  sixth  to  the  sixteenth 
haemal  nerves.  The  branches  are  given  off  from  the  anterior  side  of  the  haemal 
nerve,  close  to  the  cord.  The  neurones  lie  on  the  opposite  side  of  the  next  anterior 
neuromere,  with  those  that  supply  the  branchio-thoracic  muscles.  (Fig.  60,  D,l.ab.) 
In  the  seventh  and  eighth  metameres,  the  plexus  appears  to  be  continuous  with 
that  going  to  the  intestine. 

The  General  Cutaneous  Nerves  are  largely,  if  not  wholly,  sensory.  They 
extend  over  the  surface  of  the  branchial  plastron,  dividing  into  numerous  branches 
on  the  margin.  (Figs.  59,  70,  g.cut.)  The  fibers  that  enter  into  these  branches 
probably  form  the  second  root,  h.r.2  (Fig.  61.) 

Cardiacs.— For  a  description  of  the  segmental  cardiacs,  see  p.  200. 
The  Branchio-thoracic,  or  Hypo-branchial  Muscles  and  Nerves. 

The  hypo-branchial  muscle  is  a  large  compound  muscle  derived  from  the 
eighth  to  thirteenth  metameres  inclusive.  The  neural  end  of  each  component  is 
separate  and  terminates  in  a  tendinous  infolding  of  the  ectoderm  at  the  base  of 
its  corresponding  appendage.  The  haemal  ends  form  a  single  massive  muscle 
which  shifts  its  position  a  long  ways  forward  into  the  haemal  region  of  the  thorax, 
where  it  is  attached  to  the  inner  surface  of  the  shield,  in  front  of  the  anterior  end 
of  the  heart  and  the  forebrain.  (Fig.  78,  B.) 

The  muscle  aids  in  the  performance  of  the  complicated  respiratory  move- 
ments, drawing  the  bases  of  the  gills  forward  and  upward;  it  also  aids  in  flexing 
the  thorax  on  the  branchial  section  of  the  body. 

The  hypo-branchial  nerve1  forms  a  great  longitudinal  trunk  extending  over 
the  neural  surface  of  the  muscle.  (Figs.  59  and  77,  b.th.n.)  It  receives  its  fibers 
from  the  eighth  to  the  fourteenth  haemal  nerves,  via  short  communicating 
branches.  It  is,  therefore,  to  be  regarded  as  a  compound  nerve  formed  by  the 
united  branches  of  at  least  seven  segmental  nerves.  For  the  greater  part  of  its 
course,  it  forms  a  compact  longitudinal  trunk,  giving  off  at  regular  intervals 
branches  to  the  proximal  ends  of  the  muscle  slips,  near  their  tendinous  at- 
tachment to  the  base  of  the  gills.  At  its  anterior  end,  it  breaks  up  into  many 
branches  that  are  distributed  through  the  single  muscle  into  which  the  six 
separate  muscles  merge.  One  nerve  separates  from  the  anterior  end  of  the  main 
trunk  and  supplies  the  inter -tergal,  or  arthro-tergal,  muscle.  (Fig.  77,  in.t.) 

The  nerve  fibers  arise  from  clusters  of  large  D  neurones  lying  on  the  opposite 
side  of  the  cord,  on  the  posterior  margin  of  the  neuromere,  in  front  of  the  one 
where  the  nerve  enters  the  cord.  (Fig.  60.)  The  very  large  axones  cross  in  the 
posterior  haemal  commissure  and  pass  backward,  as  a  conspicuous  bundle  of 
large  nerve  tubes,  A.r.5,  to  the  haemal  nerve  root.  Some  of  the  fibers  go  to  the 
longitudinal  abdominal  nerves,  but  the  main  bundle  passes  on  with  the  haemal 
nerve,  leaving  it  farther  on,  to  enter  the  main  hypobranchial. 

The  remarkable  condition  of  the  hypobranchial  muscles  and  nerves  of 
Limulus  is,  no  doubt,  one  that  has  its  counterpart  in  other  arachnids.  At  present, 

1  Lateral  Sympathetic  of  Patten  and  Redenbaugh. 


106  PERIPHERAL    NERVES   AND    GANGLIA. 

practically  nothing  is  known  about  the  anatomy  of  these  muscles  and  nerves  in 
other  invertebrates. 

In  Limulus,  the  fact  that  is  specially  noteworthy  is  that  the  six  originally 
vertical  components  of  the  branchio-thoracic  muscle  have  been  converted  into  a 
nearly  horizontal,  or  longitudinal,  compound  muscle,  thereby  destroying  all 
correspondence  between  the  metamerism  of  the  neural  and  haemal  surfaces  as  far 
as  this  muscle  is  concerned. 

The  hypo-branchial  muscle,  by  this  change  in  position,  gains  in  effectiveness 
as  a  respiratory  and  flexor  muscle,  but  it  would  be  a  mistake,  I  believe,  to  accept 


6-dac-- 


FXG.  80. — Muscles  and  distribution  of  the  nerves  in  the  sixth  leg  of  Limulus,  from  the  anterior  side,  i-cox., 
Coxopodite,  or  first  joint;  2-bas.,  basipodite,  or  second  joint;  s-isc.,  ischiopodite,  or  third  joint;  4-car.,  mer.,  fused 
carpopodite  and  meropodite,  or  fourth  joint;  s-pro.,  propodite,  or  fifth  joint;  6-dac.,  dactylopodite,  or  sixth  joint; 
apo.,  apodeme;  bt.,  brain. 

MUSCLES:  6a  and  6b,  Plastro-coxal  muscles  inserted  upon  anterior  side  of  entocoxite;  6c  and  6d,  tergo-coxal 
muscles  inserted  upon  anterior  side  of  entocoxite;  e.2~7 ,  extensors  of  second  to  seventh  joints;/.2" 7,  flexors  of  second 
to  seventh  joints;  i.m.,  inter-tergal  muscle. 

NERVES:  a.e.n.,  Anterior  entocoxal  nerve;  e.p.n.,  external  pedal  nerve;  h.,  haemal  branch  of  integumentary 
nerve;  h.n.e,  haemal  nerve;  i.n.G,  intestinal  nerve;  in.n.6,  integumentary  branch  of  haemal  nerve;  i.p.n.,  internal  pedal 
nerve;  l.c.n.,  lateral  cardiac  nerve  \rn.c.n.,  median  cardiac  nerve;  m.e.n.,  median  ento-coxal  nerve  or  flabellar  nerve ; 
m.n.,  mandibular  nerve;  «.,  neural  branch  of  integumentary  nerve;  n.n..6,  neural  nerve;  p.,  pericardium;  p.e.n., 
posterior  ento-coxal  nerve;  s.c.n.6,  segmental  cardiac  nerves. 

that  as  a  primary  cause  of  the  change  in  position,  or  as  an  explanation  for  the 
existence  of  that  particular  function.  The  real  reason  lies  deeper,  and  is  to  be 
seen  in  those  changes  that  have  gradually  reduced  the  volume  of  the  haemal  organs 
in  the  anterior  head  region.  This  atrophy  or  reduction  of  the  haemal  surface  of 
the  head  during  the  early  embryonic  periods,  inevitably  draws  the  haemal  structures 
of  the  post-cephalic  metameres  forward,  and  is  the  initial  cause  of  that  forward 


VAGAL  AND   HYPO-BRANCHIAL   NERVES. 


107 


displacement  and  condensation  that  we  have  just  described  in  the  haemal  ends  of 
the  hypo-branchial  muscles. 

This  condition  is  a  very  ancient  one,  for  these  very  muscles,  in  this  position, 
no  doubt  cause  that  folding  of  the  thorax  onto  the  abdomen  which  is  so  common 
in  trilobites.  I  have  seen  the  same  thing  in  Bunodes,  very  much  to  my  astonish- 
ment. For  sections  of  specimens  that  appeared  to  be  headless,  showed  that  the 
cephalo-thorax  was  present,  but  doubled  over  so  as  to  lie  with  its  neural  surface 
flat  against  the  neural  surface  of  the  branchial  region. 

The  general  trend  of  the  branchio-thoracic  nerves  no  doubt  has  been  deter- 
mined by  these  morphological  changes  in  the  muscles;  but  the  union  of  these 
several  nerves  into  a  common  trunk  is  to  be  regarded  as  an  expression  of  the 
tendency  to  gain  simplicity  by  the  merging  of  several  separate  agents,  performing 
the  same  function,  into  a  single  one. 

V.  RELATION  OF  THE  VAGAL  AND  HYPO-BRANCHIAL  NERVES  IN  ARACHNIDS  TO 

THOSE  IN  VERTEBRATES. 

The  entire  system  of  nerves  belonging  to  the  vagal  and  branchial  regions  in 
the  arachnids,  represents  the  initial  stages  in  the  evolution  of  the  vagal  and  bran- 
chial complex  in  vertebrates.     We  can  already  distinguish  in  the  arachnids  the 
beginning  of  that  remarkable  segregation  of 
similar    components   into  compound  nerves, 
that  in  the  vertebrates  has  given  rise  to  the 
branchial,  hypoglossal,  cardiac,  visceral,  and 
lateral  line  nerves;  and  the  beginning  of  that 
readjustment  in  the  position  of  the  terminals 
that  has  given  to  each  set  of  components  their 
characteristic    distribution    and    direction  of 
growth.     (Figs.  57,  58.) 

The  branchio-thoracic  muscles  and  nerves 
of  Limulus  are  clearly  comparable  with  the 
hypoglossal  nerves  and  muscles  of  vertebrates. 
In  both  vertebrates  and  arachnids,  the  nerves 
arise:  a.  from  a  large  number  of  post-vagal 
neuromeres  (five  branchial  and  one  oper- 
cular);  b.  they  are  either  haemal  nerves  (ven- 
tral roots)  or  branches  of  haemal  nerves;  c.  they 
are  united  to  form  a  compound  longitudinal 
trunk,  terminating  in  an  extensive  plexus;  d.  the  distal  ends  of  both  muscles  and 
nerves  migrate  a  long  distance  forward  onto  the  anterior  haemal  surface  of  the 
head,  thus  causing  the  nerves  to  follow  their  characteristic  U-shaped  course,  and 
disguising  the  original  relation  between  the  metameric  arrangement  of  organs  on 
the  neural  and  haemal  surfaces;  e.  the  distribution  of  the  neural  nerves  (branchial 
arch  nerves)  is  not  affected  by  these  changes. 


FIG.  81. — Muscles  and  nerves  of  the  chi- 
laria  of  Limulus,  from  anterior  side.  The  ap- 
pendages are  revolved  outward  about  45°. 
b.c.7,  Capsuliginous  bar,  or  branchial  cartilage 
of  the  chilaria. 

MUSCLES:  70-*,  Plastro-coxal;  7  /  and  g, 
tergo-coxal;  i.m.,  inter-tergal;  up..m.7,  veno- 
pericardiac. 


io8 


PERIPHERAL    NERVES   AND    GANGLIA. 


In  the  arachnids  (scorpion),  four  abdominal  neuromeres  have  migrated  for- 
ward to  unite  with  the  hindbrain.  Of  these  four,  the  last  one  is  a  true  branchial 
neuromere. 

In  vertebrates,  all  the  branchial  neuromeres  have  fused  with  the  hind- 
brain,  probably  in  some  such  manner  as  that  indicated  in  Fig.  68.  The 
hypobranchial  nerves  united  to  form  the  hypoglossus,  having  the  peculiar  distri- 
bution indicated  above,  although  in  a  more  exaggerated  form.  The  neural  roots 


FIG.  82. — Muscles  and  distribution  of  nerves  in  the  first  gill  of  Limulus.  The  appendage  is  flexed  upon  the 
abdomen,  and  is  seen  from  the  neural  side.  a. e.9,  Abdominal  endochondrite;  fc.c.8  and  be. 9,  branchial  cartilages 
of  operculum  and  first  gill;  i.l.,  inner  lobe  of  gill;  rn.l.  median  lobe  of  gill  o.l.,  outer  lobe  of  gill. 

MUSCLES:  a.b.m.9,  Abductor  muscle  of  gill;  b.t.m.,  branchio-thoracic  muscles;  e.b.m.9,  external  branchial 
muscle;  i.b.m.9,  internal  branchial  muscle;  i.l.m.,  inner  lobe  muscles;  l.a.m.,  longitudinal  abdominal  muscles;  o.l.m., 
outer  lobe  muscles. 

NERVES:  a.g.,  First  abdominal  ganglion;  e.b.n.,  external  branch  of  neural  nerve;  g.n.,  branch  of  neural  nerve 
supplying  gill  book;  h.n.9,  haemal  nerves;  i.b.n.,  internal  branch  of  neural  nerve;  i.n.9,  intestinal  nerve  (two  branches 
are  shown,  a  posterior  and  an  anterior  one);  in.n.,9  integumentary  branch  of  haemal  nerve;  l.s.n.,  hypo-bran- 
chial nerve;  m.b.n.,  median  branch  of  neural  nerve;  m.n.  9,  neural  nerve;  s.c.n.,9  segmental  cardiac  nerve; 
v.c.,  ventral  cord. 

united  with  one  another,  and  with  the  posterior  neural  roots  of  the  vagus,  as  they 
have  to  a  certain  extent  in  the  scorpion,  to  form  the  series  of  nerves  supplying  the 
gill  arches.  The  nerves  supplying  the  important  sense  organs  in  the  group  of 
modified  vagal  appendages,  gave  rise  to  the  lateral  line  nerve;  and  the  combined 
cardiac  and  intestinal  components,  that  in  the  arachnids  are  confined  to  this 
region,  gave  rise  to  the  corresponding  elements  in  the  vertebrates. 

We  need  not  carry  this  comparison  any  farther,  for  the  conditions  are  ex- 
tremely complicated,  and  there  are  many  variations  peculiar  to  each  class.  But 
that  this  entire  region  has  undergone  the  same  kind  of  changes  in  vertebrates  that 


VAGAL   AND   HYPO-BRANCHIAL    NERVES.  109 

we  now  see  taking  place  in  the  arachnids  is,  I  believe,  beyond  question.  The 
fact  that  the  branchial  appendages  in  arachnids,  as  nearly  as  we  may  deter  mine, 
belong  to  the  same  group  of  metameres  as  in  vertebrates,  and  the  fact  that  the 
total  number  of  branchial  segments  in  Limulus  and  the  merostomes  is  very 
nearly  the  same  as  in  vertebrates,  i.e.,  seven  appendages,  four  or  five  of  which  are 
gill  bearing,  as  against  five  or  seven  gill  bearing  arches  for  vertebrates — is  sug- 
gestive, but  perhaps  of  less  significance  than  the  fact  that  in  both  cases,  there  is  a 
much  greater  forward  growth  and  concrescence  of  the  structures  on  the  haemal 
surface  than  of  those  on  the  neural,  thus  producing  that  apparent  lack  of  harmony 
in  the  serial  arrangement  of  nerves,  neuromeres,  gill  arches,  and  myotomes,  so 
disturbing  to  the  student  of  vertebrate  cephalogenesis. 

The  conditions  become  still  more  significant  when  we  recall  that  they  are  the 
inevitable  results  of  very  remote  factors  that  are  common  to  both  types,  such  as 
the  absence  of  lateral  plates  to  the  mesodermic  segments  in  the  anterior  part  of 
the  head,  the  gradually  increasing  size  of  the  yolk  sphere,  and  the  precocious 
development  of  the  forebrain. 


CHAPTER  VII. 


GENERAL  AND  SPECIAL  CUTANEOUS  SENSE  ORGANS. 

In  the  arachnids,  we  may  recognize  three  main  groups  of  sense  organs; 
primitive  segmental,  special  cutaneous,  and  general  cutaneous. 

a.  The  primitive  segmental  sense  organs  include  the  median  and  the  lateral 
eyes,  the  olfactory  and  the  auditory  organs.  With  the  exception  of  the  last  named, 
they  are  so  highly  developed  and  have  been  so  long  established  that  they  and 
their  nerve  centers  constitute  the  very  foundations  of  the  forehead  and  forebrain. 
b.  The  special  cutaneous  sense  organs  include  the  gustatory  buds,  slime  buds, 
and  other  chemotactic,  or  tactile  organs  that  have  well  defined  nerves,  ganglia, 
and  central  terminals,  and  that  are  located  in  definite  fields,  or  areas,  such  as 
the  coxal  spurs,  chilaria,  pectines,  flabellum,  etc.  They  attain  their  highest 
development  in  the  midbrain  and  hindbrain  regions,  c.  The  general  cutaneous 
sense  organs  may  be  of  the  same  nature  as  the  special  cutaneous,  but  they  are 
irregularly  distributed,  and  without  separate,  well  defined  nerves  or  ganglia; 
we  also  include  in  this  category  temperature  organs  and  free  nerve  endings. 
They  are  supplied  mainly  by  the  subdermal  plexus  formed  from  the  terminal 
branches  of  the  neural,  but  especially  of  the  haemal  nerves.  These  nerves  may 
arise  from  all  the  main  divisions  of  the  neuron  except  the  forebrain. 

I.  GENERAL  CUTANEOUS  SENSE  ORGANS. 

Temperature  Organs. — Reaction  to  changes  in  temperature  in  the  lower 
animals  is  probably  much  more  delicate  and  more  generally  exercised  than  has 
been  suspected.  The  temperature  sense  is  not  dependent  on  the  location  of  its 
organs  in  a  particular  part  of  the  body,  for  changes  of  temperature  are  diffusely 
distributed,  and  are  not  likely  to  affect  the  animal  at  one  point  more  than  another. 
An  effective  response  results  in  the  transfer  of  the  whole  body  to  surroundings  of 
a  different  temperature;  thus  the  temperature  organs  primarily  control  them  ove- 
ments  of  the  animal  as  a  whole,  or  its  migrations,  or  distribution  in  space. 

One  would  hardly  suspect  that  such  a  heavily  armored  animal  as  Limulus 
would  be  very  sensitive  to  changes  in  temperature,  yet  that  such  is  the  case  may  be 
easily  demonstrated.  When  placed  on  its  back  and  allowed  to  become  perfectly 
quiet,  it  may  be  fanned,  or  the  surface  of  the  carapace,  or  the  gills,  or  the  legs, 
may  be  touched  with  an  object  the  same  temperature  as  the  air,  without  causing 
any  reflexes;  but  the  instant  any  of  these  parts  are  touched  ever  so  gently  with  the 
finger,  or  if  water  a  little  warmer  or  colder  than  the  surrounding  air  falls  on  them, 

no 


TEMPERATURE   AND    GUSTATORY    ORGANS.  Ill 

or  even  if  one  gently  breathes  on  the  gills  and  under  surface  of  the  body,  the  animal 
at  once  becomes  greatly  agitated. 

The  most  sensitive  areas  are  the  margins  of  the  carapace,  and  especially  the 
margins  of  the  gill  chamber  along  which  the  main  current  of  water  passes  to  the 
gills,  and  the  anterior  surface  of  the  branchial  appendages  themselves. 

We  cannot  positively  identify  the  temperature  organs.  They  appear  to  be 
short,  spike-like  projections  in  which  terminates  a  small  tuft  of  sensory  cells. 
They  are  distributed  over  all  parts  of  the  carapace  and  are  supplied  by  the  terminal 
plexus  formed  by  the  branching  of  the  haemal  nerves. 

They  are  seen  to  best  advantage  in  the  gills  of  young  Limuli,  2-4  in.  long. 
In  those  parts  of  the  gills  that  are  most  sensitive  to  heat,  i.e.,  the  outer  surf  ace  of 
the  terminal  joints  of  the  exopodites,  one  may  see,  in  successful  methylene  blue 
preparation,  a  loose  subdermal  nerve  plexus  continuous  with  small  clusters  of 
spindle  shaped  sensory  cells.  From  each  cluster  a  very  fine  fiber  extends  outward, 
through  a  chitenous  tubule,  to  a  short  spike  situated  on  the  outer  surface  of  the 
gill.  (Fig.  86,A,t.s.) 

********* 

Free  Nerve-Ends. — In  the  abdominal  appendages  that  have  been  injected 
with  methylene  blue,  large  nerve  branches  may  be  seen  going  to  the  soft  integument 
around  the  joints  of  the  endopodites.  Each  branch  ends  in  a  group  of  bipolar 
or  multipolar  cells ;  from  them  arise  many  branching  fibers  that  form  a  rich  termi- 
nal meshwork,  lying  in  or  on  the  ectoderm,  but  without  association  with  any 
specialized  cells.  (Figs.  86 B,  87.) 

The  hyphae  of  a  parasitic  fungus  sometimes  ramify  in  all  directions  through 
or  over  the  surface  of  the  chiten.  They  usually  take  on  an  intensely  blue 
stain,  and  at  first  sight  might  be  mistaken  for  nerve  fibers. 

II.  SPECIAL  CUTANEOUS  SENSE  ORGANS. 

The  Gustatory  Organs  of  Limulus. — Gustatory  organs  are  widely  dis- 
tributed over  the  neural  surface  of  the  head,  but  they  are  most  highly  developed 
in  the  appendages  that  come  in  frequent  contact  with  the  food.  In  other  words, 
the  principal  aggregations  of  these  organs  are  located  around  the  mouth  in  segment- 
ally  arranged  fields.  This  condition  explains  their  remarkable  distribution  in  verte- 
brates. There  they  are  primarily  arranged  in  several  radiating  series  on  the  top  of 
the  head,  an  inconveniently  long  distance  from  the  present  vertebrate  mouth,  but 
close  to  the  central  areas  where  the  old  invertebrate  mouth  was  located.  (Fig.  89.) 

The  gustatory  organs  have  been  most  carefully  studied  in  Limulus,  and  they 
form  the  principal  basis  for  my  conclusions.  They  are  abundant  in  the  mandibles 
of  the  thoracic  appendages,  except  the  first  and  last  pairs,  and  in  the  tips  of  the 
thoracic  appendages.  Their  presence  in  the  mandibles  is  indicated  by  a  most 
beautiful  series  of  reflexes,  first  described  by  me  in  1892.  Organs  similar  in 


112  GENERAL  AND    SPECIAL    CUTANEOUS    SENSE    ORGANS. 

structure  to  the  ones  we  are  about  to  describe  occur  in  other  parts  of  the  head 
and  trunk,  but  stimulation  of  them  does  not  produce  any  recognizable  reflex. 

a.  Reactions  to  Stimulation. — If  an  adult  Limulus  be  placed  on  its  back,  it 
soon  becomes  quiet,  except  that  after  long  intervals  the  gills  are  raised  and  lowered 
a  few  times.  If,  during  the  quiescent  condition,  the  jaw-like  spurs,  or  mandibles, 
at  the  base  of  the  legs  are  gently  rubbed  with  some  hard  object,  such  as  a  piece 
or  wood,  glass,  or  iron;  or  if  water  the  temperature  of  the  surrounding  medium  be 
gently  poured  over  them;  or  if  the  animal  be  vigorously  fanned,  or  loud  noises 
be  made  near  it,  only  slight,  aimless  movements  of  the  legs  or  abdomen  are  pro- 
duced; usually  none  at  all.  But  if  a  very  small  piece  of  clam,  not  more  than  two 
or  three  millimeters  long,  is  gently  laid  on  the  surface,  say  of  the  third  mandible 
on  the  left  side,  care  being  taken  not  to  touch  any  other  parts,  that  leg  will  be 
repeatedly  raised  and  the  tip  bent  toward  the  mouth,  while  its  mandible  will 
move  back  and  forth,  alternating  with  the  leg  movement.  Meantime  all  the  other 
mandibles  and  appendages  are  motionless.  One  may  start  in  this  way  one  append- 
age after  the  other  (except  the  first  and  last  pairs),  until  all  of  them,  first  on  one  side 
and  then  on  the  other,  are  in  action. 

If  all  the  jaws  are  stimulated  with  food  at  the  same  time  the  normal  chewing 
reaction  takes  place  as  follows :  The  second  and  fourth  pairs  of  mandibles  move 
in  unison  inward  toward  the  median  plane,  and  downward  toward  the  mouth; 
then  back  again  in  the  reverse  order.  When  they  are  farthest  from  the  mouth 
the  corresponding  legs  (except  the  second  pair  in  both  males  and  females)  are 
quickly  raised,  flexed,  and  the  tips  carried  toward  the  mouth,  where  they  remain 
an  instant,  and  then  fall  back  on  to  the  under  side  of  the  carapace;  the  corre- 
sponding jaw  movement  then  begins  again.  The  third  and  fifth  pairs  of  append- 
ages and  the  corresponding  jaws  work  in  unison  in  the  same  manner,  but  they 
alternate  with  those  of  the  second  and  fourth.  At  intervals  these  movements 
cease,  the  abdomen  is  raised,  and  the  stout  crushing  mandibles  on  the  sixth  pair 
of  appendages,  which  have  heretofore  remained  motionless,  are  slowly  closed  with 
great  force,  as  though  to  crush  some  object  too  large  to  be  swallowed  whole,  or 
to  kill  some  struggling  prey.  These  powerful  jaws  then  slowly  relax  their  con- 
vulsive grasp,  and  the  chewing  movements  are  resumed. 

All  these  movements  go  on  with  the  greatest  precision  and  regularity,  so  that 
the  food  that  was  placed  on  the  jaws  is  forced  into  the  mouth  and  down  the 
oesophagus. 

A  drop  of  clam  water  is  sufficient  to  start  the  whole  reaction,  which  is  per- 
formed in  the  same  manner  as  during  the  actual  process  of  eating. 

If  wads  of  blotting  paper  are  used,  wet  with  ammonia  or  picric  acid,  the 
chewing  movements  are  reversed,  and  the  offensive  object  may  be  snapped  up  by 
the  chelicerae  and  rejected. 

Strong  smelling  food  held  close  to  the  mouth,  or  to  the  jaws,  produces  no 
effect,  although  chewing  movements  are  instantly  produced  when  the  jaws  are 
touched  by  it. 


GUSTATORY    ORGANS.       FLABELLUM. 


If  the  mandibles  on  one  side  are  stimulated,  the  chelicera  of  that  side,  al- 
though not  stimulated  itself,  extends  rigidly  backward,  or  waves  aimlessly  back 
and  forth  snapping  its  chelae  and  thrusting  the  tip  of  the  appendage  into  the  mouth. 
If  the  jaws  on  the  opposite  side  are  now  stimulated,  the  chelicera  on  that  side 
begins  to  work  also. 

The  chewing  reactions  can  only  be  produced  by  stimulating  the  spines  on 
the  mandibles,  or  the  smooth,  under  surface  of  the  inner  mandibular  spur.  Stimu- 
lating the  skin  around  the  mouth,  or  in  it,  does  not  produce  the  chewing  reflexes. 
If  the  mandibles  are  amputated,  no  reaction  in 
the  leg  so  treated  occurs.  If  the  spines  are  shaved 
off,  the  reaction  is  produced  only  after  strong  stimu- 
lation, or  by  stimulating  the  under  surface  of  the 
inner  mandible. 

It  is  thus  evident  that  we  are  dealing  with  true 
taste  organs,  and  that  they  must  be  located  in  the 
mandibular  spines. 

b.  Structure  of  the  Gustatory  Organs.  —  The 
mandibular  spines  are  thickly  covered  with  minute 
pores  arranged  in  vertical  lines.  (Fig.  83,  A.)  The 
pores  lead  into  canals,  each  of  which  contains  a 
long,  slender  chitenous  tubule  that  terminates  flush 
with  the  outer  surface.  The  chitenous  tubule 
contains  an  exceedingly  fine,  hair-like  prolongation 
of  a  gustatory  cell.  Toward  its  inner  end,  the 
tubule  expands  into  a  peculiar  spindle,  beyond 
which  lies  the  nucleated  cell  body.  The  gustatory 
cells  are  united  into  spindle-shaped  clusters,  each 
cluster  corresponding  to  a  single  line  of  pores. 
The  central  ends  of  the  cells  are  continued  as  nerve 
fibers  into  the  main  gustatory  nerve,  which  extends 
over  the  surface  of  the  pedal  ganglia,  through  the 

gUStatOry     tracts,     tO     the     COmmon     Centers    in    the      of  Limulus,  showing  linear  arrangement 

cheliceral  lobes  and  hemispheres.     (Figs.  65-114.) 


FIG.  83.  —  A,  Gustatory  coxal  spine 


the  gustatory  cells,  g.s.c.,  spindles,  sp, 

*  *  and  chitenous  end  tubes,  sch.t.  highly 

magnified.       C.D,    Details    of    surface 

The  flabellum  is  a  large  spatulate  organ,  one    terminals 

to  one  and  one-half  inches  long,  attached  to  the  outer  side  of  the  coxal  joint 
of  the  sixth  leg.  It  lies  in  a  channel  leading  into  the  respiratory  chamber,  so 
that  the  water  going  to  the  gills  passes  over  its  flat  anterior  surface.  The  latter 
is  perforated  with  innumerable  canals  that  afford  an  opening  for  the  elements 
of  the  underlying  sense  organ,  the  most  voluminous  one  in  the  whole  body. 

Each  canal  contains  the  outer  end  of  a  pear-shaped  sense  bud  composed 
of  eight  to  twelve,  or  more,  sense  cells.     The  buds  are  loosely  united  into  small 

8 


GENERAL   AND    SPECIAL    CUTANEOUS    SENSE    ORGANS. 

groups  that  are  surrounded  by  an  ill  defined  sheath  and  supplied  by  a  single 
nerve.     (Fig.  84.) 

The  slender  outer  ends  of  the  sensory  cells  unite  to  form  a  dense,  conical 
body,  enclosed  in  a  bulb-like  enlargement  at  the  base  of  a  chitenous  tubule. 
Before  uniting,  the  cell  ends  become  especially  distinct  and  each  one  develops  a 
minute,  bead-like  swelling. 


if 


FIG.  84. 


FIG.  85. 


FIGS.  84  and  85. — A,  Section  through  the  anterior  surface  of  the  flabellum  of  an  adult  Limulus,  showing  four 
flabellar  sense  organs — von  Rath's  preparation;  B,  section  throngh  one  of  the  gill  warts  of  an  adult  Limulus,  show- 
ing the  peculiar  bell- shaped  terminal  "  hairs"  and  the  associated  cluster  of  sensory  cells  and  chitenous  tubules — von 
Rath's  preparation;  C,  terminal  hair  of  a  gill  wart,  more  highly  magnified;  D,  diagram  of  a  slime  bud,  Limulus;  E, 
taste  bud  from  the  pharynx  of  an  embryo  Catastomus  (after  Johnston) ;  F,  a  taste  organ  from  the  skin  of  an 
adult  Lampetra  (ajter  Johnston);  G,  a  neuromast  from  the  skin  of  Catastomus  (after  Johnston) ;  H,  diagram  of  an 
arachnid  sense  bud. 

The  apex  of  the  cone  extends  outward  as  an  exceedingly  minute  fiber,  through 
a  small  chitenous  tubule,  probably  as  far  as  the  outer  surface  of  the  flabellum. 
Between  the  slender  necks  of  the  organs  are  a  few  elongated  cells,  and  similar 
ones,  but  smaller,  are  seen  in  the  canals  through  which  the  chitenous  tubules 
pass  to  the  exterior. 


BRANCHIAL    WARTS.  115 

The  thick  epidermis  is  heavily  pigmented,  and  pigment  is  frequently  seen 
in  the  body  of  the  sensory  cells. 

The  sense  buds  are  so  numerous  that  their  inner  ends  are  crowded  together 
several  rows  deep.  The  inner  surface  of  the  sensory  field  is  very  vascular,  and 
the  narrow  crevices  between  the  organs  are  often  crowded  with  blood  corpuscles. 

The  posterior  wall  of  the  flabellum,  in  marked  contrast  to  the  anterior,  con- 
tains few  or  no  sensory  perforations,  and  the  epidermis  is  thin,  nearly  colorless, 
and  with  few  blood-vessels. 

The  flabellum  is  supplied  by  a  very  large  nerve  the  root  of  which  passes  over 
the  neural  surface  of  the  sixth  pedal  ganglion  and  joins  the  main  gustatory  tracts. 
It  does  not  differ  from  the  fascicles  coming  from  the  gustatory  cells  in  the  man- 
dibles, except  that  it  is  larger.  It  appears  to  form  the  greater  part  of  the  conspic- 
uous neuropile  enlargements  seen  on  the  median  face  of  each  crus.  (Fig.  65.) 

The  flabellum  doubtless  serves  to  test  the  quality  of  the  water  that  is  drawn 
into  the  gill  chamber.  I  have  not  been  able  to  detect  any  characteristic  reactions 
when  it  is  stimulated. 

The  flabellum  probably  represents  the  exopodite  of  the  sixth  pair  of  append- 
ages. Traces  of  similar  organs  are  seen  for  a  short  time  at  the  base  of  the  other 
appendages.  (Fig.  141.) 


The  Branchial  Warts. — The  branchial  warts  are  blister-like  elevations  about 
four  mm.  in  diameter,  located  on  the  endopodites  of  the  branchial  appendages. 
They  are  covered  by  a  soft,  bluish  chiten,  and  lie  either  folded  over  the  margin, 
half  on  each  side  of  the  gill,  or  in  pairs,  one  member  on  the  anterior,  the  other 
opposite  to  it  on  the  posterior  surface  of  the  appendage.  (Fig.  82.) 

The  outer  surface  is  thickly  and  uniformly  covered  with  goblet,  or  bell- 
shaped  hairs,  deeply  set  in  conical  recesses.  There  are  two  distinct  sizes,  evenly 
distributed  in  about  the  proportion  of  five  small  ones  to  one  large.  The  large 
bell-shaped  hairs  lie  over  the  outer  ends  of  large  canals  which  contain  spirally 
coiled,  and  very  distinct  chitenous  tubules.  (Fig.  85,  Br.  and  C.) 

The  canals  are  colorless  and,  except  for  the  tubule,  appear  to  be  empty. 
They  do  not  contain  blackened  fibers  or  nuclei  such  as  occur  in  the  flabellar 
canals.  The  tubule  springs  from  a  small,  fusiform  cluster  of  sensory  cells  lying 
well  below  the  surface.  Thick  nerve  bundles,  remarkable  for  the  large  ganglion 
cells  scattered  over  them,  leave  the  inner  ends  of  the  cell  clusters  and  uniting  with 
the  other  bundles  form  a  loose  nerve  plexus,  which  is  continuous  with  termina 
branches  of  the  branchial  nerve. 

The  smaller  goblet  hairs  are  without  visible  tubules,  and  their  faint,  under- 
lying canals  do  not  perforate  the  outer  chitenous  layers.  They  do  not  appear 
to  be  connected  with  nerves. 

The  inner  surface  of  the  flexible  chiten  that  covers  the  branchial  warts  ex- 
tends inward  in  the  form  of  thin,  vertical  walls  that  form  a  coarse,  polygonal 


IZ6  GENERAL   AND    SPECIAL    CUTANEOUS    SENSE    ORGANS. 

network  when  seen  in  surface  views.  They  are  covered  with  a  thick  epithelium* 
giving  them  in  sections  a  false  appearance  of  sensory  infoldings. 

Between  two  opposing  organs  is  a  loose,  areolar  tissue  and  a  conspicuous 
venous  chamber. 

These  organs  are  clearly  of  a  very  special  kind.  In  addition  to  their  unusual 
naked  eye  appearance,  they  are  peculiar  in  the  shape  of  the  terminal  goblet  hairs; 
in  the  absence  of  cells  or  fibers  in  the  underlying  canals;  in  the  thick  walled,  spiral, 
chitenous  tubule;  in  the  ganglionated  nerve  branches,  and  in  their  inflated  elastic 
walls  that  lie  on  opposite  sides  of  the  appendage. 

Their  peculiar  structure  indicates  that  they  may  be  provisionally  regarded 
as  a  kind  of  pressure  guage  which  aids  in  the  control  of  the  heart  beat  and  the 
respiratory  movements. 


The  Slime  Buds. — Slime  buds  are  spherical  masses  of  glandular  cells,  mingled 
with  nervous  or  sensory  ones,  and  richly  supplied  with  nerves. 

They  vary  greatly  in  their  grade  of  development,  and  in  the  relative  number 
and  size  of  their  sensory  and  glandular  cells.  They  are  scattered  over  the  whole 
surface  of  the  body,  but  are  especially  abundant  in  certain  regions,  or  areas,  that 
are  known  to  be  highly  sensitive. 

While  at  first  sight  they  appear  to  be  merely  integumentary  glands,  closer 
examination  raises  many  important  questions  that  are  difficult  to  answer,  as,  for 
example,  in  regard  to  their  function,  minute  structure,  and  development. 

It  is  probable  that  they  play  an  important  part  in  the  reactions  toward  certain 
kinds  of  stimuli,  but  whether  their  secretions  serve  to  protect  the  adjacent 
nerve  buds  against  excessive  stimulation  (which  seems  to  me  very  improbable) ,  or 
as  absorbers  and  intensifies  of  certain  substances,  has  not  been  demonstrated. 

A  familiar  illustration  of  a  similar  condition  in  vertebrates  is  the  association 
of  slime,  or  mucous  cells  with  the  sense  organs  of  the  lateral  line.  There  is  also 
an  intimate  association  of  mucous  and  sensory  cells  in  many  molluscs,  i.e.,  in 
Lima,  Area  Noae,  and  in  the  tentacles  of  Haliotis. 

The  facts  that  have  special  significance  for  our  problems  are  as  follows : 

1.  Secretion  of  mucus.     When  small  Limuli  are  violently  stimulated,  the 
slime  buds  discharge  an  abundance  of  mucus,  and,  if  the  surface  of  the  shell 
has  been  previously  wiped  dry,  it  may  be  seen  to  collect  in  small  drops  over  each 
pore.     When  allowed  to  accumulate,  it  forms  a  thick  slimy  covering  to  the  whole 
surface. 

2.  Distribution.     The  slime  buds  are  very  numerous  in  certain  well  defined 
areas  which,  from  their  location  and  abundant  nerve  supply,  have  every  indication 
of  being  highly  sensory,  as,  for  example,  in  the  olfactory  organs,  in  the  mandibular 
spurs  of  Limulus,  and  the  maxillaria  of  the  second  and  third  pairs  of  thoracic 
appendages  in  the  scorpion. 

3.  Innervation.     These  groups  of  slime  buds  are  innervated  by  special  nerves, 


SLIME    BUDS.  117 

or  by  the  same  nerves  that  supply  the  adjacent  gustatory  organs.  Those  that  are 
scattered  over  the  general  surface  of  the  body  are  supplied  by  branches  from  a  sub- 
cutaneous plexus,  formed  by  the  ramifications  of  the  general  cutaneous  branches 
of  the  haemal  nerves.  In  the  appendages,  the  subdermal  plexus  arises  from  the 
general  cutaneous  branches  of  the  neural  nerves. 

4.  Structure.  Slime  buds  are  found  in  many  Crustacea  and  arachnids,  and, 
although  but  little  is  known  about  them,  they  appear  to  have  a  similar  structure 
to  those  in  Limulus. 

The  slime  buds  differ  in  appearance  in  different  regions,  and  apparently  at 
different  times.  They  are  generally  spherical  or  oval,  with  a  small  central  space 


FIG.  86. — Anterior,  or  outer,  surface  of  the  branchial  appendage  of  a  young  Limulus,  two  inches  long.  A,  a 
portion  of  the  subdermal  plexus  of  nerve  fibers,  with  clusters  of  bipolar  sense  cells  whose  outer  ends  terminate  in 
minute  chitenous  spikes;  A,  one  of  the  sense  buds,  more  highly  magnified,  with  its  chitenous  tubule,  ch.t.,  that 
conveys  the  terminal  fibers  to  the  surface;  B,  two  apparently  isolated  multipolar  ganglion  cells,  lying  just  below, 
or  in,  the  surface  ectoderm  of  the  branchial  appendage;  C,  four  multipolar  ganglion  cells  from  the  same  region. 
Methylene  blue. 


from  which  a  chitenous  tubule  leads  to  the  exterior.  (Fig.  88.)  This  tubule 
may  or  may  not  be  convoluted  near  its  origin,  but  it  generally  terminates  in  a 
straight  delicate  tubule  that  cannot  be  distinguished  from  those  covering  the 
outer  ends  of  the  gustatory  and  temperature-cells.  All  the  tubules  aie  shed  with 
the  old  shell  at  ecdysis.  They  may  be  seen  protruding  a  considerable  distance  from 
the  inner  surface  of  the  cast  off  shell  that  have  been  cleaned  with  boiling  potash. 
The  slime  buds  contain  at  least  two  different  kinds  of  cells,  namely,  true 
slime  cells,  which  may  constitute  the  greater  part  of  the  organ,  and  one  or  more 
sensory  cells.  The  slime  cells  vary  greatly  in  appearance.  In  the  typical  ol- 
factory and  mandibular  slime  buds,  they  are  irregularly  conical  or  cylindrical, 
their  walls  are  sharply  defined,  and  they  contain,  at  their  pointed  central  ends,  a 
mass  of  refractive  colorless  spherules.  The  enlarged  peripheral  ends  of  the  cells, 


n8 


GENERAL   AND    SPECIAL    CUTANEOUS    SENSE    ORGANS. 


in  which  is  located  the  small  nucleus,  may  be  finely  granular,  staining  a  dark 
gray  or  bluish-black  in  von  Rath's  fluid.  (Fig.  88.) 

Each  slime  bud  contains  a  single  ganglionic  or  sensory  cell,  distinguished 
from  all  the  others  by  its  large  size,  dark,  finely  granular  protoplasm  and  indis- 
tinct outline.  This  cell  appears  to  be  larger  and  more  distinct  in  the  mandibular 
slime  buds  than  it  is  in  the  olfactory  buds. 

Between  the  outer  ends  of  the  slime  cells  there  are  minute,  rod-like  bodies 
with  a  dilatation  at  their  inner  ends.  They  have  the  appearance,  under  some 
conditions,  of  being  minute  sensory  cells,  but  I  have  not  been  able  to  fully  satisfy 
myself  that  such  is  the  case.  In  von  Rath's  fluid,  they  become  very  black, 
and  in  some  cases  hair- like  processes  appear  to  project  from  them  into  the  cavity 
of  the  slime  bud,  where  they  unite  to  form  a  small  star-like  body.  "(Fig-  85,  D.) 

In  some  cases,  the  buds  are  greatly  distended  and  the  cells  appear  nearly 


FIG.   88.  FIG.    87. 

FIG.  87. — A  cluster  of  ganglion  cells  terminating  in  a  sub-dermal  plexus  of  anastomosing  fibers;  from  the  soft  skin 

between  the  joints  of  the  endopodites  of  the  branchial  appendages  of  a  young  Limulus.     Methylene  blue. 
FIG.  88.— Two  slime  buds  grom  the  olfactory  region  of  an  adult  Limulus — von  Rath's  prepration.   a.  Groups  of  cells 
of  unknown  significance;  c.c,  central  coagulum  resting  on  hair-like  projections. 

colorless  and  empty,  as  though  after  a  certain  period  of  activity  they  were  about 
to  degenerate. 

New  slime  buds,  that  have  arisen  de  novo  from  the  indifferent  ectoderm, 
or  by  the  division  of  the  existing  buds,  appear  in  the  older  stages. 

5.  In  the  vertebrates,  there  is  a  similar  association  of  sensory  and  mucous 
cells  in  the  lateral  line  organs.  For  a  long  time  it  was  assumed  that  the  lateral 
line  canals  were  primarily  slime  producing  organs,  and  nothing  more.  When 
the  sense  organs  in  the  canals  were  discovered,  the  associated  mucous  cells  were 
apparently  forgotten. 

In  the  lower  vertebrates,  the  typical  taste  organs  form  small  clusters  of  sensory 
cells,  resembling  in  structure  and  innervation  the  taste  organs  and  the  flabellar 
organs  in  Limulus.  (Fig.  85,  E.)  The  typical  lateral  line  organs,  or  neuromasts, 
consist  of  short,  hair-bearing  sense  cells,  united  with  longer  so-called  "supporting," 
or  indifferent  cells,  which  may  or  may  not  secrete  mucous  (Maurer).  Thisassocia- 


SLIME    BUDS.  119 

tion  of  mucous  cells  and  hair  cells  in  one  organ  is  comparable  with  the  association 
of  sensory  and  mucous  cells  in  the  slime  buds  of  arachnids  and  in  the  olfactory 
organs  of  insects  (Necrophorus,  Dahlgren  and  Kepner).  If  the  short,  rod-like 
bodies  in  Limulus  are  true  sensory  cells,  then  the  morphological  resemblance 
between  an  arachnid  slime  bud  and  a  vertebrate  neuromast  is  very  striking. 
(Figs.  85,  D-H.) 

According  to  Maurer,  there  are  some  cases  in  the  vertebrates  where  the 
lateral  line  organs  still  remain  in  a  condition  approaching  that  in  the  arachnids, 
for  he  regards  the  slime  buds  in  Myxine  and  Bdellostoma  as  probably  representing 
modified  lateral  line  organs  of  Petromyzon.  In  other  words,  in  Myxine  and 
Bdellostoma,  the  mucous  sacs  are  sense  buds,  in  which  all  or  nearly  all  the  cells 
secrete  mucous. 

In  the  vertebrates,  however,  the  secreting  function  is  usually  relegated  to 
separate  cells  in  the  adjacent  ectoderm,  the  "supporting"  cells  apparently  re- 
taining their  secreting  function  only  in  exceptional  cases  (Maurer).  In  reply 
to  an  inquiry  on  this  point,  Prof.  C.  Judson  Herrick  writes  me  that  "the  line 
organs  of  vertebrates  are  so  exceedingly  variable  that  I  would  not  venture  to 
generalize,  with  my  present  knowledge,  on  the  relation  between  the  sensory  and 
the  mucous  cells;  but  certainly  in  some  cases,  and  I  think  as  a  rule,  they  are  closely 
associated.  The  mucous  cells  are  I  think  generally  absent  in  the  non-sensory 
parts  of  the  lining  of  the  canals.  As  to  the  function  of  the  mucus,  I  have  hitherto 
regarded  it  as  like  the  mucus  of  the  general  body  surfaces,  protective.  But 
in  view  of  Parker's  work  on  the  function  of  the  lateral  canal  sense  organs  as  re- 
ceptors for  slow  vibrations,  it  may  be  that  the  mucus  and  the  cilia  of  the  hair 
cells  both  enter  into  the  formation  of  the  cupula  which  overlies  the  lateral  line 
organs  much  as  in  the  case  of  ampullae  of  the  internal  ear  and  that  the  whole  cupula 
assists  in  the  stimulus  of  the  sensory  cells." 

However,  it  is  clear  that  we  must  go  farther  back  than  primitive  vertebrates 
for  our  explanation.  In  Limulus,  for  example,  we  have  the  same  kind  of  gland 
cells  intimately  associated  with  cutaneous  sense  organs,  and  it  is  extremely  improb- 
able that  the  abundant  mucus  there  serves  either  to  protect  an  already  practically 
impervious  covering,  or  to  assist,  by  slow  vibrations,  in  the  stimulation  of  the 
sensory  cells. 

6.  The  function  of  the  slime  buds  in  Limulus  is  not  apparent.  The  presence 
of  the  mucoid  secretion  is  obvious  enough  in  both  vertebrates  and  arachnids,  but 
a  satisfactory  explanation  of  its  purpose  is  not  available;  and  it  is  difficult  to 
account  for  the  rich  innervation  of  these  organs  in  Limulus,  or  for  the  presence  of 
sensory  or  nerve  cells  in  them,  or  for  their  association  with  other  sense  organs, 
on  the  ground  that  they  are  mucous  glands  and  nothing  more. 

The  only  conclusion  open  to  us  at  present  is  that  first  suggested  by  me  in  1889, 
namely  that  in  the  arachnids  and  primitive  vertebrates,  the  mucous  secretion  serves 
to  absorb  certain  chemical  substances  held  in  solution,  and  to  thus  intensify 
their  action  on  the  nerve  ends.  This  explanation  would  account  for  the  abund- 


120  GENERAL    AND    SPECIAL    CUTANEOUS    SENSE    ORGANS. 

ance  of  mucous  in  the  olfactory  and  gustatory  organs,  and  for  its  absence  in  the 
tactile  or  auditory  ones. 

The  mandibular  slime  buds  are  sufficiently  numerous  to  suggest  that  they 
are  in  the  nature  of  salivary  glands.  This,  however,  does  not  seem  probable,  since 
there  is  no  way  to  get  the  secretions  into  the  mouth  with  the  food;  and  the  mem- 
branes immediately  within,  or  surrounding  the  mouth  are  entirely  devoid  of  these 
organs.  Moreover,  it  is^certain  that  the  precisely  similar  slime  buds  in  the  ol- 
factory fields,  and  in  the  integument  of  the  back  or  branchial  chamber,  cannot  be 
regarded  as  salivary  organs. 

7.  The  slime  buds  of  Limulus  and  other  arachnids  are  found  in  segmentally 
arranged  fields,  or  groups,  that  are  supplied  by  special  nerves,  the  most  conspicuous 
groups  being  those  in  the  olfactory  organ,  in  the  mandibles  of  the  second  to  the 
fifth  thoracic  appendages,  and  in  the  rudimentary  vagus  appendages  (scorpion). 
These  organs  appear  at  an  early  embryonic  period  as  thickenings  of  the  ectoderm 
and  in  close  association  with  the  cranial  ganglia. 

The  Auditory  Organ. — In  my  first  contribution,  1889,  I  maintained  that 
the  large  segmental  sense  organ,  which  in  Limulus  embryos  lies  opposite  the  fourth 
pair  of  legs,  was  the  probable  forerunner  of  the  vertebrate  ear.  I  see  no  reason 
to  change  my  opinion  on  this  point.  Although  the  evidence  in  favor  of  this  con- 
clusion is  not  voluminous,  it  is  sufficiently  precise  as  far  as  it  goes.  In  Limulus, 
the  organ  in  question  is  a  large  discoidal  placode,  of  a  sensory  nature  (Figs.  131, 
140  to  153),  strikingly  like  the  auditory  placode  of  vertebrates  in  its  general  outward 
appearance,  in  its  minute  structure,  and  in  the  fact  that  it  is  located,  as  nearly  as 
one  may  determine,  on  the  same  segment  of  the  head,  using  as  a  guide  either 
the  history  of  the  oral  arches  (Figs.  29-34),  or  the  number  of  the  corresponding 
brain  neuromere.  (Fig.  57.) 

It  is  assumed  that  in  the  primitive  vertebrates  this  particular  placode,  which 
lies  at  the  head  of  the  posterior  division  of  the  thorax,  formed  a  simple,  sac-like 
infolding,  similar  to  the  auditory  sac  in  decapods,  and  that  from  this  sac  developed 
the  inner  ear  of  vertebrates. 

The  placode  belongs  to  the  same  series  as  the  visual  and  olfactory  organs. 
(Figs.  140-148,  s.o4.)  It  increases  in  size  up  to  the  time  of  hatching.  During  the 
early  trilobite  stage,  the  cells  become  slightly  pigmented,  take  on  a  sensory  ap- 
pearance, and  a  lens-like  thickening  of  the  overlying  chiten  is  formed  over  it. 
(Fig.  131,5.)  The  organ  disappears  completely  at  the  close  of  the  trilobite  stage. 

That  is  as  far  as  the  evidence  goes.  There  is  no  evidence  that  the  placode  in 
Limulus  is  auditory;  or  that  it  is  serially  homologous  with  the  antennal  auditory 
organs  of  decapods,  although  that  is  not  improbable. 

Gaskell  regards  the  flabellum  of  Limulus,  or  the  pectines  of  the  scorpion,  as 
the  precursor  of  the  vertebrate  auditory  organ ;  but  they  lie  much  too  far  back  in 
the  head  to  be  compared  with  the  ear  of  vertebrates.  His  description  of  the 
minute  structure  of  the  flabellum  is  very  inaccurate,  and  his  intimation  that  it  is 


LATERAL   LINE    ORGANS.  121 

an  auditory  organ,  possibly  homologous  with  the  pectines  of  scorpions,  is  con- 
trary to  well  established  facts. 

Lateral  Line  Organs  of  Vertebrates.  Summary  and  Comparison.— 
The  lateral  line  organs  of  vertebrates  consist  of  several  distinct  groups  that  arise 
at  an  early  embryonic  period  from  the  neural  surface  of  the  head.  Each  line  of 
organs  makes  its  appearance  as  an  oval  thickening  of  the  ectoderm,  located  be- 
tween the  dorsal  extremity  of  a  gill  arch  and  the  lateral  margin  of  the  medullary 
plate.  (Figs.  26-34.)  The  thickening  gradually  extends  in  a  peripheral  direc- 
tion, and  as  it  does  so  it  separates  into  a  superficial  linear  series  of  sense  buds 
and  an  accompanying  underlying  nerve  and  ganglion.  Subsequently  an  in- 
folding of  the  ectoderm  may  take  place  along  the  line  of  growth,  forming  first  an 
open  groove  and  then  a  canal,  in  which  the  organs  are  located  at  regular  intervals. 
Finally  the  several  canals  may  unite,  forming  a  continuous  system,  but  each  part 
that  was  originally  a  distinct  canal  is  innervated  by  a  special  cranial  nerve. 
(Fig.  89.) 

It  has  been  suggested  that  the  anlage  of  each  canal  represents  a  very  ancient 
sense  organ  (the  so-called  branchial  sense  organ),  but  so  far  as  I  know,  no  ex- 
planation has  been  offered  for  the  extraordinary  fact  that  these  ancient  organs 
must  have  originated,  not  around  or  close  to  the  vertebrate  mouth,  as  one  would 
naturally  suppose,  but  from  the  opposite  or  aboral  side  of  the  head;  and  not  from 
a  single  anlage,  but  from  several. 

This  condition,  however,  is  perfectly  intelligible  as  soon  as  we  recognize  that 
the  whole  system  of  taste  buds  and  lateral  line  organs  of  vertebrates  represents 
the  thoracic  and  vagal  coxal  sense  organs  of  arachnids,  which  there  lie  on  the  neural 
surface  of  the  head  around  the  primitive  mouth,  the  latter  having  closed  up  and 
disappeared  in  the  vertebrates. 

When  the  sense  organs  of  the  arachnids  are  projected  on  the  neural  surface 
of  the  cephalothorax,  the  principal  groups,  each  one  containing  many  organs, 
appear  as  oval  or  circular  areas  arranged  around  the  mouth.  (Fig.  89,  A.)  We 
may  recognize  three  sets :  the  gustatory  organs  and  slime  buds  located  side  by  side 
in  the  thoracic  and  vagal  appendages,  and  the  chemotactic  general  cutaneous 
organs  located  on  the  neural  flanks  of  the  thoracic  and  branchial  regions,  and 
supplied  by  a  great  longitudinal  nerve  arising  from  one  of  the  anterior  thoracic 
neuromeres,  l.n. 

Taste  buds  predominate  in  the  second,  third,  fourth,  and  fifth  thoracic  coxae, 
i.e.,  those  immediately  surrounding  the  mouth.  Sense  cells  of  the  same  general 
type  are  abundant  in  the  flabellum  and  in  the  vagal  appendages,  but  there  they 
may  serve  as  tactile  organs,  or  for  some  other  purpose. 

The  organs  of  the  gustatory-tactile  type  and  the  slime  buds  may  arise  side 
by  side  from  the  same  anlagen,  and  they  may  be  supplied  by  the  same  nerve 
trunks  and  ganglia.  Their  later  phylogenetic  history  appears  to  follow  along  the 
same  lines  in  both  cases,  but  there  is  apparently  a  tendency  to  separate,  more  and 


122  GENERAL   AND    SPECIAL    CUTANEOUS    SENSE    ORGANS. 

more,  the  two  kinds  of  organs,  so  that  each  kind  assembles  in  particular  areas 
and  is  supplied  with  distinct  nerves  arising  from  distinct  brain  tracts.  We  shall 
here  refer  to  the  common  anlagen  of  both  sets  of  organs  as  coxal  and  vagal  sense 
organs. 

There  is  a  sharp  distinction  morphologically  between  the  anlagen  of  the 
thoracic  organs  and  those  of  the  vagal  region.  The  thoracic  anlagen  are  always 
directed  forward  and  outward  and  are  located  well  on  the  sides  of  the  thorax. 
The  vagal  anlagen  are  always  crowded  close  to  the  median  line  and  are  directed 
backward,  approximately  parallel  with  the  nerve  cord.  (Fig.  89.)  The  location 
and  direction  of  growth  of  these  organs  is  determined  by  that  of  the  appendages  to 
which  they  belong  and  is  prophetic  of  their  condition  in  vertebrates. 

When  the  oral  appendages  were  transferred  to  the  haemal  surface  (see  Chapter 
XV),  it  is  probable  that  the  anlagen  of  the  coxal  organs  were  drawn  forward  and 
outward  into  a  narrow  band,  each  one  giving  rise  to  a  row,  or  linear  series 
of  taste  organs,  the  general  course  or  direction  of  the  organs,  and  the  accompany- 
ing nerves  and  ganglia,  indicating  the  path  of  migration  of  the  corresponding 
appendage. 

The  vagal  appendages  of  the  arachnids  are  always  carried  backward,  relative 
to  the  other  parts  of  the  same  segments,  as  shown  by  the  invariable  direction  of 
their  nerves  and  ganglia.  The  conditions  that  controlled  their  movements  have 
no  doubt  continued  to  direct  the  line  of  growth  of  the  vagal  group  of  anlagen  in 
the  embryos  of  their  vertebrate -descendants. 

There  is  nothing  to  indicate  what  conditions  determined  the  backward  growth 
of  the  immense  longitudinal  cutaneous  nerve,  which  in  Limulus  arises  from  the 
first  post-oral  neuromere.  (Figs.  70,  89,  l.n.) 

The  embryological  history  of  the  lateral  line  organs  in  primitive  vertebrates 
bears  out  this  interpretation.  We  may  recognize  there  two  principal  groups  of 
organs,  one  lying  in  front  of  the  auditory  organ  and  belonging  to  the  oral  arches, 
the  other  lying  behind  the  auditory  organ  and  belonging  to  the  branchial  region 
and  trunk.  The  former  repiesent  the  coxal  organs  of  the  arachnid  thorax,  the 
latter,  the  organs  of  the  vagal  appendages.  These  two  groups  of  organs  grow  in 
the  same  general  direction  in  the  vertebrates  that  they  do  in  the  arachnids,  but 
they  have  extended  very  much  farther  in  the  former. 

In  the  vertebrates  the  several  pairs  of  anlagen  tend  to  run  together,  and  it 
is  not  clear  just  how  many  there  are  in  either  region,  or  which  ones  of  those  in  the 
arachnids  they  represent. 

The  second,  third,  fourth,  and  to  a  less  degree,  the  fifth  pairs  of  coxal  anlagen 
en  the  arachnids  are  probably  in  part  retained  in  the  vertebrates,  forming  the 
rudiments  of  lines  of  canal  organs  for  their  corresponding  appendages,  which 
have  themselves  furnished  the  basis  of  the  premaxillary,  maxillary,  mandibular 
and  hyoid  arches.  One  or  more  groups  of  vagal  sense  organs  gave  rise  to  the 
lateral  lines  of  the  branchial  region  and  the  trunk. 

********* 


LATERAL    LINE    ORGANS. 


I23 


Let  us  now  consider  the  several  lines  of  canal  organs  as  they  appear  in  ostra- 
coderms  and  primitive  vertebrates. 

In  the  ostracoderms,  they  undoubtedly  occur  in  the  most  primitive  condition 
known  in  the  adult  of  any  vertebrate-like  animal. 

In  Tremataspis  (Fig.  236),  the  organs  were  apparently  located  in  short,  shal- 
low surface  grooves;  in  Bothriolepis  (Fig.  247),  in  continuous  open  grooves. 
When  expressed  in  a  simple  diagrammatic  form,  the  sensory  grooves  of  the  ostra- 
coderms  appear  to  originate  in  the  occipital  region  and  to  radiate  from  it  in  the 
following  lines:  There  is  a  main  suborbital  (Fig.  89,  B.),  i.o.L,  continued  for- 
ward as  the  rostral  line,  r.L,  in  front  of  the  olfactory  organs.  In  Bothrio- 
lepis, a  branch  line  arises  from  it  and  extends  haemally  over  the  surface  of  the 


1UX  I 


A.  B.  C.  D. 

FIG.  89. — Schematic  figures  showing  the  location  of  the  lateral  and  median  eye,  olfactory,  auditory,  gusta- 
tory, and  canal  organs,  in  the  arachnids,  ostracoderms,  arthrodiri,  and  primitive  vertebrates.  All  figures  seen 
from  the  neural  surface. 

premaxillae.  A  mandibular  line  is  not  recognized  in  any  ostracoderm,  prob- 
ably owing  to  the  small  size  of  the  mandibles.  There  is  no  true  supra-orbital 
line,  probably  owing  to  the  median  location  of  the  lateral  eyes,  although  the 
short-post  orbital  line  of  Tremataspis  and  the  longer  one  in  Bothriolepis  possibly 
represent  the  proximal  end  of  such  a  line,  s.o.l. 

The  orbital  line  appears  to  be  continuous  with  the  lateral  line  of  the  branchial 
region  and  of  the  trunk,  by  means  of  short  glosso-pharyngeal  sections,  g.p.  Judg- 
ing from  the  embryological  conditions  in  vertebrates,  this  section  represents  a 
separate  line,  supplied  solely  by  the  glosso-pharyngeal  nerve.  The  main  lateral 
line  extends  along  the  branchial  region  and  in  Bothriolepis  may  be  traced  for  a 
short  distance  on  to  the  trunk.  There  are  two  accessory  dorso-branchial  lines 
in  Tremataspis  and  one  in  Bothriolepis. 


124  GENERAL   AND    SPECIAL    CUTANEOUS    SENSE    ORGANS. 

In  the  Arthrodira  (Fig.  89,  C),  the  most  important  advance  is  in  the  appear- 
ance of  a  distinct  supra-orbital  line,  s.o.L,  extending  forward  between  the  now 
widely  separate  lateral  eyes;  and  a  distinct  hyomandibular  line  h.ml.  extending 
toward,  and  probably  onto,  the  greatly  enlarged  mandibles.  In  the  arthrodira 
the  neuromasts  apparently  never  form  a  series  of  separate  dots  and  dashes,  but 
lie  in  continuous  grooves  of  varying  depth. 

In  true  vertebrates,  no  important  changes  or  new  conditions  arise.  The  sev- 
eral lines  may  be  deeply  infolded  and  joined  at  their  proximal  ends  to  form  a 
united  series  of  canals,  with  the  sense  organs  located  in  them  at  regular  intervals, 
suggesting  the  interrupted  surface  grooves  of  the  ostracoderms.  (Fig.  89, 
B  and  D.) 


CHAPTER  VIII. 

LARVAL  OCELLI  AND  THE  PARIETAL  EYE. 

I.  THE  DIFFERENT  KINDS  OF  EYES  IN  ARTHROPODS  AND  VERTEBRATES. 

Since  the  principal  facts  in  their  embryonic  development  became  known, 
it  has  been  generally  assumed  that  the  vertebrate  eyes  originated  inside  the  brain 
chamber,  and  that  the  retina  was  a  highly  specialized  part  of  the  brain  wall. 

There  are  fundamental  objections  to  this  interpretation,  namely:  a.  it  reverses 
the  usual  order  of  histological  development,  for  nerve  cells  are  to  be  regarded  as 
specialized  sensory  cells,  not  vice  versa;  and  b.  it  fails  to  establish  any  connection 
or  relation  between  the  eyes  of  vertebrates  and  those  that  are  almost  universally 
present,  and  often  highly  developed,  in  the  invertebrates.  In  fact,  it  neither  ex- 
plains how  the  eyes  got  into  the  brain  chamber  from  without,  nor  under  what 
conditions  they  developed  "de  novo"  from  within. 

The  arthropod  theory  is  not  open  to  these  objections,  for  we  shall  show  that 
the  evolution  of  a  cerebral  eye  has  already  taken  place  in  the  arachnids,  and  that 
the  principal  steps  in  the  process  are  recorded  there  in  great  detail. 

Eyes  of  Arthropods. 

In  the  arthropods,  we  may  recognize  four  types  of  eyes,  namely:  paired  lar- 
val ocelli;  parietal  eyes;  frontal  ocelli,  or  stemmata;  and  the  lateral  or  compound 
eyes. 

The  larval  ocelli,  of  which  there  may  be  six  pairs,  two  for  each  of  the  fore- 
brain  segments,  are  present  in  the  active  larvae  of  most  insects,  but  disappear 
during  the  metamorphosis  (coleoptera,  lepidoptera,  neuroptera,  hymenoptera). 
They  are  cup-like  infoldings  of  the  ectoderm,  with  upright  or  horizontal  retinal 
cells  or  rods.  In  the  insects,  the  retinal  cells  are  never  completely  inverted, 
and  the  ocelli  never  form  unpaired  eyes  enclosed  in  a  common  chamber  or  vesicle. 

The  Parietal  Eye. — In  the  Crustacea  and  arachnids,  two  pairs  of  ocelli  unite 
to  form  an  unpaired  ocellar  vesicle,  or  parietal  eye.  The  ocellar  placodes  remain 
more  or  less  distinct  and  form  the  side  walls  of  the  dilated  anterior,  or  distal  end 
of  the  vesicle.  The  proximal,  or  posterior  end  is  generally  tubular  and  may  open 
on  the  outer  surface  of  the  head;  or  it  may  merge  with  the  palial  folds  and  open 
into  the  forebrain  vesicle.  The  parietal  eye  usually  persists  through  life,  and  it 
may  be  the  largest  and  most  important  one  functionally. 

The  frontal  eyes  or  stemmata  of  insects  consist  of  two  pairs  or  placodes 

125 


126  LARVAL  OCELLI  AND  THE  PARIETAL  EYE. 

that  form  a  median,  tri-oculate  group.  They  arise  during  the  metamorphosis,  or 
at  any  rate  after  the  embryonic  period,  and  are  quite  independent  of  the  primitive 
ocelli.  They  are  never  involved  in  a  palial  fold  or  in  a  common  vesicle,  and 
the  retinal  cells  are,  apparently,  always  upright.  They  are  functional  eyes  only 
in  adult  insects,  or  in  the  late  larval  stages. 

In  the  arachnids  and  Crustacea  (phyllopods,  entomostraca),  the  frontal  ocelli 
are  present  in  a  highly  modified  form,  as  two  sets  of  frontal  organs  two  paired  and 
one  unpaired.  In  Limulus,  they  become  the  olfactory  organs.  In  spiders  and 
scorpions,  they  are  apparently  absent.  Their  nerve  roots  arise  from  the  median 
anterior  surface  of  the  forebrain,  or  from  the  anterior  surface  of  the  optic  ganglia 
and  hemispheres  (Limulus). 

The  lateral  or  compound  eyes  are  found  in  adult  insects,  Crustacea,  and 
arachnids,  including  the  trilobites  and  merostomes.  Like  the  stemmata,  their 
relation  to  the  primary  head  segments  cannot  be  easily  determined,  because  at  the 
time  the  cephalic  lobes  are  most  clearly  segmented,  as  in  the  embryonic  stages  of 
Acilius  and  the  scorpion,  the  lateral  eyes  are  absent,  and  they  do  not  appear,  if  at 
all,  till  near  the  close  of  larval  life.  In  Limulus  they  belong  to  the  cheliceral 
segment;  in  insects,  they  appear  to  belong  to  the  antennal  segment. 

The  development  of  the  lateral  eyes  is  essentially  the  same  in  all  arthropods. 
They  are  derived  from  large  crescentic  placodes  lying  near  the  posterior  lateral 
margin  of  the  cephalic  lobes  close  to  the  edge  of  the  infolding  for  the  optic  gang- 
lion; but  they  never  lie  inside  the  fold,  and  the  visual  cells  are  never  inverted. 
The  entire  visual  layer  is  formed  from  a  single  layer  of  primitive  ectoderm. 

The  placodes  are  frequently  divided,  or  may  be  entirely  separated,  into 
two  distinct  parts,  which  differ  in  their  histological  characters,  and  in  function 
(hymenoptera,  neuroptera,  coleoptera).  One  part  may  be  especially  developed 
in  males  (ephemeridae) ,  or  one  may  serve  for  vision  under  watei,  and  the  other  for 
vision  in  air. 

Cerebral  Eyes  of  Vertebrates. 

In  vertebrates  we  recognize  as  belonging  to  the  forebrain,  the  median  or 
parietal  eyes,  the  lateral  eyes,  and  the  olfactory  organs.  At  an  early  embryonic 
period  they  lie  on  the  outer  margins  of  the  open  neural  plate,  in  similar  positions 
to  the  ones  they  occupy  in  arthropods. 

The  Parietal  Eye. — There  are  probably  two  pairs  of  ocellar  placodes  that 
for  a  short  time  occupy  this  marginal  position.  Later,  they  are  caught  in  the 
palial  overgrowth  and  carried  on  the  inner  limb  of  the  closing  neural  crests  to 
the  median  line.  There  they  form  a  group  of  one,  or  two,  or  three  placodes  lying 
in  the  membranous  roof  of  the  brain.  During  or  after  the  closing  of  the  cerebral 
vesicle,  the  brain  roof  is  evaginated  at  the  place  where  the  ocelli  are  located,  thus 
forming  a  sac  or  tube  in  the  blind  end  of  which  the  ocellar  placodes  lie. 

The  extraordinary  way  in  which  the  vertebrate  parietal  eye  develops  is, 


CEREBRAL  EYES  OF  VERTEBRATES.  127 

therefore,  essentially  like  that  of  the  parietal  eye  in  Limulus  and  the  scorpion. 
This  fact,  and  many  others  to  be  brought  out  later,  demonstrates  that  the  parietal 
eye  of  the  Crustacea  and  arachnids  is  a  true  cerebral  eye  in  the  vertebrate  sense, 
and  is  identical  with  the  parietal  eye  of  vertebrates. 

The  lateral  eyes  of  vertebrates  represent  the  compound  or  convex  eyes  of 
arthropods  that  have  been  transferred  to  the  interior  of  the  cerebral  vesicle.  In 
the  arthropods  the  lateral  eyes  lie  near  the  margin  of  the  cephalic  lobes,  on  the 
outer  edge  of  a  deep  ganglionic  infolding.  In  vertebrates,  they  are  first  seen  in  a 
very  similar  position  on  the  lateral  margin  of  the  open  medullary  plate.  Later 
they  are  swept  into  the  infolding  brain,  turning  the  retinas  inside  out.  They  then 
grow  out  laterally  on  the  end  of  membranous  tubes,  in  much  the  same  manner  as 
the  median  eyes.  In  arthropods,  the  lateral  eyes  usually  have  a  crescentic,  or 
kidney-shaped  outline;  in  vertebrates,  this  shape  is  retained,  giving  the  retinas 
their  characteristic  crescentic  outline  during  the  early  stages.  When  the  two 
limbs  of  the  crescent  unite,  a  circular  retina  is  produced,  giving  rise  to  the  choroid 
fissure  and  the  centrally  located  optic  nerve  that,  together  with  the  inverted 
rods  and  cones,  have  long  been  such  inexplicable  features  of  the  lateral  eyes  in 
vertebrates. 

The  olfactory  organ  in  vertebrates  arises  from  three  placodes  situated  on  the 
anterior  margin  of  the  cephalic  lobes.  They  are  not  drawn  into  the  brain  cham- 
ber, but  remain  permanently  in  the  surface  ectoderm.  They  move  forward 
along  the  median  line  followed  by  two  pairs  of  olfactory  nerves,  that  in  the  lower 
vertebrates  may  remain  separate  up  to  the  adult  stages.  Its  structure,  develop- 
ment, and  innervation  is  therefore  similar  to  that  of  the  frontal  organs  of  the 
Crustacea,  and  the  olfactory  organ  of  Limulus. 

II.  THE  EYES  AS  SEGMENTAL  SENSE  ORGANS. 

The  larval  ocelli,  lateral  eyes,  auditory  organs,  stemmata  and  olfactory  or- 
gans appear  to  be  local  modifications  of  a  series  of  primitive  sense  organs  belong- 
ing to  the  procephalic  and  first  six  thoracic  metameres. 

In  insects  and  arachnids,  the  larval  ocelli  of  the  procephalic  lobes  present  a 
clearly  defined  segmental  arrangement.  (Fig.  14.)  In  scorpion  and  Limulus, 
in  addition  to  these  ocelli,  there  is  a  transient  series  of  segmental  sense  organs  in 
the  thorax,  which  appears  to  be  a  continuation  of  that  in  the  forebrain.  (Figs.  15, 
16,  140-142.) 

In  Limulus,  the  first  pair  of  the  thoracic  series  are  the  lateral  eye  placodes,  I.e. 
The  fourth  pair,  s.o4,  are  large,  circular  placodes,  distinctly  sensory  in  character, 
that  are  retained  through  the  first  larval  or  trilobite  stage,  after  which  they  dis- 
appear. This  organ  is  probably  the  forerunner  of  the  auditory  organ  of  verte- 
brates for  it  has  the  same  shape  and  general  appearance  as  the  auditory  placode 
in  vertebrate  embryos,  and  as  nearly  as  may  be  determined,  lies  on  the  same 
segment.  The  other  placodes  are  less  distinct  and  are  visible  for  a  very  short 
period  only. 

In  scorpions,  on  the  outer  margins  of  each  coxal  joint  (Figs.  15-16),  there 


128 


LARVAL    OCELLI  AND    THE    PARIETAL    EYE. 


are  two  transitory  sense  organs  which  appear  to  represent  the  thoracic  series  of 
Limulus.  They  disappear  before  hatching,  after  contributing  an  important  mass 
of  ganglion  cells  to  the  pedal  nerves. 

The  series  of  procephalic  and  thoracic  sense  organs  just  described  should  not 
be  confused  with  the  segmentally  arranged  gustatory  organs,  which  belong  to  a 
different  system,  and  which  are  always  located  on  the  median  side  of  the  base  of 
the  appendages. 

After  this  preliminary  survey,  we  may  consider  the  several  organs  under  dis- 
cussion in  more  detail. 

III.  THE  OCELLI  OF  INSECTS. 

A  very  primitive  and  suggestive  condition  is  seen  in  Acilius,  where  the  early 
history  of  the  ocelli  is  best  known.  Here  the  cephalic  lobes  are  clearly  divided 
into  three  segments,  each  one  containing  a  segment  of  the  brain,  one  of  the  optic 
ganglion,  and  one  of  the  optic  plate.  (Fig.  14.)  Three  deep  infoldings,  iv,  l~3, 


FIG.  90.— The  ocellus  of  an  insect  larva,  Acilius  (eye  V).     This  ocellus  looks  forward  and 


outward. 


form  on  the  median  side  of  the  plate,  carrying  the  three-lobed  optic  ganglion  be- 
low the  surface.  The  openings  soon  close,  without  the  formation  of  a  palial  fold 
like  that  which  covers  the  whole  forebrain  in  the  scorpion. 

ft.  The  ocelli  are  formed  by  separate,  pit-like  infoldings  of  the  optic  plates,  the 
retina  forming  from  the  bottom  of  the  pits  and  the  dioptric  apparatus  from  the 
lips  of  the  closed  vesicles.  (Figs.  90-91  and  102.) 

At  the  close  of  larval  life,  the  ocelli  break  away  from  the  surface  ectoderm 


PARIETAL    EYE    OF    THE    SCORPION. 


I29 


and  become  lodged  deep  in  the  head,  on  the  surface  of  the  optic  ganglia,  where 
they  degenerate. 

The  frontal  ocelli  are  new  formations,  usually  appearing  at  the  beginning  of 
the  metamorphosis,  and  differing  from  the  larval  ocelli  in  their  mode  of  develop- 
ment, time  of  appearance,  and  relation  to  the  brain. 

IV.  THE  PARIETAL  EYE. 


Parietal  Eye  of  the  Scorpion. 

The  development  of  the  parietal  eye  in  the  scorpion  and  spiders  furnishes 
the  best  picture  of  the  process  by  which  ocelli  are  carried  into  the  brain 
chamber  to  form  a  true  parietal  eye  like  that  in 
vertebrates. 

The  evolution  of  the  brain  chamber  and 
the  parietal  eye  is  essentially  the  same  in  scor- 
pions and  spiders.  (Figs.  15,  20,  21.)  I  will 
describe  the  condition  in  the  former. 

The  cephalic  lobes  soon  divide  into  three 
segments  that  have  a  very  constant  and  charac- 
teristic form  in  the  arachnids.  (Fig.  15.)  One 
may  distinguish  the  centrally  located  brain  neu- 
romeres,  br.*~3,  two  prominent  optic  ganglia,  and 
a  marginal  plate,  with  deep  infoldings  between  it 
and  the  ganglia. 

The  whole  of  the  first  segment  forms  a  dark 
infolded  band,  extending  across  the  anterior 
margin  of  the  cephalic  lobes.  From  it  is  formed 
the  olfactory  lobes  (organ  stratifie  of  St.  Remy). 

The  lateral  lobe  of  the  second  segment  forms 
the  optic  ganglion  of  the  median  eyes,  p.e.g., 
and  the  one  behind  forms  the  ganglion  of  the 
lateral  eyes,  l.e.g. 

Between  the  two  ganglia  and  the  lateral  margin  of  the  cephalic  lobes  are  two 
infoldings,  the  floor  of  which  is  formed  by  the  lateral  portions  of  the  optic  gan- 
glia, iv2-iv3. 

The  median  ocelli  will  develop  from  the  extreme  lateral  margins  of  the 
cephalic  lobes,  opposite  the  second  pair  of  infoldings,  and  the  lateral  ocelli  oppo- 
site the  third  pair.  The  ocelli,  however,  are  not  visible  till  later. 

The  arachnid  cephalic  lobes  are  clearly  comparable  with  those  of  Acilius,  the 
principal  differences  lying  in  the  union  of  the  parts  of  the  first  segment  to  form  the 
olfactory  lobe,  and  the  small  size  and  late  appearance  of  the  ocellar  placodes. 

9 


FIG.  91. — The  ocellus  of  an  insect  larva, 
Acilius  (eye  7).  This  eye  looks  directly 
upward. 


130  LARVAL    OCELLI   AND    THE    PARIETAL    EYE. 

Another  important  difference  lies  in  the  method  of  closing  the  ganglionic 
infoldings,  which  is  as  follows:  in  the  scorpion,  the  openings  to  the  two  pairs  of 
marginal  infoldings  lengthen  till  they  merge  with  each  other  and  with  the  one  in 
the  olfactory  lobes.  A  continuous  groove,  varying  in  depth,  is  thus  formed 
around  the  sides  and  anterior  margin  of  the  cephalic  lobes.  The  edge  of  the 
optic  plates  projects  over  the  groove  forming  a  thin-walled  fold,  which  repre- 
sents the  beginning  of  the  palial  fold,  its  free  margin  being  the  neural  crest. 

The  margin  of  the  palial  fold  now  advances  inward  and  backward  over  the 
outer  surface  of  the  forebrain.  At  the  same  time  the  olfactory  lobes  sink  below 
the  surface,  and  slide  backward,  underneath  the  second  segment,  leaving  only 
a  small,  median  part  visible  from  above. 

As  the  palial  fold  advances,  the  optic  plate  is  rolled  inward,  transferring  the 
median  eye  placodes  from  the  outer  limb  of  the  fold  to  the  inner.  When  the 
placodes  have  been  carried  about  half-way  across  the  surface  of  the  brain,  pig- 
ment develops  in  them  that  may  be  seen,  in  surface  views,  through  the  overlying 
ectoderm.  (Fig.  16,  A.)  As  the  edge  of  the  palial  fold  moves  still  farther  back- 
ward, the  outline  of  the  two  eye  sacs  becomes  distinctly  visible.  (Fig.  16,  B.) 

Finally  both  sacs  merge  into  a  single  bi-lobed  sac,  with  a  narrow  neck,  or 
epiphysis,  that  opens  to  the  exterior  through  a  small  pore,  which  we  shall  call 
the  anterior  neuropore.  (Fig.  18,  a.n.p.) 

The  neck  to  the  eye  sac  elongates  somewhat,  its  walls  thicken  and  become 
lined  with  chiten.  It  is  still  open  in  young  scorpions,  and  remnants  of  it  may 
persist  through  life.  (Fig.  43,  e.t.) 

The  posterior  edge  of  the  completed  palial  fold  extends  straight  across  the 
posterior  boundaries  of  the  forebrain.  (Fig.  18.)  When  the  latter  is  bent  back- 
ward onto  the  haemal  surface  of  the  egg,  the  edge  of  the  fold  forms  the  anterior 
edge  of  the  cephalo-thoracic  shield.  (Figs.  17,  43.) 

By  the  time  the  eye  tube  and  palial  fold  are  completed,  the  anterior  portion 
of  the  palium,  that  is  the  part  overlying  the  hemispheres,  and  the  part  originally 
connected  with  the  anterior  wall  of  the  inferior  lobes,  has  thinned  out  and  is  no 
longer  recognizable.  The  position  it  would  have,  if  retained  up  to  that  period, 
is  indicated  in  Fig.  43,  pi. 

It  is  clear  that  the  anterior  neuropore  in  the  scorpion  represents  the  point 
over  the  forebrain  toward  which  the  palial  folds  converge  and  finally  unite.  The 
pore  leads,  not  only  into  the  proximal  end  of  the  eye  stalk,  but  also  into  the  fore- 
brain  vesicle  and  into  the  olfactory  lobes.  Furthermore,  it  is  clear  that  there 
is  no  real  difference  between  this  method  of  forming  a  parietal,  or  cerebral  eye, 
and  that  in  vertebrates.  In  the  latter  animals,  the  eye  tube  usually  appears  at  a 
relatively  later  stage,  as  an  outgrowth  of  the  completed  palium  or  roof  of  the  brain, 
near  the  place  where  the  anterior  neuropore  closed.  In  arthropods,  the  same 
final  condition  is  shown,  and  in  addition,  all  the  preliminary  steps  by  which  the 
eyes  were  transferred  from  their  original  position  to  the  brain  roof. 

The  lateral  ocelli  lie  for  a  considerable  period  on  the  external  surface  of  the 


THE    PARIETAL    EYE    OF    LIMULUS.  131 

procephalic  lobes,  close  to  the  margin  of  the  palial  fold,  but,  unlike  the  median 
ocelli,  they  are  not  swept  into  the  infolding,  and  hence  onto  the  brain  roof.  They 
develop  into  typical  external  eye-pits,  which  permanently  remain  in  their  original 
position  as  regards  the  procephalic  lobes.  But  in  the  adult,  after  the  forebrain 
has  been  folded  back  onto  the  haemal  surface,  they  lie  on  the  anterior  lateral 
margins  of  the  cephalo-thoracic  shield,  on  the  haemal  surface  of  the  body,  instead 
of  the  neural.  (Fig.  17.) 

The  Parietal  Eye  of  Limulus. 

In  Limulus,  the  cephalic  lobes,  at  first  sight,  bear  no  resemblance  to  those  of 
the  scorpion,  or  of  Acilius,  but  a  more  careful  examination  will  show  that  the 
essential  features  are  the  same  in  all  of  them. 


FIG.  92. — A,  Ocellus  of  Lycosa  (middle  one  of  the  three  lateral  ocelli);  B,  retinal  portion  of  the  same,  more 
highly  magnified,  showing  the  retinal  cells,  each  with  a  large  outer  nucleus,  «,  and  a  smaller  inner  one,  nl;  the 
lateral  rods,  rd,  are  in  parallel  rows,  fenced  off  by  vertical  walls  of  dense  pigment;  concave  reflecting  membranes 
underlie  each  double  row  of  rods. 

Development. — The  cephalic  lobes  at  first  fomTtwo  wing-like  expansions  of 
the  neural  plate,  with  the  stomodaeum  on  the  extreme  anterior  margin  and  the 
chelicerae  on  the  posterior  one.  (Fig.  140.)  No  division  into  segments  is  visible 
at  this  stage. 

A  little  later  (Fig.  141),  one  may  recognize  the  various  parts  that  belong  to 
these  segments,  viz.,  two  large  infoldings  in  the  olfactory  lobes  representing  the 
first  segment,  ol.L ;  two  pairs  of  minute  pores  representing  the  marginal  infoldings 
for  the  median  ocelli  on  the  second  segment,  p.e.\  a  large  olfactory  placode  on  the 


132  LARVAL    OCELLI   AND    THE    PARIETAL    EYE. 

anterior  edge  of  the  lateral  eye  ganglion,  representing  the  sense  organ  of  the 
third  segment,  ol.  o.\  and  the  compound  eye  placode  itself,  about  opposite  the 
chelicerae,  and  belonging  to  the  fourth  segment,  I.e. 

The  compound  eyes  arise  just  behind  the  true  cephalic  lobes;  apparently  they 
are  not  represented  in  the  scorpion  or  in  spiders,  or  in  the  embryonic  cephalic 
lobes  of  those  insects  that  undergo  a  metamorphosis. 

During  the  following  stages,  the  two  pairs  of  ocellar  tubes  unite  in  the  median 
line  in  front  of  the  olfactory  lobes,  forming  a  single  median  tube  or  epiphysis, 
directed  forward,  below  the  skin.  (Fig.  142,  e.p.)  Its  distal  end  is  dilated  and 
contains,  as  shown  by  its  structure  in  the  later  stages,  four  ocellar  placodes,  two 
paired  and  two  practically  unpaired  ones;  its  posterior  end  opens  on  the  surface 
of  the  head  by  an  oval  pore  situated  just  in  front  of  the  hemispheres,  an.  p. 

Meantime  the  two  paired  olfactory  placodes  move  mesially  and  a  new  un- 
paired olfactory  placode  appears  just  in  front  of  the  pore  of  the  eye  tube.  The 
compound  eyes  migrate  in  the  opposite  direction,  toward  the  posterior  haemo- 
lateral  surface  of  the  thorax. 

We  may  harmonize  these  conditions  with  those  in  Acilius  by  assuming 
that  the  two  pairs  of  ocelli  of  the  second  segment  are  the  only  larval  ocelli  of  the 
acilius  type  retained  in  Limulus;  and  that  the  three  olfactory  placodes  and  the 
compound  eyes  represent  respectively  the  three  stemmata  and  the  compound 
eyes  of  insects,  which  do  not  appear  there  till  the  close  of  the  larval  life.  In 
other  words,  in  Limulus  the  secondary,  or  imaginal,  set  of  eyes,  and  the  primary, 
or  larval  set,  appear  at  the  same  period,  the  more  recent  organs  being  reflected 
back  into  the  same  embryonic  period  as  the  more  ancient  ones. 

While  these  events  are  taking  place,  the  palial  fold  is  forming  in  separate 
sections,  one  being  directed  backward  and  inward  over  each  lateral  eye  ganglion 
op.g.-,  another  over  each  infolding  for  the  olfactory  lobes,  ol.l,  and  a  third  over  the 
ocellar  plates,  an. p. 

The  margins  of  all  three  folds  gradually  move  toward  the  anterior  margin  of 
the  hemispheres  where  they  unite  to  form  a  common  opening,  the  anterior  neuro- 
pore.  This  pore  appears  to  be  merely  the  opening  to  the  united  ocellar  tubes, 
but  in  reality  it  represents  more  than  that.  It  is  obviously  comparable  with  the 
anterior  neuropore  of  the  scorpion,  differing  from  it  only  in  that  it  lies  farther 
forward.  In  both  cases,  the  main  opening  represents  the  point  toward  which  all 
the  epithelial  overgrowths  of  the  forebrain  converge  and  the  last  point  to  be 
covered  by  them.  This  interpretation  is  no  doubt  the  correct  one,  for  it  is  clear 
that  the  opening  offers  access,  as  it  does  in  the  scorpion,  not  only  to  the  eye  tubes, 
but  also  to  the  cavities  of  the  olfactory  lobes,  the  spaces  between  the  hemispheres 
and  the  palial  wall,  and  the  spaces  between  the  under  surface  of  the  hemispheres 
and  the  floor  of  the  forebrain.  (Fig.  47,  B.) 

Change  of  Position. — The  distal  end  of  the  ocellar  tube  is  at  first  diiected 
horizontally  forward  toward  the  ectoderm  that  forms  the  anterior  margin  of 
the  procephalon.  (Fig.  142.) 


THE    PARIETAL    EYE    OF    THE    LIMULUS. 


As  the  embryo  develops,  this  layer  of  ectoderm  extends  forward,  carrying 
the  ocelli  with  it  and  drawing  out  the  ocellar  sac  into  a  long  epithelial  tube  or 
epiphysis.  The  procephalic  ectoderm  then  forms  a  vertical  wall  covering  the 
median  anterior  surface  of  the  egg;  still  later  it  is  bent  backward  onto 


10 


FIG.  93 . — Various  forms  of  retinophorae,  isolated  by  maceration  and  showing  the  position  and  shape  of  the  retinal 
rods.  Cross- sections  of  the  rods  are  shown  over  each  figure,  the  place  where  the  section  is  taken  being  indicated  by 
the  letter  S.  i.  Upright  terminal  rod  from  ocellus  V  of  Acilius;  2,  horizontal  terminal  rod  from  sides  of  ocellus  II  of 
Acilius;  3,  a  giant  retinal  cell  with  short  horizontal  rod,  from  ocellus  II;  4,  retinal  cell,  with  lateral  rod  from  com- 
pound eye  of  Limulus;  5,  retinula  cell  from  the  compound  eye  of  Tabanus;  6,  retinal  cell  from  the  ocellus  of  Lycosa; 
7,  retinula  cell,  with  serrated  rod,  from  the  compound  eye  of  Pinaeus;  8,  inverted  retinal  cell  from  the  eye  of  Pecten; 
9,  rod  cell  from  retina  of  an  amphibian  (species  of  Diemyctylus) ,  showing  two  nuclei,  n'and  n,  and  indications  of 
division  of  rod  into  two  parts  with  either  a  canal  or  fiber  running  through  a  part  of  the  rod;  10,  cone  cell  from 
same  animal,  showing  double  nature  of  the  cell  as  well  as  of  the  cone.  The  body  corresponding  to  the  second 
nucleus  lies  at  n' 

the  haemal  surface  of  the  buckler,  where  it  represents  the  exposed  surface 
of  the  anterior  end  of  the  primitive  head,  or  procephalon;  the  posterior  end 
of  the  head,  coincident  with  the  forebrain,  remaining  on  the  neural  surface. 
(Figs.  152,  153,  pc.c.) 


134  LARVAL    OCELLI  AND    THE    PARIETAL    EYE. 

While  this  is  taking  place  the  ends  of  the  anterior  liver  lobes  unite  in  front 
of  the  cephalic  lobes,  thus  apparently  isolating  that  part  of  the  head  containing 
the  ocelli,  from  the  neural  portion  containing  the  olfactory  organs  and  cephalic 
lobes.  (Figs.  149,  151.) 

At  this  stage,  the  surface  contours  of  the  forehead  cannot  be  clearly  dis- 
tinguished. But  during  the  early  trilobite  stages,  after  boiling  in  caustic  potash, 
a  distinct  suture  is  visible  on  the  cephalo-thoracic  shield,  marking  the  boundaries 
of  the  primitive  procephalon.  (Fig.  152,  pr.c.) 

This  suture  quickly  disappears,  and  in  all  subsequent  stages  the  only  part 
of  the  primitive  fore-head  visible  on  the  haemal  surface  is  a  narrow  patch  bearing 
the  ocelli.  (Fig.  155.) 

Appearance  of  the  Placodes. — We  may  now  confine  our  attention  to  the  later 
stages  of  the  parietal  eye. 

After  the  trilobite  stage,  one  pair  of  ocellar  placodes  form  the  lateral  walls 
of  a  terminal  dilatation,  that  may  be  called  the  ecto-parietal  eye.  (Fig.  102,  D.) 
Their  cells  become  invested  with  black  pigment  and  they  take  on  the  character  of 
typical  visual  cells.  (Fig.  94,  l.ec.p.e.) 

The  other  pair  form  the  walls  of  a  second  median  dilatation  that  we  shall 
call  the  endo-parietal  eye,  en.p.e.  It  lies  below  the  surface,  and  on  the  proximal 
side  of  the  ecto-parietal  eye.  Its  cells  are  unlike  the  usual  retinal  cells  in  shape, 
arrangement,  and  pigmentation;  but  they  are  provided,  temporarily,  with  plate- 
like  visual  rods  or  rhabdoms. 

Nerves. — In  young  Limuli,  three  to  four  inches  long,  four  nerve  fascicles  may 
be  seen  at  the  distal  end  of  the  eye  tube,  one  for  each  retina  of  the  ecto-parietal 
eye,  and  two  for  the  unpaired  endo-parietal  eye.  (Figs.  94,  101,  A.) 

In  the  middle  section  of  the  tube,  the  four  nerves  unite  to  form  a  common 
layer  of  fibers  outside  the  epithelial  walls  of  the  tube.  Toward  its  proximal  end, 
the  nerve  fibers  separate  from  the  epithelial  walls  of  the  tube  and  again  divide 
into  four  fascicles,  or  two  pairs  of  roots,  the  larger  pair  ending  in  two  conical 
ganglia  on  the  haemal  surface  of  the  olfactory  lobes,  the  smaller  one  in  two  smaller 
ganglia  situated  a  little  farther  back.  (Fig.  51,  ey.r1,  ey.r2.) 

Thus  the  evidence  afforded  by  the  infoldings  on  the  cephalic  lobes,  the  struc- 
ture of  the  terminal  sac,  of  the  eye  tube  and  the  four  nerve  roots,  show  that  the 
" unpaired  eye"  of  Limulus  is  formed  by  the  partial  fusion  of  two  separate  pairs 
of  ocelli. 

Structure  of  the  Retinas. — From  the  earliest  larval  stages,  the  difference  in 
structure  between  the  endo-  and  ecto-parietal  eyes  is  very  striking.  The  ecto- 
parietal  retinas  contain,  besides  numerous  indifferent  cells,  well  defined  ommatidia 
consisting  of  from  five  to  seven  cells  with  the  visual  rods  arranged  in  star-shaped 
rhabdoms  near  their  outer  ends.  (Fig.  94.)  The  visual  cells  contain  a  relatively 
small  amount  of  reddish-brown  pigment,  and  little,  or  none,  of  the  white  pigment. 
The  endo-parietal  eye,  in  young  Limuli  three  to  four  inches  long,  is  a  thick- walled, 
pear-shaped  vesicle  lying  well  below  the  surface  and  almost  inaccessible  to  light. 


THE    PARIETAL   EYE    OF   THE    LIMULUS. 


135 


An  unpaired  tubercle,  or  a  more  transparent  spot  in  the  chiten,  usually  marks 
its  location  from  the  exterior.  (Fig.  201.) 

The  thick  inner  wall  of  the  vesicle,  en.p.e.,  now  consists  of  irregular  elongated 
cells  with  small  nuclei.  The  cells  may  show  an  obscure  arrangement  into  large 
groups,  and  are  completely  filled  with  minute  granules,  which  are  snow-white 
by  reflected,  and  greenish-black  by  transmitted  light. 

The  outer  wall  consists  of  a  few  prominent  sensory  cells,  whose  pointed 
outer  ends  terminate  in  nerve  fibers.  They  are  devoid  of  either  white,  or  colored 
pigment,  or  of  visual  rods,  rt2. 

This  eye  reaches  the  height  of  its  development  in  the  young  animals  from  four 
to  six  inches  long,  and  from  that  stage  on  it  appears  to  undergo  a  slow  histological 
degeneration,  but  without  perceptible  diminution  in  size. 


C 


FIG.  94. — The  three  chambered  parietal  eye  vesicle  of  Limulus.     A,  from  above;  B,  in  cross- section;  C,  in  long- 
itudinal section.     Semi-diagrammatic. 


In  the  older  animals,  the  distinction  between  the  inner  and  outer  walls  dis- 
appears, and  the  entire  eye  then  consists  of  a  solid  mass  of  large  vesicular  cells, 
with  minute  nuclei,  crowded  with  " white  pigment." 

After  the  early  larval  stages,  all  traces  of  the  epithelial  walls  to  the  primitive 
eye  sacs  have  disappeared;  the  eyes  appear  to  be  separate  organs,  except  in  so 
far  as  they  are  innervated  by  separate  branches  of  a  common  nerve. 

The  development  of  the  eyes  has  shown  us  that  the  epithelium  of  the  eye 
tube  merely  represents  the  tract  of  ectoderm  that  separated  the  ocellar  placodes 
from  the  brain,  before  they  were  enclosed  in  the  brain  chamber;  along  the  inner 
surface  of  this  tract  the  nerve  fibers  passed  from  one  to  the  other.  When  the  pla- 
codes were  infolded,  the  connecting  paths  of  ectoderm  were  infolded  also,  forming 
the  walls  of  the  eye  tube  or  epiphysis.  When  the  fibers  of  the  optic  nerve  grew 
from  the  eye  to  the  brain,  or  from  the  brain  to  the  eye,  they  were  compelled  to 


136  LARVAL    OCELLI  AND    THE    PARIETAL    EYE. 

follow  the  old  paths,  that  is,  the  outer  surface  of  the  eye  tube.  When  the  nerve 
fibers  separate  from  it,  the  tube  is  left  as  functionless  epithelium,  which  may,  in 
whole  or  in  part,  disappear. 

The  dilated  proximal  end  of  the  eye  tube,  from  which  the  nerve  roots  have 
separated,  remains  for  a  long  time  (up  to  three  to  four  inches  long),  adhering  to 
the  anterior  surface  of  the  hemispheres,  beneath  the  thick  neurilemma  sheath. 
In  the  adult  all  traces  of  it  have  disappeared.  The  distal  end  of  the  tube  like- 
wise disappears,  so  that  finally  the  three  ocelli  are  united  to  the  brain  by  a  single 
solid  nerve  with  four  terminal  branches  and  two  pairs  of  roots,  each  of  the  four 
roots  ending  in  a  distinct  ganglion. 


The  parietal  eye  of  Limulus  differs  from  that  of  the  scorpion  in  the  great 
length  of  the  eye  tube,  in  the  presence  of  the  endo-parietal  eye,  and  in  the  location 
of  the  ganglia  on  the  haemal  surface  of  the  brain,  instead  of  the  neural.  These 
differences,  although  sufficiently  striking,  are  not  fundamental,  but  due  merely 
to  differences  in  the  relative  rate  of  growth  of  the  adjacent  organs  in  the  two 
animals.  One  cause  of  the  difference  was  the  closing  of  the  anterior  neuropore 
in  front  of  the  hemispheres  in  Limulus,  and  behind  them,  in  the  scorpion.  More- 
over, as  the  parietal  eye  in  the  scorpion  lies  (morphologically)  behind  the  hemis- 
pheres, and  over  the  neural  surface,  the  ocellar  ganglia  are  drawn  upward, 
toward  the  median  neural  side,  as  near  to  the  eye  as  possible.  In  Limulus,  the 
parietal  eye  has  migrated  forward,  and  then  backward  on  the  haemal  surface, 
drawing  the  nerves  and  ganglia  forward  and  haemally.  (Fig.  47,  A  and  B.) 

The  Parietal  Eye  of  Branchipus. 

The  early  stages  in  the  development  of  the  median  ocellus  of  phyllopods 
and  other  Crustacea  are  imperfectly  known.  But  its  structure  in  the  adult  in- 
dicates very  clearly  that  it  is  the  same  kind  of  an  eye  as  the  median  one  in  Limulus 
and  other  arachnids,  and  probably  develops  in  a  similar  manner.  That  is,  it 
consists  of  two  pairs  of  ocelli  enclosed  in  a  median  sac  that  opens  to  the  exterior 
for  a  time  at  least  by  a  short,  median  duct,  or  epiphysis.  (Fig.  102,  A.) 

The  parietal  eye  of  Branchipus  is  probably  typical  of  many  Crustacea. 
Its  characteristic  features  appear  in  the  nauplius  at  a  very  early  period.  In 
Branchipus  it  is  a  tri-lobed  vesicle  consisting  of  two  communicating  sacs. 
(Figs.  95  and  96.)  The  larger,  outer  one,  or  ecto-parietal  eye,  has  thick,  lateral 
walls  representing  two  ocellar  placodes  or  retinas.  The  distal  ends  of  the  retinal 
cells  are  directed  inward  and  are  capped  with  minute  lateral  rods,  or  plates.  The 
cavity  of  the  sac  is  coated  with  a  layer  of  dense  black  pigment,  apparently  the 
product  of  two  large  cells  whose  nuclei  are  seen  in  the  posterior  lateral  part,  pg.c. 

The  inner  sac,  or  endo-parietal  eye  en.p.e.  is  conical  and  with  a  minute  cen- 
tral canal  or  crevice  toward  which  the  inner  ends  of  the  retinal  cells  converge  from 


THE    PARIETAL    EYE    OF   BRANCHIPUS. 


137 


all  sides.     It  is  colorless,  and  doubtless  represents  one  completely  fused  pair  of 
ocelli,  corresponding  with  the  inner,  colorless  eye  sac  of  Limulus. 

In  sagittal  sections,  the  pigmented  floor  of  the  outer  sac,  in  younger  specimens, 
appears  to  be  continuous  with  the  outer  ectoderm,  leaving  a  narrow  pore  or  crevice 


to. 


FIG.  95. — Parietal  eye  vesicle  of  a  young  Branchipus,  with  the  adjacent  frontal  organs  (or  lateral  olfactory  organs). 

Composite  frontal  section.     Camera  outline. 

by  which  the  cavity  of  the  eye  sac  communicates  with  the  exterior.  (Fig.  102,  A.) 
This  opening  is  doubtless  comparable  with  the  eye  tube  of  scorpion  and  Limulus. 
As  the  tube  itself  is  very  short  the  opening  leads  directly  into  the  common  eye 
chamber  containing  the  three  ocellar  placodes. 

There  is  no  lens  for  any  part  of  the  eye,  light  having  free  access  to  the  shallow 
retina  of  the  outer  sac  from  either  side,  and  to  the  inner  sac  from  all  sides. 


FIG.  96. — Sagittal  section  of  the  parietal  eye  of  a  young  Branchipus.     Camera  outlines. 

The  ecto-parietal  eye  has  two  distinct  nerves  distributed  over  the  outer 
surface  of  the  retinas,  n.ec.e.  They  arise  from  ganglionic  enlargements  of  the 
anterior  median  portion  of  the  brain.  The  endo-parietal  eye  has  a  single  nerve, 
n.en.e. 


138 


LARVAL    OCELLI  AND    THE   PARIETAL   EYE. 


The  Parietal  Eye  of  Apus. 

In  Apus,  the  conditions  are  a  little  more  complicated.  Here,  as  in  many  other 
phyllopods,  there  is  a  remarkable  skin  fold  directed  forward,  forming  a  broad, 
shallow  chamber  over  both  the  compound  eyes  and  the  ocelli.  (Fig.  102,  B.)  It 
opens  to  the  exterior  by  a  narrow  pore  plugged  with  chiten.  (Fig.  98,  O.)  This 
opening  should  not  be  confused  with  the  epiphyseal  pore  of  scorpion  and  Limulus. 
The  parietal  eye  forms  a  closed  chamber  with  a  retinal  placode  on  each  side 
wall,  and  two  unpaired  placodes,  one  on  its  posterior,  the  other  on  its  inner  wall. 
(Figs.  97-99-) 

Each  placode  consists  of  a  single  row  of  large,  colorless,  columnar  cells. 
Their  distal  ends  are  buried  in  a  dense  mass  of  dark  brown,  or  black  pigment; 
their  proximal  ends  are  colorless. 


o  at 


c.e  v. 


FIG.  97. — Sagittal  section  of  the  parietal  eye  vesicle  of  an  adult  Apus.  m,  Fold  covering  the  lateral  eyes;  O, 
opening  of  the  lateral  eye  vesicles,  c.e.v ;  o.at.,  remnants  of  canal  leading  into  parietal  eye  vesicle;  a.t..  cavity  of 
the  same;  p.rt.,  posterior  retina;  a.rt.,  anterior  ratina. 

As  in  Branchipus,  there  are  two  large  cells  which  appear  to  give  rise  to  the 
greater  part  of  the  pigment  that  fills  the  cavity  of  the  vesicle.  (Fig.  98,  p.g.c.) 

When  the  pigment  is  partially  dissolved,  it  is  seen  that  each  retinal  cell  is 
capped  with  a  large  brush-like  mass  of  fine  fibers  (retinidium) ,  apparently  the 
free  ends  of  nerve  fibers  passing  through  the  interior  of  the  cells,  or  over  their 
outer  surfaces.  They  are  comparable  with  the  nervous  network  described  by 
me  in  the  visual  rods  of  Pecten,  Acilius,  Lycosa,  etc.,  except  that  they  are  not  reg- 
ularly arranged,  and  are  not  imbedded  in  a  dense,  transparent  matrix,  which 
usually  forms  the  most  conspicuous  part  of  a  visual  rod. 

The  parietal  eye  sac  of  Apus  probably  contains  the  retinas  of  four  distinct 
ocelli,  which  during  development  migrated  from  the  sides  of  the  head  toward 
the  median  line.  There  they  became  enclosed  in  a  common  sac,  that  opened  to 


PARIETAL   EYE    OF  APUS. 


139 


the  exterior  by  a  short  duct  or  pore.  The  remnant  of  this  duct  is  seen  in  the 
adult  in  the  deep  recess  on  the  posterior  outer  margin  of  the  eye  sac.  (Fig.  97, 
o.  at.,  and  102,  B.) 

The  fold  of  skin  that  covers  both  lateral  and  median  eyes  was  no  doubt  a 
later  formation,  having  nothing  to  do  with  the  original  parietal  eye  infolding. 

The  parietal  eye  of  Apus  lies  entirely  below  the  surface.  There  are  no  over- 
lying lenses,  or  thickenings  of  the  adjacent  ectoderm,  to  control  the  direction  of 
the  light.  The  latter  may  enter  the  paired  retinas  from  the  sides,  and  the  un- 
paired ones  from  in  front,  or  from  behind. 


c  e 


"  a  rt 
FIG.  98. — Parietal  eye  vesicle  of  Apus,  in  cross-section. 


-a.rt 


FIG.  99. — Same  as  preceding  figure,  with 
pigment  removed,  showing  the  coarsely 
fibrillated  visual  rods,  r.t.d.,  on  the  inner 
ends  of  the  retinal  cells. 


There  is  no  reason  to  doubt  that  the  tri-oculate  median  eye  of  decapods, 
copepods,  trilobites,  and  merostomes,  in  structure  and  development  is  essentially 
like  that  of  Limulus,  scorpions,  spiders,  Apus,  and  Branchipus.  The  evidence 
presented  clearly  indicates  that  this  group  of  ocelli  is  very  constant  throughout 
the  Crustacea  and  arachnids,  and  that  it  has  certain  remarkable  features  which 
distinguish  it  from  all  other  visual  organs.  There  is  no  parallel  to  the  way  in 
which  these  ocelli  develop  except  in  the  parietal  eye  of  vertebrates,  and  there  is 
no  explanation  available  for  the  condition  seen  in  vertebrates  except  the  one  offered 
by  the  arachnids. 

The  Parietal  Eye  of  Vertebrates. 

The  parietal  eye  of  vertebrates  was  long  ago  demonstrated  to  be  a  vestigial 
eye,  although  there  are  some  authors  who  still  refer  to  it  as  a  mysterious  organ  of 


I40  LARVAL    OCELLI   AND    THE    PARIETAL    EYE. 

unknown  function.  It  is  difficult  to  understand  how  any  one  familiar  with  visual 
organs,  could  fail  to  recognize  in  the  parietal  eye  of  Petromyzon,  or  Hatteria,  or 
Lacerta,  a  visual  organ  of  some  kind.  The  pigment,  lens,  retinal  cells,  and  nerves, 
are  unmistakably  parts  of  an  organ  that  served  at  one  time  as  an  eye,  whatever 
its  function  may  be  now. 

The  conflicting  accounts  of  the  parietal  eye  are  due  in  part  to  the  various 
conditions  in  which  it  appears  in  different  groups  of  vertebrates,  but  mainly 
to  a  fundamental  misconception  of  the  ground  plan  of  the  organ  and  how  it 
happens  to  get  inside  the  brain. 

It  has  not  been  clearly  recognized  that  the  parietal  eye  is  a  paired  organ 
arising  originally  outside  and  beyond  the  boundaries  of  the  brain;  that  it  contains 
several  distinct  sensory  placodes;  that  there  is  a  fundamental  distinction  between 
the  sensory  placodes  and  the  non-sensory  epithelial  tube  that  connects  them  with 
the  brain;  and  it  has  been  very  difficult  to  eliminate  the  idea  that  the  paraphysis  is 
an  eye  or  a  part  of  one,  or  that  it  produces  some  part  of  the  parietal  eye. 

From  our  new  point  of  view,  the  parietal  eye  of  vertebrates  is  a  most  signifi- 
cant and  illuminating  organ.  The  best  insight  into  its  meaning  may  be  obtained 
by  studying  its  structure  and  development  in  the  lamprey. 

Petromyzon.— My  observations  on  the  development  of  the  eyes  of  this 
animal  in  the  main,  agree  with  those  of  Sterzi,  especially  in  regard  to  the  nature 
of  the  early  epyphysial  outgrowth. 

The  Parietal  Eye  Vesicle. — In  larvae  about  6  mm.  long,  a  single  median 
parietal  eye  tube  is  seen  just  in  front  of  the  superior  commissure.  The  dilated 
end  of  this  tube  appears  to  divide  into  two  lobes,  the  larger  one  lying  outside  of, 
and  a  little  in  front  of  the  other.  The  inner  one  lies  somewhat  to  the  left  of  the 
median  line,  the  outer  one  to  the  right;  the  displacement,  however,  is  not  enough 
to  indicate  that  the  two  sacs  are  right  and  left  lobes  of  a  single  pair. 

I  have  no  material  representing  the  stages  between  6  mm.  and  30  mm. 
larvae,  and  I  do  not  know  just  what  takes  place  at  this  critical  period,  but  the  next 
following  stages  seem  to  indicate  clearly  enough  that,  meantime,  the  inner  sac  has 
become  separated  from  the  main  tube,  giving  rise  to  an  endo-parietal,  or  "  para- 
pineal"  eye;  while  the  outer  sac  remains  connected  with  the  primitive  eye  tube, 
giving  rise  to  the  " pineal"  or  ecto-parietal  eye.  (Fig.  100,  ec.p.e.&nd  en. 
p.e.)  The  floor  of  each  sac  is  now  divided  by  a  deep  longitudinal  groove,  consist- 
ing of  undifferentiated  epithelium,  into  two  symmetrically  placed,  concave  discs, 
each  disc  probably  representing  a  retinal  placode.  (Fig.  101.)  Both  sacs  develop 
a  small  amount  of  brownish  pigment,  which  is,  however,  masked  by  a  large 
quantity  of  the  characteristic  white  granules.  The  entire  organ,  when  seen  with 
the  naked  eye,  is  a  glistening  white  spot  that  looks  precisely  like  the  endo-parietal 
eye  of  Limulus.  In  both  Limulus  and  Petromyzon,  the  granules  are  soluble  in 
weak  acid. 

The  outer  eye  sac  presents  the  most  characteristic  retinal  structure.  In 
six  inch  ammoccetes,  the  retinas  consist  of  a  layer  of  sensory  cells,  each  bearing 


PARIETAL    EYE    OF    PETROMYZON. 


141 


a  long,  fibrous,  colorless  rod  suggestive  of  those  seen  in  Apus.  (Figs.  99  and  100.) 
A  layer  of  nuclei  and  fibers  is  seen  below  the  columnar  cells.  Its  outer  wall,  in 
its  central  portion,  consists  of  similar  cells  and  rods.  They  have  been  regarded 
as  forming  an  imperfect  lens,  but  their  histological  structure  indicates  that  they 


r 

FIG.  ioo. — The  parietal  eye  vesicle  of  a  young  lamprey,  6mm.  long.  A.  Sagittal  section;  B .  cross- section. 


FIG.  101. — Plan  of  the  parietal  eye  vesicle  with  its  nerves,  ganglia,  and   epiphysis,  seen  from  the  neural  surface. 

A,  Young  Limulus;  B,  young  lamprey. 

represent  the  remnants  of  visual  cells,  although  they  are  not  so  well  developed  as 
those  on  the  floor  of  the  sac.  The  two  sets  of  rods  meet  in  the  middle  of  the  sac, 
their  distorted  ends  forming  a  distinct  cleavage  band.  On  the  periphery  of  the 
eye  the  walls  consist  of  a  single  layer  of  short,  columnar  cells. 


142  LARVAL    OCELLI   AND    THE    PARIETAL    EYE. 

The  amount  of  pigment,  and  its  distribution,  varies  greatly  in  different 
individuals  and  at  different  stages.  In  many  cases,  the  cells  of  the  outer  walls 
are  colorless,  and  the  inner  wall,  and  especially  the  two  layers  of  rods,  are  densely 
crowded  with  pigment,  a  condition  similar  to  that  seen  in  Apus  and  Branchipus. 

The  inner  sac  en.p.e.  resembles  the  outer  one,  except  that  its  retinal  cells  are 
less  highly  specialized,  and  its  outer  wall  consists  of  a  thin  layer  of  indifferent, 
columnar  cells. 

The  groove  on  the  floor  of  the  outer  sac  is  hardly  recognizable  anteriorly, 
but  it  gradually  deepens  toward  the  posterior  margin,  where  it  leads  into  the  en- 
larged, distal  end  of  the  epithelial  eye  tube  or  epiphysis.  This  part  of  the  tube 
persists  in  the  adult  as  the  conical  " atrium"  of  Studniaka.  The  proximal  part 
of  the  tube  likewise  persists  as  a  small  solid  cord,  extending  over  the  outer  surface 
of  the  ganglion  habenula.  A  trace  of  its  original  opening  may  be  seen  as  a  conical 
recess,  in  front  of  the  superior  commissure.  (Figs.  100  and  101,  141.) 

A  similar  groove,  leading  into  a  short  blind  tube,  is  seen  in  the  floor  of  the 
inner  sac,  et2.  This  tube  leads  toward  the  base  of  the  "  atrium,"  but  at  this  stage 
does  not  unite  with  it.  It  undoubtedly  represents  the  remnants  of  the  connection, 
existing  during  the  early  stages,  between  the  inner  sac  and  the  main  eye  tube. 

After  the  metamorphosis  the  parietal  eye  loses  the  clear  cut  histological 
details  seen  in  the  early  stages,  and  is  then  undoubtedly  of  less  importance 
functionally. 

The  Parietal  Eye  Ganglia,  or  Ganglia  Habenulcz,  consist  of  a  main  right  and 
left  ganglion,  each  consisting  of  an  anterior  and  a  posterior  lobe.  We  may  there- 
fore, recognize  four  lobes,  or  two  pairs  of  ganglia,  for  the  parietal  eye,  a  condition 
in  complete  harmony  with  the  presence  of  two  pairs  of  retinal  placodes  in 
the  eye. 

The  left  ganglion  is  smaller  than  the  right  and  differs  from  it  in  minor, 
histological  details.  It  gradually  moves  forward  and  mesially,  till  the  anterior  lobe 
lies  close  to  the  posterior,  inner  wall  of  the  inner  sac,  with  which  it  is  connected 
by  a  large  bundle  of  nerve  fibers.  This  nerve  divides  into  two,  one  passing  on 
either  side  of  the  median  groove.  (Fig.  100,  n2.) 

The  larger,  outer  eye  is  said  to  be  connected  by  nerve  fibers  with  the  larger, 
or  right  ganglion.  I  have  not  been  able  to  satisfy  myself  that  this  was  the  case. 
In  fact  the  nerves  to  the  outer  sac  are  small  and  very  difficult  either  to  identify, 
or  to  follow  to  their  terminals. 

The  right  and  left  ganglia  are  connected  by  at  least  two  commissures  that 
originate  in  two  large  cores  of  neuropile.  (Fig.  101.)  From  the  latter,  two 
pairs  of  nerve  tracts  arise,  the  anterior  pair,  a.tr.,  passing  downward  and  forward 
to  the  median  face  of  the  olfactory  lobes;  the  posterior  pair,  p.tr.,  downward  and 
backward  to  the  floor  of  the  midbrain. 

It  is  a  surprising  fact  that  the  two  anterior  bundles  are  of  approximately 
equal  dimensions,  while  of  the  posterior  pair,  the  right  is  very  much  larger  than  the 
left. 


PARIETAL    EYE    OF    THE    PETROMYZON.  143 

The  inner  and  outer  sacs  of  Petromyzon  and  the  two  similar  ones  in  teleosts 
have  been  regarded  as  right  and  left  mates  of  a  single  pair,  on  the  ground  that 
they  are,  for  a  short  time  at  least,  somewhat  asymmetrical  in  position,  one  being 
a  little  to  the  left,  the  other  to  the  right  of  the  median  line;  furthermore,  it  is  claimed 
that  in  the  lamprey  the  larger  outer  sac  is  innervated  mainly  from  the  right 
ganglion  habenulae,  and  the  inner  one  from  the  smaller,  left  ganglion  habenulae. 

The  evidence  however,  is;  by  no  means  conclusive.  My  own  observations 
lead  me  to  the  conclusion  that  the  inner  and  outer  parietal  eyes  are  just  what  they 
appear  to  be,  namely,  two  unpaired  organs  of  slightly  unequal  value,  one  of  which 
has  been  crowded  away  from  the  median  line. 


olo. 


ec.pe. 


FIG.  102. — Semi-diagrammatic,  sagittal  sections  of  the  head,  showing  the  relative  position  and  character 
of  the  parietal  eye,  the  epiphysis,  neuropore,  etc.,  in  A,  Branchipus;  B,  Apus;  C,  scorpion;  D,  Limulus;  E,  vertebrate. 
On  the  right  the  eyes  are  shown  on  a  larger  scale. 

Evidence  for  this  conclusion  is  afforded  by  the  parietal  eye  of  the  cyclostomes 
and  less  directly  by  the  parietal  eye  of  arachnids. 

i.  In  the  first  place,  in  the  lampreys,  during  the  earliest  stages,  one  sac 
lies  directly  behind  the  other,  and  there  is  nothing  to  indicate  that  one  is  the  right 
or  left  mate  of  the  other.  Whatever  asymmetry  appears  in  the  eye  sacs  is  seen 
later,  and  is  comparatively  slight.  The  same  condition  appears  to  prevail  in 
teleosts,  according  to  Hill's  observations,  although  he  interprets  them  differently. 


144 


LARVAL    OCELLI   AND    THE    PARIETAL    EYE. 


2.  The  two  parietal  eye  sacs  in  the  cyclostomes  not  only  stand  very  nearly, 
in   the  median  plane,  but  each  sac  contains  a  right  sensory  placode,  or  retina, 
separated  from  a  left  one  by  a  median  groove,  or  by  an  unspecialized  band  of 
tissue.     Thus  there  are  two  symmetrical  retinal  placodes  in  each  parietal  eye. 

3.  In  young  lampreys  about  two  inches  long,  each  ganglion  habenula  is 
divided  into  a  smaller  anterior  lobe,  united  by  two  nerves  with  the  inner  sac, 
and  a  larger  posterior  one,  probably  united  in  a  similar  manner  with  the  outer 
sac.     Thus  there  are  apparently  four  ganglia  corresponding  to  the  four  placodes, 
These  facts  are  incompatible  with  the  assumption  that  one  sac  is  the  right  or 
left  mate  to  the  other. 

4.  The  asymmetry  of  the  ganglia  is  pronounced.     The  left  anterior  lobe 
ultimately  takes  up  a  central  position  below  the  inner  sac,  and  remains  com- 
paratively small.     The  other  three  lobes  become  very  large,  especially  the  two 
on  the  right;  but  the  reason  for  this  unequal  development  is  not  apparent,  since 
the  nervous  connection  with  the  right  sac  is  insignificant. 

5.  A  comparison  with  the  parietal  eye  of  arachnids    (Fig.  101),  shows  that 
the  inner  sac  of  petromyzon  (parapineal  eye)  corresponds  to  the  endo-parietal 
eye  of  Limulus,  both  sacs  agreeing  in  position,  in  their  lower  grade  of  histological 
structure,  in  their  innervation,  and  in  their  relation  to  the  epiphysis.     The  outer 
sac  of  the  lamprey  corresponds  with  the  outer  one  of  Limulus,  both  sacs  agreeing 
in  relative  position,  in  being  symmetrically  bi-lobed,  and  in  the  presence  of  the 
more  highly  specialized  visual  cells  and  rods. 

The  Lenses  of  the  Parietal  Eye. 

It  will  be  recalled  that  in  the  simple  isolated  ocelli  of  insects,  the  chitenous 
lens  and  the  thick  transparent  ectoderm  that  serves  as  a  vitreous  body  are  parts 
of  the  optic  cup,  or  of  the  lips  of  the  cup.  (Figs.  90,  91.) 

When  there  are  well  defined  lenses  to  the  parietal  eye,  as  in  many  arachnids, 
they  are  formed  from  isolated  thickenings  of  the  ectoderm  and  of  the  overlying 
chiten,  wherever  the  distal  end  of  the  eye  tube  reaches  the  surface  of  the  head, 
however  remote  that  point  may  be  from  the  one  where  the  retinal  placodes  first 
appeared. 

In  the  scorpions,  the  parietal  eye  has  a  highly  developed  vitreous  body  and 
two  lenses.  (Fig.  105.) 

In  Limulus,  there  are  two  well  developed  lenses,  one  for  each  retina  of  the 
outer  sac  (Fig.  94).  But  the  inner  sac  never  has  over  it  a  true  chitenous  lens,  or 
any  ectodermic  thickening  which  may  represent  the  remnants  of  a  vitreous  body, 
although  there  may  be  a  tubercle  like  thickening  of  the  chiten,  or  a  semi-transpar- 
ent spot.  (Fig.  201.) 

In  the  phyllopods,  although  the  parietal  eye  is  often  very  highly  developed, 
it  lies  well  below  the  surface,  and  there  is  no  thickening  whatever  of  the  adjacent 
ectoderm,  or  of  the  chiten,  to  form  a  lens  or  vitreous  body  for  them.  The  fre- 


LENSE    OF    THE    PARIETAL   EYE. 


145 


FIG.  103. — Cross- sections  of  the  procephalic  lobes  of  an  embryo  scorpion.  E,  Posterior  part  of  stomodaeal  region, 
showing  the  third  cephalic  neuromere,  en3,  the  lateral  eye  ganglion,  l.e.g.,  and  the  corresponding  invagination,  and 
the  posterior  margin  or  the  palial  fold;  Ef,  same  stage  farther  forward,  showing  the  stomodaeal  ganglion  and  its 
commissure,  the  second  cerebral  neuromere,  the  parietal  eye  ganglion  and  its  corresponding  infolding,  iv2.  Com- 
pare Fig.  15,  B.  F  and  F'  are  corresponding  sections  in  an  older  stage,  Fig.  16,  A,  showing  the  crowding  of  the 
lateral  eye  ganglion  over  the  cerebral  neuromeres  and  the  appearance  of  the  parietal  eye,  p.a.e,t  on  the  inner 
limb  of  the  palial  fold.  Camera  outlines. 


FIG.  104. — Selected  sections  from  a  continuous  series  through  the  procephalic  lobes  of  an  embryo  scorpion,  in  stag 

G  (Fig.  1 6,  B).     Camera  outlines. 
IO 


146 


LARVAL    OCELLI  AND    THE    PARIETAL    EYE. 


quent  absence  of  a  lens  and  vitreous  body,  in  the  otherwise  well  developed  parietal 
eye  of  arthropods,  is  remarkable,  since  it  does  not  occur  in  the  other  types  of 
arthropod  ocelli.  The  fact  is  all  the  more  significant  when  we  recall  that  in 
vertebrates  true  lenses  to  the  parietal  eye  are  never  present.  In  place  of  them,  we 
find  a  transparent  spot,  or  tubercle,  or  a  thin  place  in  the  overlying  tissues.  The 
thickening  of  the  outer  wall  of  the  eye  vesicle,  which  may  possibly  serve,  in  ex- 
ceptional cases,  as  a  lens  (reptiles),  is  probably  the  remnant  of  a  retinal  placode. 

Location  of  the  Placodes. — The  location  of  the  retinal  placodes  in  the 
parietal  eye  vesicle  varies  greatly.  In  the  arthropods,  they  may  lie  in  the  side 
walls  (Branchipus),  or  in  the  outer  wall  (scorpion  and  Limulus),  or  in  the  inner 
wall  or  floor,  as  in  Apus.  The  prevailing  position  in  arachnids  is  in  the  outer  wall, 


A  B 

FIG.  105. — A,  Section  through  the  posterior  margin  of  the  parietal  eye  in  stage  H,  showing  the  approaching 
union  of  the  two  retinas,  and  the  palial  folds;  B,  section  through  the  parietal  eye  of  a  newly  born  scorpion,  showing 
the  parietal  eye  vesicles,  and  the  ventricle,  V,  formed  by  the  optic  ganglia,  the  palial  folds,  and  the  forebrain 
neuromeres.  The  ventricle  extends  forward  and  downward,  into  the  cavity  of  the  olfactory  lobes,  ol.v. 

thus  inverting  the  cells,  and  turning  the  rod  bearing  end  toward  the  cavity  of 
the  vesicle.  But  the  retinal  cells  have  a  remarkable  method  of  readjustment, 
so  that  in  the  later  stages,  they  appear  to  be  standing  in  an  upright  position. 

In  vertebrates,  the  placodes  in  general  appear  to  occupy  the  floor  of  the 
vesicle,  but  they  may  develop  on  both  walls,  as  in  Petromyzon. 

Minute  Structure. — In  the  arthropods,  there  is  nothing  constant  in  the 
histological  structure  of  the  parietal  eye  retinas.  The  principal  elements  are 
columnar,  sensory  cells  arranged  either  in  a  continuous  layer,  with  terminal 
rods  projecting  into  the  eye  chamber  (Apus,  Branchipus),  or  they  may  be 
arranged  in  definite  groups,  or  ommatidia,  consisting  of  from  two  to  five  or 
more  cells  with  plate-like  rods  attached  to  the  side  walls  of  each  cell  (Limulus, 
scorpion,  Phalangium,  Lycosa). 

Where  the  eye,  to  all  appearances,  has  become  functionless,  i.e..  endo-parietal 
eye  of  adult  Limulus,  the  cells  form  a  confused  mass,  without  any  definite  arrange- 
ment in  layers,  or  in  respect  to  the  source  of  whatever  light  may  reach  them. 
The  black  pigment  is  then  absent  and  the  cells  are  filled  with  a  dense  mass  of 
glistening  white  granules.  Even  in  this  degenerate  condition,  the  visual  rods 
may  be  retained  as  irregular  plates,  singly  or  in  groups,  attached  to  the  side  walls 
of  the  retinal  cells. 


PARIETAL   EYE.  147 

In  the  scorpion  and  other  arachnids  (Limulus,  Galeodes  and  Phalangium) 
a  transformation  takes  place  in  the  arrangement  of  the  retinal  cells,  shortly  after 
the  eye  assumes  its  definite  form.  In  the  scorpion,  owing  to  the  method  of  in- 
folding, the  retinal  cells  are  inverted,  the  nerves  being  distributed  over  the  outer 
surface  of  the  sac,  and  the  rods  turned  toward  the  lumen  of  the  vesicle.  Later, 
however,  the  nerves,  entering  from  the  side,  appear  to  penetrate  the  retina  about 
midway  between  the  inner  and  outer  surfaces.  In  the  adult,  the  rods  are  located 
on  the  sides  of  the  cells,  near  their  outer  ends,  and  the  nerves  then  enter  the  oppo- 
site, or  inner  end.  Just  how  this  apparent,  or  actual,  reversal  of  the  retinal 
cells  takes  place,  I  have  not  been  able  to  determine. 

In  the  scorpion,  Limulus  and  Phalangium,  the  rods  lie  in  isolated  groups,  on 
the  sides  of  the  cells,  just  below  the  outer  surface  of  the  retina.  But  in  the  parietal 
eye  of  Galeodes  and  of  spiders,  where  the  same  method  of  development  prevails, 
the  rods  form  in  the  adult  a  continuous  layer  outside  the  retinal  cells,  and  there 
is  no  indication  as  to  what  was  the  nature  of  the  of  the  post-embryonic  trans- 
formation that  brought  the  rods  and  nerve  ends  into  that  position. 

I.  Summary. 

We  may  summarize  our  conclusions  in  regard  to  the  parietal  eye  as  follows: 

1.  All  vertebrates  possess  remnants,  more  or  less  distinct,  of  a  median  or  parie- 
tal eye  which  in  some  forms  contains  true  retinal  cells  and  visual  rods,  and  is 
connected  by  several  (4  ?)  distinct  nerves  with  as  many  ganglia. 

2.  There  is  but  one  median  or  parietal  eye  consisting,  however,  of  several 
parts. 

3.  The  eye  proper  consists  of   three  or  four   sensory  placodes,  each  one 
representing  the  retina  of  a  simple  ocellus  of  the  arthropod  type.     The  placodes 
form  the  walls  of  a  sac  on  the  end  of  a  membranous  tube  projecting  from  the 
roof  of  the  tween-brain. 

4.  The  placodes  have  a  paired  arrangement  and  probably  represent  two  pairs 
of  ocelli,  located  originally  in  the  ectoderm,  just  outside  the  lateral  margins  of 
the  open  medullary  plate. 

5.  They  were  ultimately  forced  into,  or  carried  into,  the  brain  chamber  by  the 
same  forces  that  produced  the  brain  infolding.     The  placodes  are  carried  on  the 
crest  of  the  brain  infolding  toward  the  median  line,  meantime  shifting  from  the 
outer,  to  the  inner,  limb  of  the  fold.     When  the  crests  unite,  the  four  placodes 
form  a  compact  group  on  the  membranous  roof  of  the  brain.     At  that  point  a 
tubular  outgrowth  of  varying  length  is  formed  which  has  a  vesicle  or  dilatation  at 
its  distal  end,  in  the  walls  of  which  the  placodes  lie.     This  vesicle  with  its  four 
placodes  is  the  parietal  eye. 

6.  The  primary  vesicle  may  now  be  constricted,  forming  two  unpaired  lobes, 
or  the  lobes  may  separate,  forming  two  separate  sacs,  a  larger,  anterior  and  outer 
one,  the  ecto-parietal  eye,  containing  the  two  most  highly  developed  placodes, 


148  LARVAL    OCELLI  AND    THE    PARIETAL   EYE. 

and  an  inner  posterior  one,  or  endo-parietal  eye,  containing  the  remaining  two 
placodes,  now  completely  united  into  one  organ,  and  with  greatly  reduced  struc- 
tural details. 

7.  The  membranous  tube,  or  epiphysis  may  disappear  in  whole  or  in  part, 
leaving  the  terminal  eye  sacs  either  isolated,  or  united  by  distinct  nerves  with  the 
parietal  eye  ganglia,  or  the  ganglia  habenulae. 

8.  The  parietal  eye  of  vertebrates  is  homologous  with  the  parietal  eye  of  such 
arthropods  as  Limulus,  scorpion,  spiders,  phyllopods,  copepods,  trilobites,  and 
merostomes,  but  not  with  the  frontal  stemmata  or  other  ocelli  of  insects. 

9.  In  the  arthropods,  various  stages  in  the  evolution  of  a  cerebral  eye  are 
shown  in  detail,  from  functional  eyes  on  the  outer  margin  of  the  cephalic  lobes, 
to  a  median  group  of  ocelli  enclosed  within  a  tubular  outgrowth  of  the  brain  roof. 

The  most  primitive  type  of  a  parietal  eye  is  seen  in  the  nauplii  of  phyllopods 
and  entomostraca,  where  the  eye  is  a  pear-shaped  sac,  opening  by  a  median  pore 
or  tube  on  the  outer  surface  of  the  head.  (Fig.  272,  308.)  In  the  higher  arachnids, 
the  process  of  forming  an  embryonic  eye  vesicle  merged  with  the  process  of  form- 
ing a  cerebral  vesicle,  the  external  opening  of  the  forebrain  vesicle  and  that  of  the 
parietal  eye  tube,  forming  a  common  opening  or  anterior  neuropore. 

10.  The  parietal  eye  of  arthropods  is  an  important  visual  organ  until  the 
lateral  eyes,  which  represent  a  later  product,  are  fully  developed.     It  may  then 
diminish  in  size  and  activity,  but  it  rarely,  if  ever,  wholly  disappears. 

n.  During  the  evolution  of  vertebrates  from  arachnids,  there  was  a  consider- 
able period  during  which  the  lateral  eyes  were  adjusting  themselves  to  their  new 
position  inside  the  brain  chamber,  and  when  they  were  in  functional  abeyance. 
At  this  period,  ancestral  vertebrates  were  mon-oculate,  that  is  they  were  dependent 
solely  on  the  parietal  eye,  which  had  come  to  them  from  their  arachnid  ancestors 
as  an  efficient  and  completely  formed  organ. 

When  the  lateral  eyes  again  became  functional,  the  parietal  eye  began  to 
decrease  in  size  and  effectiveness. 

The  parietal  eye  is  the  only  one  now  present  in  tunicates.  In  the  oldest 
ostracoderms,  like  Pteraspis,  Cyathaspis,  Palaeaspis,  the  lateral  eyes  are  absent,  or 
at  least  do  not  reach  the  surface  of  the  head,  the  only  functional  one  being  the 
parietal  eye,  which  is  of  unusual  size. 

In  the  lampreys  we  see  the  same  conditions,  the  parietal  eye  being  very  well 
developed  in  the  larvae,  while  the  lateral  eyes  are  deeply  buried  in  the  tissues  of 
the  head,  and  useless.  During  the  transformation,  the  lateral  eyes  again  become 
functional,  and  the  parietal  begins  to  atrophy,  finally  losing  many  of  its  structural 
details  and  its  function,  although  still  retaining  very  nearly  its  original  form. 


CHAPTER  IX. 

THE  COMPOUND  EYES  OF  ARTHROPODS  AND  THE  LATERAL 
EYES  OF  VERTEBRATES. 

Froriep  (Hertwig's  Handbook  of  Embryology)  states,  quoting  Kessler,  1877, 
that  K.  E.  von  Baer's  discovery  that  the  eye  in  the  chick  is  a  hollow  outgrowth  of 
the  forebrain  vesicle,  is  the  most  interesting  fact  in  the  development  of  the  eye 
that  could  have  been  obtained,  and  is  without  a  parallel.  He  also  quotes  with 
approval  Gegenbaur's  expression  of  astonishment  that  in  the  entire  range  of 
vertebrates  there  are  no  lower  stages  in  the  development  of  such  a  complex  organ 
as  the  eye.  The  vertebrate  eye,  he  says,  Athene-like,  makes  its  appearance  com- 
pletely formed,  and  comparative  anatomy  and  embryology  are  powerless  against  it. 

The  problem,  however,  is  not  as  hopeless  as  this,  for  we  have  shown  that  the 
parietal  eye  of  arachnids  furnishes  a  very  striking  parallel  to  the  development  of  a 
vertebrate  cerebral  eye.  The  arachnid  theory  also  provides  a  satisfactory  ex- 
planation for  the  sudden  appearance  of  lateral,  cerebral  eyes  in  vertebrates,  and  for 
their  most  striking  peculiarities.  It  is  clear,  on  the  arachnid  theory,  that  the  lateral 
eye  was  delivered  to  the  vertebrates  in  a  high  stage  of  perfection.  There  are  no 
transitional  stages  between  the  external,  convex  eye  of  arthropods  and  the  internal, 
concave  eye  of  vertebrates,  because  there  can  be  no  half-way  stages  between  an 
eye  that  stays  outside  the  brain  chamber,  and  one  that  during  development  is 
carried  into  the  chamber.  The  eye  must  either  get  in  early,  before  the  brain 
closes,  or  stay  out.  Either  position,  at  once  and  definitely,  determines  its  char- 
acter and  the  way  it  does  its  work.  Whatever  moulding  influence  the  new  en- 
vironment had  on  the  enclosed  eye  was  felt  immediately,  and  the  necessary  read- 
justments, no  doubt,  at  first  followed  rapidly  and  then  ceased,  leaving  the  eye 
more  stable  than  before,  because  enclosed  in  less  variable  surroundings. 


I.  COMPOUND  EYES  OF  ARTHROPODS. 

A.  Serial  Location.  —  The  lateral  eyes  of  arthropods  are  such  essential  and 
constant  parts  of  the  head  that  it  is  important  to  determine  their  origin,  and  to 
what  metamere  they  belong.  This,  however,  is  a  very  difficult  thing  to  do. 

The  view,  often  expressed,  that  the  compound  eyes  are  compact  groups  of 
larval  ocelli  is  untenable,  since  the  primitive  larval  ocelli  (coleoptera  and  lepi- 
doptera)  degenerate  and  take  no  part  in  the  formation  of  the  lateral  eyes.  More 
over,  in  the  very  early  larval  stages  of  phyllopods,  copepods,  and  many  other 
Crustacea  the  larval  ocelli  and  compound  eyes  are  present  at  the  same  time  and 
clearly  arise  independently  of  each  other. 

149 


1 5° 


THE    EYES    OF   ARTHROPODS. 


In  forms  like  Acilius,  that  give  us  a  most  detailed  picture  of  ancestral  con- 
ditions, not  a  trace  of  the  lateral  eyes  appears  till  late  in  the  larval  stages,  when  it  is 
impossible  to  certainly  determine  their  relations  to  the  cephalic  lobes,  or  to  other 
segmental  structures.  They  are  first  seen  on  the  haemal  side  of  the  head  of  the 
oldest  larvae,  median  to  the  ocelli.  The  latter,  during  the  metamorphosis,  are 
torn  away  from  the  ectoderm,  apparently  by  the  relative  shortening  of  the  optic 
nerves,  and  are  finally  lodged  on  the  surface  of  the  optic  ganglion,  where  they  may 
be  seen  in  a  degenerate  condition,  long  after  the  lateral  eyes  have  become  func- 
tional. In  the  early  embryonic  stages  of  insects  that  do  not  pass  through  a 
metamorphosis,  and  in  many  Crustacea,  the  lateral  eyes  are  seen  on  the  posterior, 
lateral  margins  of  the  cephalic  lobes,  just  lateral  to  an  infolding  that  gives  rise  to 
the  optic  ganglion.  Here  also  their  relation  to  the  metameres  has  not  been 
determined. 

Llmulus  is  the  only  form  in  which  the  larval  ocelli,  frontal  ocelli  (olfactory 
organs),  and  the  lateral  eyes,  are  all  present  at  the  same  time  in  an  early  embryonic 
stage.  Here  it  is  clear  that  the  lateral  eyes  arise  from  the  cheliceral  or  first 
thoracic  segment.  (Figs.  141  and  142.) 

I  see  no  serious  objections  to  regarding  the  lateral  eyes  of  insects  as  also 
belonging  to  the  first  appendage  bearing  segment,  and  if  the  "organ  of  Tomos- 
vary,"  in  the  myriapods  represents  the  rudiment  of  the  lateral  eyes,  as  I  have 
suggested,  1892,  then  that  also  would  have  a  similar  position,  since  it  is  situated 
at  the  base  of  the  antennae,  and  its  nerve  is  attached  to  the  ganglion  of  the  larval 
ocelli  in  the  same  way  the  compound  eye-nerve  is  in  Acilius.  (See  optic  ganglion 
in  Acilius.) 

These  facts  indicate,  therefore,  that  the  lateral  eyes  of  arthropods  stand 
serially  behind  both  the  primitive  cephalic  lobes  and  the  larval  ocelli,  and  belong 
to  the  most  anterior  appendage-bearing  segments  of  the  primitive  body  or  thorax. 
I  can  find  no  evidence  in  the  structure  or  development  of  the  lateral  eyes  to  indi- 
cate that  they  are  modified  appendages. 

B.  Development. — Although  the  lateral  eyes  are  often  post-embryonic 
structures,  they  may,  in  some  forms,  arise  during  the  embryonic  stages. 

In  such  cases,  Vespa,  Astacus,  Limulus  and  others,  the  lateral  eye  placodes 
lie  on  the  external  margin  of  a  deep  infolding  which  gives  rise  to  the  optic  ganglia, 
in  the  same  manner  that  the  infolding  in  Acilius  gives  rise  to  the  ganglia  of  the 
larval  ocelli.  (Fig.  14.)  The  lateral  eyes,  however,  are  never  involved  in 
this  infolding.  It  soon  closes,  and  the  placodes  move  away  from"  the  margin  of 
the  cephalic  lobes  onto  the  posterior  haemal  surface  of  the  cephalothorax  (Limulus, 
many  trilobites  and  merostomes),  or  in  some  cases,  onto  its  anterior  margin, 
or  they  may  remain  in  their  original  position  on  the  neural  surface,  (Cladocera). 
(Fig.  78.)  The  position  of  the  lateral  eyes  in  the  adult,  therefore,  varies  greatly, 
and  is  either  determined  by  the  prevailing  position  of  the  animal  in  relation  to 
the  source  of  light,  or  the  location  of  the  eyes  determines  the  position  of  the 
animal. 


LATERAL   EYES    OF    VERTEBRATES.  151 

In  Vespa,  after  the  ganglionic  infolding  has  closed,  the  lateral  eye  placodes 
are  themselves  deeply  infolded  and  partly  covered  by  thin  membranous  folds, 
but  the  latter  soon  disappear  and  take  no  part  in  the  formation  of  the  eye.1 

In  insects,  Crustacea,  and  Limulus,  the  eye  proper,  or  ommataeum,  including  the 
cyrstalline  cone  cells  and  retinulae,  is  formed  from  the  single  layer  of  columnar, 
ectodermic  cells  that  constitutes  the  lateral  eye  placode.  The  infolding  described 
by  Reichenbach  and  others,  in  the  crayfish,  as  forming  the  deeper  layers  of  the 
eye,  is  merely  the  infolding  that  produces  the  lateral  eye  ganglion. 

II.  LATERAL  EYES  OF  VERTEBRATES. 

In  my  first  contribution  to  the  origin  of  vertebrates,  1889,  I  pointed  out  the 
remarkable  resemblance  between  the  early  position  of  the  eye  placodes  in  verte- 
brates and  arthropods,  and  the  similar  way  in  which  the  neural  crests  enclose  the 
forebrain  vesicle. 

Many  contributions  have  been  made  since  that  time,  especially  in  regard 
to  the  vertebrates,  that  confirm  my  observations  and  my  interpretations  of  them, 
yet  no  one  appears  to  have  clearly  understood  the  facts  or  their  significance. 
It  is  hoped  that  a  fuller  description,  with  numerous  additional  figures,  will  make 
these  important  data  intelligible  and  convincing. 

While  the  lateral  eyes  of  arthropods  are  never  caught  in  the  infoldings  of 
the  embryonic  forebrain,  as  the  larval  ocelli  are,  they  lie  very  close  to  the  edge 
of  such  folds,  so  that  any  marked  deepening  or  extension  of  them,  brought  about 
by  the  increasing  size  and  precocity  of  the  brain  and  its  ganglia,  would  be  likely 
to  include  the  lateral  eye  placodes  in  the  infolding,  and  thus  transfer  them  to  the 
inner  walls  of  the  brain  chamber.  In  this  new  position,  they  would  be  subject 
to  entirely  new  conditions,  and  they  would  doubtless  quickly  undergo  important 
structural  changes. 

The  structure  and  development  of  the  vertebrate  eye  indicate  that,  in  some 
of  the  intermediate  forms  between  vertebrates  and  arthropods,  these  events  have 
actually  taken  place. 

Location. — It  has  long  been  known  that  the  lateral  eye  placodes  of  verte- 
brates are  visible  at  a  very  early  stage  on  the  outer  margin  of  the  open  medullary 
plate,  (selachians,  amphibia,  birds).  (Figs.  34  and  35.) 

As  the  neural  crests  advance  toward  the  median  line,  the  placodes  are  trans- 
ferred to  the  inner  limb  of  the  fold,  and  finally  come  to  lie  in  the  walls  of  the 
brain  chamber,  in  precisely  the  same  manner  that  the  parietal  eye  placodes  reach 
a  similar  position. 

Origin  of  the  Choroid  Fissure  and  the  Blind  Spot.— After  they  are  thus 
enclosed  in  the  brain  walls,  they  assume  the  shape  and  position  which  is  so 
characteristic  of  vertebrates,  and  which  becomes  so  significant  when  compared 
with  the  same  features  in  the  lateral  eyes  of  arthropods. 

1  See  the  peculiar,  hood-like  fold  over  the  lateral  eyes  of  Apus  and  other  phyllopods. 


152  THE    EYES    OF   ARTHROPODS. 

It  will  be  recalled  that  in  arthropods  the  compound  eye  is  rarely  circular  in 
outline.  It  is  usually  crescentic  or  kidney-shaped,  the  convex  margin  being  turned 
toward  the  source  of  light.  A  characteristic  condition  is  seen  in  forms  like  Limulus, 
trilobites  and  merostomes,  where  the  eyes  are  located  on  the  sloping  haemal 
surface  of  the  bucklers,  the  light  coming  from  above,  when  the  animal  is  in  its 
normal  crawling  position.  Here  the  eyes  take  the  form  of  convex  crescents,  or 
some  slight  modification  of  them,  because  such  a  form  distributes  the  maximum 
number  of  ommatidia  to  best  advantage  in  reference  to  the  direction  and  the  in- 
tensity of  light.  For  similar  reasons  of  economy,  the  optic  nerve  reaches  the  eye 
at  its  topographical  center,  that  is,  near  the  middle  of  its  neural  or  concave  margin, 
and  all  the  fibers  are  distributed  from  that  point  by  the  shortest  paths  to  their 
respective  terminals.  (Fig.  106,  A.) 


FIG.  106. — Diagram  to  explain  the  origin  of  the  choroid  fissure  in  the  lateral  eye  of  vertebrates.  A.  The 
extra  cerebral,  kidney- shaped  compound  eye  of  a  marine  arachnid;  B,  the  same  eye,  as  is  appears  seen  through 
the  walls  of  the  head  in  vertebrate  embryos.  In  its  transfer  to  the  wall  of  the  cerebral  vesicle,  the  eye  is  turned 
upside  down  and  inside  out.  The  arms  of  the  horseshoe- shaped  retina  then  unite,  forming  a  choroid  fissure,  while 
the  optic  nerve,  entering  near  the  middle  of  the  retina,  distributes  its  fibers  over  what  is  now  its  outer  surface. 

If  such  a  kidney-shaped  eye,  lying  during  the  early  stages  near  the  margins 
of  the  cephalic  lobes,  were  actually  involved  in  the  brain  folds,  as  the  larval  ocelli 
are,  it  would  still  tend  to  retain  the  same  shape  and  to  occupy  the  same  posi- 
tion that  it  did  while  on  the  outer  surface  of  the  body,  that  is,  the  eye  would 
eventually  grow  out  from  the  brain  wall,  on  the  end  of  a  tube  directed  back- 
ward toward  the  original  position  of  the  eye. 

But  the  kidney-shaped  eye,  owing  to  its  inversion  during  the  infolding,  would 
now  form  a  kidney-shaped  retina,  or  sensory  placode,  with  its  convex  surface 
directed  inward,  instead  of  outward,  and  its  concave  margin  directed  haemally, 
instead  of  neurally.  (Compare  Figs.  32  to  34  and  106.)  In  other  words, 
the  inverted  compound  eye  would  have  the  same  peculiar  shape  and  position  that 
the  vertebrate  retina  has  at  an  early  embryonic  period.  But  in  its  new  position 
and  under  the  new  conditions  prevailing  within  the  head,  the  open  crescentic 
form  of  the  placode  would  probably  not  be  retained.  It  would  be  likely  to 
follow  its  original  method  of  growth  unchecked,  till  a  new  position  of  equilibrium 
was  attained;  that  is,  it  would  continue  to  grow  more  rapidly  on  one  margin  than 
on  the  other,  till  the  two  limbs  of  the  crescent  unite,  forming  a  concave,  circular 
retina,  with  a  "choroid  fissure"  directed  haemally,  and  with  a  centrally  located 
nerve  at  the  apex  of  the  fissure,  distributing  its  fibers  radially  over  the  concave 
surface  of  the  retina.  (Figs.  107  and  108.) 


LATERAL    EYES    OF    VERTEBRATES.  153 

The  Retinal  Cell  Pattern. — In  Limulus,  and  no  doubt  similar  conditions 
prevail  in  trilobites  and  merostomes,  the  lateral  eyes  consist  of  numerous  chitenous 
lenses,  under  each  of  which  is  an  "ommatidium,"  consisting  of  a  circle  of  fifteen 
or  twenty  rod  bearing  cells,  surrounding  a  central  one  that  appears  to  be  more 
highly  specialized  and  to  have  a  richer  nerve  supply  than  the  others. 

The  ommatidia  are  separated  from  one  another  by  circles  of  unspecialized 
columnar  epithelial  cells.  The  crystalline  cone  cells  and  the  corneagen  cells  of 
other  arthropods  are  absent. 

When  such  a  simple  kind  of  facetted  eye  was  enclosed  in  the  brain  walls 
of  vertebrates,  not  only  was  the  primitive  shape  of  the  whole  eye  retained,  but 
the  characteristic  pattern  in  the  arrangement  of  the  two  different  kinds  of  cells 
was  also  retained.  That  is,  the  circles  of  rod-bearing  cells  surrounding  a  single 
central  one,  is  probably  represented  in  vertebrates  by  the  circles  of  rod  cells  sur- 
rounding a  cone  cell.  (Fig.  106.) 

The  histological  changes  involved  in  this  transformation  are  comparatively 
small,  the  most  important  one  being  a  transfer  of  the  retinular  rods  from  a  lateral 
to  a  terminal  position.  Such  changes  as  this  frequently  occur  in  the  arthropods. 
Compare,  for  example,  the  striking  differences  in  the  structure  of  the  retinal  cells 
in  the  parietal  eye  of  Apus,  Branchipus,  Buthus,  Galeodes,  and  Limulus. 

The  Retinal  Ganglion. — In  Limulus,  there  is  a  loose  layer  of  ganglion  cells 
lying  just  beneath  the  inner  surface  of  the  lateral  eye;  and  a  similar  one  is  present 
in  the  eyes  of  many  other  arthropods,  e.g.,  retinal  ganglion  of  insects  and  Crustacea. 
When  the  lateral  eye  of  vertebrates  was  involved  in  the  palial  fold,  this  layer  went 
with  it,  forming  the  nerve  cells  that  lie  outside  the  stratum  of  rod  and  cone  cells. 
(Figs.  107  and  108,  r.g.} 


The  Lens. — A  striking  feature  of  the  lateral  eye  is  the  development  of  a  lens 
vesicle  from  the  surface  ectoderm  and  its  union  with  a  retinal  placode  which  grows 
out  from  the  brain  walls  to  meet  it. 

The  origin  of  the  image  forming  organ  at  a  remote  time  and  place  from  that 
of  the  sensory  receptive  surface,  has  led  many  writers  to  the  conclusion  that  they 
represent  two  originally  different  classes  of  organs,  secondarily  united  into  one. 
Thus  the  lens  vesicle  has  been  interpreted  as  a  specialized  gill  pocket,  or  as  a 
segmental  sense  organ  serially  homologous  with  those  of  the  hindbrain  region. 
These  views  are  untenable  because  they  are  not  called  for  by  the  facts  as  we  now 
understand  them.  In  Limulus  and  scorpion,  we  have  shown  that  the  cuticular 
lens  and  the  lentiginous  ectoderm  of  the  parietal  eye  are  formed  wherever  the 
vesicle  reaches  the  surface  ectoderm,  no  matter  how  remote  that  point  may  be 
from  the  original  position  of  the  retinal  placode.  (Figs.  101  and  102.)  It  is 
clear  enough,  in  these  cases,  that  the  lens  cups  are  an  original  part  of  the  eye,  and 
cannot  be  thought  of  as  existing  apart  from  it.  Precisely  the  same  condition,  it 
seems  to  me,  prevails  in  the  lateral  eye  of  vertebrates.  We  would  therefore 


154  THE    EYES    OF   ARTHROPODS. 

eliminate  the  lens  vesicle  from  the  category  of  organs  foreign  to  the  eye.  It  may 
be  comparable  with  the  thick-walled  ectodermic  cup  that  secretes  the  chitenous 
lenses  for  the  parietal  eyes  of  arthropods.  (Fig.  108.)  When  the  chitenous 
exoskeleton  atrophied,  the  old  lens  cup  probably  remained  to  form  the  new  lens 
of  the  vertebrate  eye. 

Origin  of  the  "Imperfections"  in  the  Vertebrate  Eye. — Thus  the  com- 
pound eye  of  arachnids,  which  lies  on  the  outer  surface  of  the  head,  is  represented 
in  vertebrates  by  the  retinal  placode  which  there  lies  in  the  walls  of  the  brain 
chamber.  The  vertebrate  retinal  placode  still  retains  essentially  the  same  con- 
tour, surface  curvature,  and  arrangement  of  visual  cells  and  nerve  cells  as  that  of 
its  arachnid  prototype,  but  owing  to  the  inversion  of  the  placode,  which  took  place 
when  it  was  transferred  to  the  brain  chamber,  the  concave  surface  and  the  con- 
cave margin  of  the  retinal  placode  face  in  nearly  opposite  directions  in  vertebrates 
from  what  they  do  in  the  arachnids. 

Thus  arose  those  extraordinary  imperfections  of  the  vertebrate  eye,  which 
have  so  often  excited  the  comments  of  the  physicist,  anatomist,  and  philosopher. 
The  inverted  retina,  the  choroid  fissure,  and  the  blind  spot  caused  by  the  awkward 
entrance  of  the  optic  nerve,  are  the  inevitable  result  of  a  combination  of  condi- 
tions, some  of  which,  originally,  had  no  relation  whatever  to  the  eye.  These  con- 
ditions were  established  in  the  arthropods  long  before  the  vertebrate  stock  ap- 
peared, and  it  was  a  purely  incidental,  or  accidental,  result  of  these  conditions  that 
the  eye  was  swept  into  the  brain  chamber,  where  it  did  not  originally  belong.  In 
other  words,  the  fate  of  the  lateral  eye  was  not  decided  by  what  was  best  for  the 
eye,  as  an  instrument,  or  by  any  selective  action,  in  which  the  eye  itself  played 
a  part.  The  eye  was  a  purely  passive  victim  of  its  location,  and  of  its  more  power- 
ful neighbor,  the  brain.  But  it  survived,  in  spite  of  its  unfortunate  location,  al- 
though it  will  forever  bear  the  marks  of  a  displaced  and  made  over  organ. 

III.  THE  OPTIC  GANGLIA. 

In  reconstructing  the  history  of  the  vertebrate  brain,  the  structure  and  posi- 
tion of  the  optic  ganglia  of  arthropods  is  no  less  significant  than  that  of  the  eyes. 

Location. — We  have  already  shown  that  in  the  embryos  of  Acilius  the  optic 
ganglia  consist  of  three  lobes  lying  on  the  lateral  margins  of  the  neural  plate,  each 
lobe  lying  opposite  a  forebrain  neuromere.  (Fig.  14.) 

When  the  larval  ocelli  of  insects  degenerate,  the  ocellar  ganglia,  without 
noticeable  transformation,  become  the  ganglia  of  the  lateral  eyes.  But  in  Limulus 
and  in  the  arachnids  generally,  the  ganglia  of  the  parietal  eye  and  those  of  the 
lateral  eyes  are  separate.  In  most  insects  and  "Crustacea,  the  ganglia  retain  their 
lateral  position  through  life.  This  is  also  the  condition  in  young  Limuli,  but  later 
the  great  overlapping  lobes  of  the  hemispheres  crowd  the  lateral  eye  ganglia  to- 
ward the  haemal  surface.  (Figs.  36-39.)  Limulus  is  the  only  arthropod,  to  my 
knowledge,  in  which  the  ganglia  occupy  this  position. 


THE    OPTIC    GANGLIA.  155 

In  the  phyllopods  and  arachnids,  they  are  generally  drawn  upward,  so  that 
they  lie  on,  or  over,  the  neural  surface  of  the  brain. 

Sections  and  surface  views  of  scorpion  embryos  show  how  this  is  done. 
(Figs.  15,  16, 18, 41,  103,  104.)  It  will  be  seen  that  as  the  palial  folds  advance,  the 
optic  ganglia  move  upward  and  inward  till  they  lie  on  the  neural  surface  of  the 
forebrain  neuromeres,  the  parietal  eye  ganglion  lying  in  front  of  the  lateral  eye 
ganglion.  In  this  position  they  give  us  the  clue  to  the  interpretation  of  the  optic 
centers  in  vertebrates  for,  clearly,  one  represents  the  ganglion  habenula,  the  other 
the  tectum  opticum,  or  the  roof  of  the  midbrain.  (Figs.  43,  44.) 

Parietal  Eye  Ganglia. — We  have  already  shown  that  the  two  pairs  of  gan- 
glia, from  which  the  roots  of  the  parietal  eye  nerves  arise,  are  represented  in 
petromyzon  by  a  four  lobed  ganglion  habenula.  (Fig.  104,  B.)  The  latter 
occupies  the  same  relative  position  as  in  the  scorpion,  but  a  very  different  one  from 
that  in  Limulus.  However,  the  difference  is  more  apparent  than  real,  because  the 
anterior  roots  of  the  ganglia  habenulae  are  directed  downward  and  forward 
toward  the  olfactory  lobes,  showing  not  only  the  direction  in  which  the  ganglia 
have  been  shifted,  but  that  their  original  point  of  union  with  the  brain  is  the 
same  as  it  is  in  Limulus.  (Compare  Fig.  47,  A}  B,  and  C.) 

The  difference  in  position  of  the  parietal  eye  ganglia,  in  scorpion  and  Limulus, 
is  due  to  the  fact  that  in  Limulus  the  eye  is  drawn  forward  and  haemally  by  the 
extraordinary  size  of  the  cephalic  shield,  and  by  the  rapid  growth  of  the  hemi- 
spheres, which  have  grown  up  behind  the  epiphysis,  instead  of  in  front  of  it  as  in 
all  other  arthropods.  (Compare  Figs.  43,  44,  46,  and  47.) 

Lateral  Eye  Ganglia. — The  characteristic  shape  of  the  optic  ganglia  is  well 
seen  in  large-eyed  insects,  as  Vespa,  where  they  consist  of  three  principal  lobes. 
(Fig.  107,  A.)  i.  A  proximal  one,  that  is  roughly  spherical;  2.  a  median  one, 
consisting  of  an  immense  concave  crescentic  disc,  and  3.  a  long  narrow  one, 
extending  around  the  distal  margin  of  the  middle  lobe.  Each  lobe  contains  a 
mass  of  felted  fibers  of  the  same  general  shape  as  the  lobe,  and  is  covered,  on  its 
outer  surface,  by  a  thick  layer  of  ganglion  cells. 

The  hemispherical  middle  lobe  is  the  most  conspicuous  one,  and  the  one 
which,  by  its  contour  and  dimensions,  reflects  most  accurately  the  variations  in 
the  eye  to  which  it  belongs.  This  significant  fact  has  also  been  observed  in  the 
ocelli  of  Acilius,  where  each  one  of  the  six  pairs  of  eyes  has  a  special  shape,  or  some 
peculiarity  in  the  arrangement  of  retinal  cells,  which  is  accurately  repeated  in  the 
size,  form,  and  structure  of  the  neuropile  core  of  the  corresponding  ganglion. 

Minute  Structure  in  Limulus. — In  Limulus,  the  ganglia  have  a  similar  con- 
figuration to  those  of  insects.  (Figs.  37-39,  51, 66.)  The  inner  lobes  (Figs.  51  and  52) 
contain  large  association  neurites,  op.g*  and  op.g.*;  the  two  outer  ones,  the  central 
ends  of  the  optic  nerve  fibers,  op.f.,  and  two  relays  of  optic  neurones,  op.g.1  and 
op.g.* 

Each  of  the  two  outer  lobes  is  a  disc- shaped  mass  of  fibers,  one  surface  covered 
with  a  thick  layer  of  nerve  cells,  the  other  bare.  In  certain  cases,  successfully 


IE?  6  THE    EYES    OF    ARTHROPODS. 

impregnated  with  methylene  blue,  only  the  terminals  of  the  optic  nerves  are  stained. 
They  are  then  seen  as  small  bundles  passing  in  definite  order  through  the  gang- 
lion, between  the  medullary  core  and  the  nerve  cell  layer.  (Fig.  51.)  Each 
bundle  of  fibers,  op.f.,  probably  represents  the  terminals  of  a  definite  ommatidium. 
On  reaching  the  proximal  edge  of  the  lobe,  two  delicate  fibers  are  given  off,  one 
on  each  side,  that  penetrate  the  first  core  and  end  there  in  a  few  straggling 
branches.  (Fig.  51,  A,  op.f.)  Just  beyond  them,  a  compact  tuft  of  varicose 
fibers  is  formed  on  the  proximal  outer  surface  of  the  core.  The  main  fiber  then 
passes  to  the  under  surface  of  the  second  core,  forming  with  other  fibers  a 
characteristic  chiasma,  x.,  and  then,  bending  upward,  ends  in  widely  distributed 
dendrites. 


op.n 


FIGS.  107-108. — Diagrams  to  illustrate  the  relation  between  the  brain,  optic  ganglia  and  lateral  eyes  of  an 
arthropod  and  a  vertebrate.  In  both  cases  the  parts  are  projected  onto  the  same  transverse  plane.  FIG.  107.— 
arthropod.  FIG.  108. — vertebrate,  where  the  same  parts,  by  the  infolding  of  the  medullary  plate,  have  been 
transferred  to  the  walls  of  the  cerebral  vesicles.  The  optic  ganglia  are  inverted,  forming  the  roof  of  the  mid- 
brain;  the  compound  eye,  with  its  visual  cells  and  underlying  ganglionic  cells,  r.I.,  forms  the  inverted  retina. 


Many  other  fibers,  like  the  one  just  described,  enter  on  the  opposite  side  of 
the  first  core  and  extend  along  its  inner  face,  giving  off  the  varicose  dendrites;  they 
then  pass  to  the  outer  face  of  the  second,  ending  in  the  double  set  of  dendrites 
on  its  proximal  margin.  (Fig.  52.)  In  figure  51,  these  fibers  are  seen  as  dotted 
strands  running  diagonally  across  the  inner  face  of  the  first  lobe,  and  appearing 
as  continuous  strands  on  the  outer  surface  of  the  second.  On  passing  from  one 
lobe  to  the  other,  the  two  sets  of  fibers  form  the  well  known  chiasma  (Fig.  52,^.) 
When  seen  from  the  surface,  the  whole  effect  is  that  of  a  single  lobe  that  has  been 
twisted,  through  about  one  revolution,  into  two  lobes. 

The  surface  neurones  of  each  lobe  send  their  fibers  into  the  other  medullary 
core.  For  example,  the  fiber  from  cell  a  (Fig.  52),  extends  along  the  outer  surface 
of  the  second  core,  parallel  with  the  optic  nerve  fibers,  to  the  under  surface  of  the 
first  and  then  upward,  ending  in  the  central  part  of  the  distal  margin  of  the  core. 
Neurones  b  take  the  reverse  course.  The  remaining  ones  take  intermediate 
courses. 


COMPARISON    WITH    VERTEBRATES.  157 

The  proximal  lobe  consists  of  two  parts;  an  anterior  one,  composed  of  large 
neurones,  sending  their  main  fibers  into  the  olfactory  lobes,  and  collaterals  into 
the  second  lobe  of  the  optic  ganglion.  (Figs.  51  and  52,  op.g.4);  and  a  posterior 
part  consisting  of  a  spherical  mass  of  somewhat  smaller  neurones,  sending  their 
main  fibers  backward  into  the  longitudinal  tract  of  the  brain  (op.  f.3) ,  and  their 
collateral  branches  into  the  second  lobe  of  the  ganglion  (op.g.3). 

The  optic  ganglia  are  united  with  other  parts  of  the  brain  by  the  following 
tracts :  a,  a  distinct  bundle  of  fine  fibers  that  passes  without  interruption  through 
the  optic  ganglion  into  the  crura,  and  run  the  whole  length  of  the  brain  (Figs.  51  and 
66  op.tr.)]  b.  a  tract  uniting  the  ocellar  ganglia  with  the  second  lobe  of  the  lateral 
eye  ganglia  (oc.tr.) ;  c.  a  commissural  tract  through  the  olfactory  lobe,  formed  by 
the  neurones  of  the  fourth  lobe  (op.f.^^d.  a  longitudinal  tract  formed  by  the  neu- 
rones of  the  third  optic  lobe,  and  which  extend  backward  the  whole  length  of  the 
crura  (op.f.3) ;  e.  and  finally,  an  important  tract,  the  source  of  whose  fibers  is 
unknown  extending  from  the  second  lobe  of  the  optic  ganglion  into  the  cerebral 
hemispheres  (Fig.  52,  op.  tr.  H.). 


IV.  COMPARISON  WITH  VERTEBRATES. 

We  have  already  shown  that  the  four  ganglia  of  the  parietal  eye  in  Limulus  are 
comparable  with  the  four  lobed  ganglia  habenulae  of  the  cyclostomes.  There  is 
also  a  striking  resemblance  between  the  lateral  eye  ganglia  of  arachnids  and  the 
optic  lobes  of  vertebrates,  especially  when  we  make  due  allowance  for  certain 
peculiarities  of  position  and  structure. 

We  may  best  explain  the  origin  of  the  optic  lobes  of  vertebrates  on  the 
assumption  that  the  optic  ganglia  of  some  form  like  Limulus  have  been  bent 
backward  and  upward  onto  the  neural  surface  of  the  brain,  in  the  manner  shown 
in  Figs.  44,  57,  58,  and  108.  In  this  position,  which  is  similar  to  the  one 
they  occupy  in  the  scorpion  and  other  arachnids,  they  have  the  same  general 
form  and  the  same  relation  to  the  rest  of  the  brain  that  the  optic  lobes  have  in 
vertebrates;  the  three  lobes  of  the  optic  ganglia  corresponding  respectively  to  the 
colliculus,  the  tectum,  and  the  torus. 

It  will  be  observed  that  in  this  new  position  (Fig.  io8,5),  the  general  contour 
of  each  of  the  ganglia  is  retained,  but  the  general  appearance  of  the  whole  series 
is  greatly  disguised,  owing  to  the  change  of  curvature  of  the  second  and  third 
lobes,  and  to  the  diagonal  movement  of  the  ganglia  in  a  caudad  and  neurad 
direction.  (Fig.  46.)  Both  the  distal  and  proximal  ends  of  the  series  are  fixed  at 
the  optic  commissure,  the  original  point  of  attachment  of  the  ganglia  to  the  basal 
lobes  of  the  forebrain.  The  convergence  of  the  fiber  tracts  of  the  optic  lobes 
toward  the  optic  chiasma  shows  that  the  optic  lobes  belong  primarily  to  the  hemi- 
sphere neuromeres,  and  that  their  position  in  vertebrates  is  a  secondary  one. 


158  THE    EYES    OF   ARTHROPODS. 

It  is  clear  that,  as  regards  compactness  and  economy  of  space,  the  change 
is  an  advantageous  one. 

The  optic  lobes  of  vertebrates  have  been  forced  into  their  present  position 
by  the  general  trend  of  several  growth  forces  which  appear  at  an  early  embryonic 
period  in  the  arthropod  head.  We  have  already  referred  to  some  of  these  condi- 
tions. Those  that  are  most  persistent  and  which  most  affect  the  position  of 
the  optic  ganglia  are :  i.  the  overgrowth  of  the  neural  crests,  which  tend  to  carry  the 
ganglia  from  the  margins  of  the  medullary  plate  toward  the  median  line;  2.  the 
central  location  of  the  eyes  on  the  neural  surface  of  the  head  in  many  free  swimming 
arthropods  and  in  ostrocoderms;  3.  the  tendency  of  the  entire  brain  to  move  forward 
beneath  the  integument,  while  the  rostrum  and  other  superficial  neural  structures 
move  backward.  (Compare  Fig.  46.) 

When  the  optic  ganglia  are  once  established  in  a  median  neural  position  be- 
hind the  hemispheres,  the  increasing  size  of  the  latter,  and  of  the  optic  lobes 
themselves,  exaggerates  still  more  the  backward  movement  of  the  neural  portion 
of  the  ganglia  and  of  the  primitive  cerebellar  commissure. 

Thus  the  roof  and  sides  of  the  mesencephalon  represent  the  ganglia  of  the 
compound  eyes  of  arthropods  that  have  worked  back  into  the  territory  behind 
the  hemispheres  by  that  struggle  for  space  between  growing  organs  which  ad- 
justs and  readjusts,  till  each  part  falls  into  the  place  of  least  resistance. 

Whether  the  optic  lobes  helped  in  the  closure  of  the  old  mouth,  or  the  disap- 
pearing mouth  and  rostrum  made  a  place  for  the  lobes,  cannot  be  determined. 
Doubtless  these  events  are  part  of  a  general  movement,  where  it  becomes  im- 
possible to  distinguish  cause  from  effect. 

Dr.  L.  Griggs,  working  at  Dartmouth,  has  been  able  to  locate  the  optic 
lobes  on  the  margins  of  the  forebrain  region,  in  the  open  neural  plate  stage  of 
Amblystoma,  and  has  followed  their  course  backward  and  upward  till  they 
reach  their  permanent  position.  The  movements  of  the  lobes,  as  he  describes 
them,  afford  a  striking  confirmation  of  the  interpretation  given  above. 

Conclusion. 

1.  The   lateral   eyes   are   homologous   throughout   the  insects,  Crustacea, 
arachnids,  and  vertebrates. 

2.  In  the  arthropods  they  develop  historically  later  than  the  larval  ocelli,  and 
from  a  more  posterior  segment,  namely  the  first  appendage  bearing  segment 
behind  the  primitive  cephalic  lobes. 

3.  In  the  arthropods  the  lateral  eye  placodes  lie  for  a  time  on  the  lateral 
margins  of  the  cephalic  lobes,  close  to  the  deep  infoldings  that  form  a  part  of  the 
brain.     In  vertebrates,  the  eyes  at  first  have  a  similar  position,  but  a  precocious 
enlargement  of  the  cephalic  lobes  and  the  neural  crests  leads  to  the  enclosure  of 
the  compound  eye  placodes  in  the  brain  chamber,  so  that  they  appear  to  form 
a  part  of  the  brain. 


CONCLUSION.  159 

4.  The  characteristic  shape  of  the  arthropod  eye  and  the  arrangement  of  its 
retinal  cells  is  retained  in  an  exaggerated  form  in  the  vertebrate  retina,  and 
affords  us  the  only  satisfactory   explanation  of  its   inversion,  its   contour  and 
mode  of  growth,  its  choroid  fissure,  its  arrangement  of  rod  and  cone  cells,  and  its 
centrally  located  optic  nerve. 

5.  The  parietal  eyes  of  vertebrates  belong  to  the  second  forebrain  neuromere, 
the  lateral  eyes  to  the  third  or  fourth. 

6.  The  optic  lobes  of  primitive  vertebrates  represent  the  compound  eye 
ganglia  inverted  and  transferred  to  a  position  overlying  the  mesencephalic  neuro- 
meres.     Their  genetic  relations,  as  well  as  their  most  intimate  functional  and  an- 
atomical relations,  are  with  the  procephalic  neuromeres. 

7.  The  ganglia  habenulae  of  vertebrates  represent  the  ganglia  of  the  parietal 
eyes  of  arachnids,  united  in  the  middle  line  over  the  region  of  the  diencephalon. 
They  were  primarily  associated  with  the  olfactory  lobes. 


CHAPTER  X. 
THE  OLFACTORY  ORGANS  AND  THE  OLFACTORY  LOBES. 

The  agreement  between  the  olfactory  organ  of  Limulus  and  that  of  verte- 
brates may  be  traced  in  respect  to  so  many  different  characters  that  the  existence 
of  a  genetic  relationship  between  the  marine  arachnids  and  the  vertebrates  is 
placed  beyond  a  reasonable  doubt.  Indeed  there  is  a  greater  difference  in 
respect  to  this  organ,  between  Limulus  and  other  invertebrates  than  there  is 
between  Limulus  and  vertebrates. 

The  olfactory  organ  of  Limulus,  in  certain  respects,  stands  in  a  class  by 
itself.  Nevertheless  it  represents  a  modification  of  organs  very  widely  distributed 
in  the  arthropods,  and  known  in  insects  as  the  frontal  ocelli,  or  stemmata,  and  in 
the  phyllopods  and  other  Crustacea,  as  the  frontal  sense  organs. 

The  history  of  these  organs  is  an  important  lesson  in  evolution.  It  affords  an 
impressive  illustration  of  the  essentially  unalterable  character  of  the  procephalic 
sense  organs,  and  it  distinctly  sharpens  our  perspective  of  the  long  series  of  inter- 
mediate forms  that  connect  the  most  primitive  segmented  animals  with  the  modern 
ones. 

I.  THE  OLFACTORY  ORGAN  OF  LIMULUS. 

Structure  in  Adult  Limulus. — Gross  Structure.— In  an  adult  Limulus,  the 
olfactory  organ  (subfrontal  schlerite  of  Lankester)  is  a  bi-lobed,  wart-like  thicken- 
ing of  the  cuticula,  from  5-8  mm.  wide,  situated  in  the  median  line,  30-40  mm. 
in  front  of  the  mouth.  (Figs.  38,  39,  70,  ol.o.)  It  is  innervated  by  three  large 
nerves,  a  median  and  two  lateral  ones. 

The  olfactory  cuticula  is  provided  with  a  central  cluster  of  sensory  spines  and 
is  perforated  by  many  sensory  and  glandular  openings.  (Fig.  109,  A.)  The 
under-lying  ectoderm  is  pigmented,  and  just  beneath  it  are  many  branching  nerve 
fibers,  together  with  ganglionic  or  sensory  cells,  and  a  large  number,  about 
1500,  flask  shaped,  or  spherical,  slime  buds. 

The  most  conspicuous  parts  of  the  olfactory  organ  are  the  slime  buds, 
which  are,  with  few  exceptions,  sharply  confined  within  the  area  of  the  olfactory 
schlerite.  They  have  the  usual  form  and  structure,  as  described  in  the  chapter  on 
the  gustatory  organs  (p.  116),  the  only  noticeable  peculiarity  being  the  clusters 
of  small  ganglionic  or  sensory  cells  lying  near,  or  on,  their  outer  surface.  (Fig. 
88,  a.) 

Minute  Structure. — The  minute  structure  of  the  olfactory  organ  has  not  been 
satisfactorily  determined,  especially  the  character  of  the  nerve  terminals.  So 

1 60 


OLFACTORY    ORGANS    OF    LIMULUS. 


161 


far  as  I  have  been  able  to  discover  without  a  thorough  application  of  either  the 
methylene  blue  or  the  Golgi  method,  there  are  three  ways  in  which  the  nerves 
may  terminate  in  the  region  of  the  olfactory  organ.  The  median  nerve,  be- 
fore reaching  the  organ,  breaks  up  into  numerous  small  branches  which  are 
distributed  in  the  central  portion  of  the  organ.  (Fig.  109,  m.ol.n.)  The  two 
lateral  nerves  terminate  in  oblong  masses  of  very  large  ganglion  cells,  just  be- 
neath the  lateral  margin  of  the  organ.  From  these  ganglia  numerous  branches 
arise  that  are  distributed  to  the  olfactory  organ  and  to  a  considerable  area  of  the 
surrounding  epidermis. 

The  finer  branches  from  both  sources  form  a  sub-epithelial  plexus,  from  which 
still  smaller  branches  are  distributed  over  the  surface  of  the  slime  buds.     Others, 


loin.- 


orri 


C  •*&.<•• 


FIG.  109. — The  olfactory  organ  of  Limulus.     A,  Olfactory  organ  of  the  adult,  seen  from  the  outer  surface; 

B,  cross-section  through  the  olfactory  organ  of  a  young  Limulus,  about  seven  inches  long  (Flemming's  solution) ; 

C,  longitudinal  section  through  the  root  of  the  lateral  olfactory  nerve  of  a  young  Limulus,  showing  the  ommatidia- 
like  clusters  of  large  cells  with  rod-like  enclosures,  derived  from  the  primitive  segmental  sense  organs. 

possibly  derived  exclusively  from  the  lateral  olfactory  nerves,  end  in  peculiar 
ill  defined  masses  of  cells  that  are  either  wedged  in  between  the  slime  buds,  or  lie 
against  the  epidermis.  (Fig.  88.)  They  may  possibly  be  connected  with  slender 
sensory  cells,  similar  to  those  in  the  gustatory  organs,  that  extend  into  the 
hollow  spines  and  into  the  narrow  canals  leading  to  the  outer  surface. 

Finally  there  appears  to  be  a  system  of  fine  nerve  strands  that  penetrate 
the  soft  chitenous  exoskeleton  surrounding  the  olfactory  organ,  where  they  form 
loose  meshworks  in  superimposed  layers.  These  fibers  resemble  those  seen  in 
the  cornea  of  mammals,  and  although  of  very  uniform  caliber  they  appear  to  dif- 
fer from  the  branching  hyphae  of  the  parasitic  fungus  (Macrocystis)  that  is  fre- 
quently seen  in  this  region.  The  hyphae  take  up  the  methylene  blue  in  a  similar 
manner  to  nerve  fibers,  and  at  first  sight  might  be  easily  mistaken  for  them. 
However,  recent  preparations  in  von  Rath's  fluid  show,  in  addition  to  the  hy- 
phae above  mentioned,  branching  fibers  that  appear  to  be  true  nerve-fibers, 
ii 


1 62          THE  OLFACTORY  ORGANS  AND  THE  OLFACTORY  LOBES. 

Although  the  minute  structure  and  the  function  of  this  organ  need  further 
study,  there  is  no  question  that  it  is  a  true  sense  organ  of  great  morphological 
significance. 

The  Development  of  the  Olfactory  Organ  and  Nerves. — The  Olfactory 
Placodes. — The  primary  olfactory  organ  of  Limulus  represents  a  segmental  sense 
organ  serially  homologous  with  the  lateral  eyes  and  the  ocelli.  It  is  first  seen  as  a 
pair  of  sensory  thickenings  on  the  anterior  margin  of  the  lateral  eye  ganglion, 
behind  the  median  eye  tubes.  (Fig.  141,  ol.o.)  It  is  connected  with  the  middle 
lobe  of  this  ganglion  by  the  lateral  olfactory  nerve.  (Figs.  36-39.) 

Each  organ  soon  separates  bodily  from  the  ectoderm.  Although  there  is  no 
visible  infolding,  the  cells  which  have  the  appearance  of  visual  cells,  are  inverted 
in  the  process  and  become  filled  with  a  dense  mass  of  white  pigment  (guanin  ?) . 
(Fig.  37,  A.)  At  the  same  time  certain  cells  filled  with  the  same  kind  of  pigment 
migrate  forward  from  each  placode,  forming  a  gradually  widening,  sub-epithelial 
plexus  of  branching  pigment  cells  connected  with  the  anterior  margin  of  the 
placode  by  a  short  thick  stalk.  (Fig.  36,  p.st.)  During  the  early  embryonic 
stages  the  placodes  move  toward  the  remnants  of  the  anterior  neuropore  and 
there  unite  in  the  median  line,  meantime  acquiring  a  connection  with  the  anterior 
surface  of  the  cerebral  hemispheres  and  the  olfactory  lobes.  (Fig.  142.) 

Lateral  Olfactory  Nerve. — In  the  following  stages,  the  united  placodes  move 
forward  beneath  the  integument  toward  their  position  in  the  adult.  During 
this  process,  the  lateral  olfactory  nerves  become  greatly  elongated  and  the  cells 
of  the  original  placodes  are  now  scattered  as  ganglion  cells  along  the  nerve, 
but  forming  a  special  enlargement  at  either  end.  These  terminal  masses  consist 
of  irregular  clusters  of  five  or  six  large  pear-shaped  cells  which  greatly  resemble 
the  ommatidial  cells  of  the  paired  ocelli,  not  only  in  their  shape  and  arrange- 
ment, but  in  the  presence  of  the  clear  refractive  rods,  or  rhabdoms,  on  their  side 
walls.  (Fig.  109,  C.) 

In  young  Limuli  (2-3  in.),  the  peripheral  end  of  the  lateral  olfactories  still 
terminates  in  a  compact,  club-shaped  mass  of  metamorphosed  visual  cells.  (109, 
B.l.ol.n.)  It  also  sends  out  several  fine  nerve  branches  which  ramify  widely  under 
the  skin,  in  the  region  surrounding  the  main  olfactory  organ.  (Fig.  70.)  At 
the  same  time  the  terminal  group  of  cells  breaks  up  into  irregular  clusters  scattered 
among  the  branches  of  the  nerve.  In  the  ordinary  methods  of  preparation,  each 
cluster  has  the  appearance  of  an  isolated  ommatidium  composed  of  large  pear- 
shaped  ganglion  cells,  whose  proximal  ends  form  coarse  nerve  tubes.  There  is 
another  group  of  cells,  similar  to  those  just  described,  scattered  along  the  proxi- 
mal end  of  the  main  nerve,  some  of  them  outside  the  brain  sheath,  but  the  major- 
ity within  it,  on  the  root  of  the  nerve  as  it  passes  over  the  surface  of  the  hemi- 
spheres. (Figs.  39,  48,  51,  66,  109,  ol.l.n.  and  gcl  and  gc2.) 

Both  these  cell  groups,  which  contain  granules  that  have  a  glistening  white 
appearance  in  reflected  light,  are  the  modified  descendants  of  the  cells  constitut- 
ing the  original  visual  placode.  Even  in  the  adult,  they  still  show  traces  of 


OLFACTORY    ORGAN    OF    LIMULUS.  163 

the  white  pigment,  of  the  refractive  visual  rods  or  rhabdoms,  and  of  their  primi- 
tive grouping  into  ommatidia. 

In  young  Limuli,  the  roots  of  the  lateral  olfactories  become  less  compact,  and 
as  they  were  seldom  seen  in  methylene  blue  preparations,  it  was  very  difficult  to 
follow  them.  They  appear  to  shift  their  point  of  attachment  from  the  second 
optic  ganglion,  toward  the  inner  face  of  the  olfactory  lobes,  near  the  tract  uniting 
the  median  and  lateral  eye  centers  (Fig.  51,  oc.tr.)  Whether  they  passed  through 
this  tract  to  the  olfactory  lobes  could  not  be  determined.  In  a  few  cases  (methyl- 
ene blue)  a  small  strand  of  fibers  was  seen  to  leave  the  main  root  and  pass 
mesially  toward  the  horns  of  the  olfactory  lobes.  (Fig.  51,  z.) 

Median  Olfactory  Nerve. — When  the  united  olfactory  placodes  move  for- 
ward away  from  the  brain,  a  new  outgrowth  from  the  hemispheres  and  ol- 
factory lobes  appears  which  follows  the  placodes  forward,  or  is  drawn  out  by 
them,  to  form  the  median  olfactory  nerve.  (Figs.  38,  39,  41,  48,  66,  ol.m.n.)  It 
consists  of  large  globular  masses  of  minute  ganglion  cells,  each  lobule  con- 
taining a  central  core  of  medullary  substance,  similar  to  that  in  the 
hemispheres. 

In  young  Limuli  (2  to  3  inches  long)  there  are  four  distinct  roots  to  the  median 
nerve,  two  haemal  ones  continuous  with  the  horns  of  the  olfactory  lobes,  and  two 
neural  ones,  continuous  with  the  anterior  median  lobes  of  the  cerebral  hemispheres. 
(Fig.  48.)  Each  root  contains  a  medullary  core  of  neuropile  surrounded  by  a 
cortex  of  "  granule  cells,"  the  cortex  and  neuropile  passing  without  perceptible 
change  into  the  cortex  and  the  neuropile  of  the  cerebral  hemispheres  and  the  ol- 
factory lobes.  (Figs.  48,  51.) 

In  the  adult,  the  two  haemal  stalks  disappear,  while  the  two  neural  ones  unite 
and  shift  their  attachment  in  a  neuro-posterior  direction,  so  that  they  are  ultimately 
widely  separated  from  the  apices  of  the  olfactory  lobes. 

In  larvae  about  two  inches  long  the  distal  ends  of  the  three  olfactory  nerves 
form  a  rich  plexus  of  nerves  terminating  in  a  small  patch  of  ectoderm  that  may 
then  be  recognized  as  the  definitive  olfactory  organ. 

Summary. — The  lateral  olfactory  nerves,  then,  are  characterized  as  follows: 
The  "  ganglion  cells"  are  large  and  pear-shaped,  and  arranged  in  small  ommatidia- 
like  clusters.  Granule  cells  and  neuropile  are  never  present.  The  fibers  are 
coarse  tubes,  with  distinct  sheaths.  The  nerves  terminate  in  the  lateral  portion 
of  the  olfactory  organ  and  in  the  surrounding  integument.  The  ganglion  cells  of 
the  lateral  olfactories  are  the  metamorphosed  visual  cells  of  the  initial  olfactory 
organ. 

The  median  olfactory  nerve  represents  a  later,  or  secondary,  outgrowth  of  the 
hemispheres  and  of  the  olfactory  lobes.  Its  ganglion  consists  of  lobular  masses  of 
granule  cells  and  neuropile,  and  never  contains  large  cells  of  a  sensory  nature. 
Its  end  branches  are  bundles  of  naked  fibers,  or  at  least  they  have  no  visible 
sheath.  They  terminate  in  the  central  region  of  the  olfactory  organ. 


164          THE  OLFACTORY  ORGANS  AND  THE  OLFACTORY  LOBES. 

II.  THE  OLFACTORY  LOBES  OF  ARACHNIDS. 

Development. — The  olfactory  lobes  (organe  stratifie,  St.  Remy)  are  prob- 
ably present  in  all  arthropods.  They  always  form  a  conspicuous  part  of  the  fore- 
brain  in  arachnids,  but  their  functions  and  their  relations  to  other  parts  of  the 
procephalon  are  unknown,  except  in  Limulus  where  they  are  associated  with  the 
olfactory  nerves;  their  function  is  thus  definitely  indicated.  It  is  singular  that 
Limulus  is  also  the  only  form  in  which  the  lobes  come  into  close  morphological 
relation  with  the  nerve  roots  to  the  median  ocelli. 

In  the  scorpion  and  in  spiders,  the  olfactory  lobes  arise  from  the  walls  of  a 
deep  transverse  groove  extending  across  the  anterior  end  of  the  medullary  plate. 
The  groove  probably  represents  the  whole  of  the  first  neuromere,  hence  they  repre- 
sent the  very  anterior  margin  of  the  primitive  nerve  axis.  (Figs.  15,  16,  20,  46.) 

In  the  later  stages,  the  groove  closes  and  its  walls  form  a  conspicuous  crescentic 
band  of  small,  deeply  stainable  cells,  on  the  anterior  haemal  aspect  of  the  fore- 
brain,  Figs.  41,  42  ol.l. 

The  Olfactory  Lobes  of  Limulus. 

Development. — In  Limulus,  the  olfactory  lobes  appear  as  two  separate  in- 
foldings.  (Figs.  141,  142,  ol.l.)  Later  the  lobes  unite  and  migrate  backward 
over  the  haemal  surface  of  the  brain,  gradually  changing  from  a  thick,  bi-lobed 
transverse  bar  extending  across  the  very  anterior  end  of  the  brain,  to  an  elongated 
U-shaped  disc  lying  on  its  haemal  surface.  (Fig.  36.)  In  the  adult,  the  posterior 
margin  of  the  bow  extends  backward,  well  below  the.  middle  of  the  cheliceral  seg- 
ment, farther  back  than  its  position  in  the  half  grown  specimens  shown  in 
Figs  47,  5,  48,  51. 

The  lobe  is  formed  from  the  posterior  wall  of  the  original  infolding,  the  mem- 
branous anterior  wall  disappearing  during  the  later  stages. 

The  entire  margin  of  the  lobes  consists  of  very  small,  closely  packed  cells 
resembling  the  granule  cells  of  the  cerebral  cortex.  As  they  freely  absorb  all 
kinds  of  nuclear  stains,  the  outlines  of  the  lobes  can  usually  be  seen  with  great 
distinctness. 

In  young  Limuli  the  anterior  arms  of  the  bow-shaped  lobes  are  drawn  together, 
forming  two  slender  horns  which  up  to  the  late  larval  stages,  are  continuous  with 
the  lips  of  the  anterior  neuropore.  (Fig.  36,  A  and  B.)  At  about  the  time  the 
neuropore  closes  (after  the  trilobite  stage)  there  is  a  vigorous  forward  outgrowth 
at  this  point,  apparently  originating  in  the  hemispheres.  This  forward  outgrowth 
carries  with  it  the  pointed  ends  of  the  olfactory  lobes  and  the  peculiar  tissue  of  the 
hemispheres,  giving  rise  to  the  median  olfactory  nerve  and  its  ganglion.  Thus 
the  stalk  of  the  median  olfactory  is  continuous  with  both  hemispheres  and  with 
both  horns  of  the  olfactory  lobes. 

Structure. — As  the  crabs  grow  older,  the  cells  on  the  median  portions  of  the 
lobes  become  very  large,  and  divide  into  several  distinct  clusters.  All  the  marginal 


THE    OLFACTORY   LOBES    IN    LIMULUS.  165 

cells,  however,  remain  very  small,  and  of  uniform  size  from  one  end  of  the  lobe 
to  the  other;  toward  the  center  of  the  lobes,  the  cells  gradually  increase  in  size. 
(Figs.  51  and  66.) 

The  small  marginal  cells  send  their  neurites  into  two  sharply  denned  bands 
of  very  dense  neuropile  extending  round  the  lobes.  (Fig.  48,  ol.np.)  In  the  an- 
terior horns,  these  bands  become  smaller,  and  unite  to  form  a  single  band.  The 
latter  extends  to  the  apex  of  the  horns,  and  is  continued  into  the  neuropile  axis  of 
the  median  nerve.  (Fig.  51.) 

In  methylene  blue,  either  one  or  both  of  the  bands  often  stand  out  very  clearly, 
with  only  a  single  regular  row  of  nerve  cells  visible  over  each  band.  The  dendrites 
of  these  cells  are  very  minute,  show  a  longitudinal  trend,  and  are  confined  to  their 
respective  bands.  See  the  posterior  median  part  of  the  olfactory  lobes  in  Fig.  51. 

On  the  inner  face  of  the  deeper  band  (Fig.  48,  ol.c1'4)  are  two  small  bundles  of 
longitudinal  fibers,  derived  in  part  from  medium  sized  cells  on  the  inner  margin  of 
the  lobes.  (Fig.  51,  ol.c2.)  These  bundles  are  continuous  with  the  tracts  arising 
from  the  median  eye  centers.  One  or  two  bundles  of  heavier  fibers  lie  below  and 
concentric  with  the  ones  just  described.  They  arise  from  the  cells  of  the  fourth 
optic  lobe,  op.f4.  On  reaching  the  opposite  side,  they  turn  outward  and  back- 
ward, and  join  the  main  longitudinal,  haemo-lateral  tracts,  c.op.f*. 

Of  the  larger  central  cells  of  the  olfactory  lobes,  we  may  recognize  special 
clusters  of  medium  size  cells  sending  neurites  into  the  neuropile  terminals  of  the 
two  pairs  of  median  eye  nerves,  and  hence  to  the  circular  bundles  and  to  the  tract 
connecting  them  with  the  lateral  eyes.  (Fig.  51,  ey.r*.) 

Farther  back  is  a  large  cluster  of  cells,  generally  very  conspicuous,  sending 
richly  branched  neurites  outward  and  backward,  underneath  (on  the  neural  side), 
the  main  marginal  bands  of  the  olfactory  lobes,  into  the  longitudinal  haemo-lateral 
tracts  of  the  same  side,  or  in  wide  curves  to  the  same  tracts  of  the  opposite  side. 
(Fig.  51,  olc\) 

Finally  in  the  posterior  bend  of  the  olfactory  lobes  there  are  some  very  large 
deep  lying  median  cells  that  send  their  enormous  branching  neurites  backward 
into  the  median  neuropile  mass  of  the  cheliceral  ganglion  and  hence  right  and  left 
along  the  median  haemal  side  of  each  crus  ol.  c5. 

In  the  older  crabs,  the  horns  of  the  olfactory  lobes  gradually  withdraw  in  a 
posterior  haemal  direction  and  finally  lose  their  connection  with  the  median  ol- 
factory nerve  root.  Thus  the  main  center  of  the  olfactory  lobes  becomes  relatively 
isolated,  unless,  as  it  appears  probable,  the  lateral  nerves  ultimately  establish  a 
connection  with  them,  through  the  median  and  lateral  eye  tracts. 

III.  THE  OLFACTORY  ORGANS  IN  PHYLLOPODS.     FRONTAL  ORGANS. 

The  olfactory  organ  of  Limulus  undoubtedly  represents  a  highly  specialized 
condition  of  the  characteristic  "frontal  sense  organs"  of  the  phyllopods.  Each 
resembles  the  other  in  location,  innervation,  origin,  and  histological  structure. 


i66 


THE  OLFACTORY  ORGANS  AND  THE  OLFACTORY  LOBES. 


We  may  recognize  two  sets  of  organs  in  the  phyllopods,  the  paired  dorsal 
ones  and  the  unpaired  ventral  ones.  They  probably  correspond  to  the  stemmata, 
or  frontal  ocelli  of  insects. 


Branchipus. 

In  Branchipus  the  dorsal  or  paired  frontal-organs  consist  of  a  compact  mass 
of  small  ganglion  cells,  with  one  or  two  large  ones  situated  on  either  side  of  the 
ocelli.  (Figs.  95,  no,  B.)  The  terminal  cells  are  in  contact  with  the  unthick- 


oc  df.o 


m-f.  o. 


b  r. 


FIG.  no. — The  parietal  eye  and  olfactory  organs,  or  frontal  organs,  of  Branchipus.  A,  The  median  olfactory 
nerve  of  a  young  larva,  showing  at  the  base  of  the  nerve,  the  ganglionic  enlargement,  w,  formed  on  the  anterior 
surface  of  the  forebrain;  B,  a  more  mature  specimen,  showing  the  breaking  up  of  the  lobes  into  a  nerve  plexus 
containing  ommatidia-like  clusters  of  cells;  C,  one  of  the  cell  clusters  more  highly  magnified. 

ened  epidermis  in  the  center  of  a  faint  rounded  elevation.  They  are  connected 
with  a  small  compact  nerve,  that  runs  parallel  with  the  ocellar  nerves,  and  that 
arises  from  the  anterior  surface  of  the  brain  near  the  root  of  the  lateral  eye  ganglion. 

The  embryonic  organ  is  formed  by  the  separation,  from  the  base  of  the  lateral 
eye  ganglion,  of  a  small  patch  of  neuro-epithelium,  which  then  migrates  under 
the  epidermis  toward  the  anterior  median  line  of  the  head. 

The  history  of  this  organ,  therefore,  is  practically  identical  with  that  of  the 
lateral,  or  primary  olfactory  placode  of  Limulus. 


OLFACTORY  ORGAN  OF  BRANCHIPUS  AND  APUS.  167 

The  ventral  frontal-organ  is  unpaired  and  lies  just  in  front  of  the  ocelli. 
In  larvae  about  10  mm.  long,  the  organ  is  merely  a  rounded  area,  without  any  local 
thickening  of  the  chiten  or  epidermis,  in  which  terminate  a  great  many  fine  nerve 
fibers,  B,  m.f.o.  In  very  young  larvae  the  latter  arise  from  the  united  anterior 
ends  of  two  thick  ridges,  or  lobes,  on  the  anterior  surface  of  the  forebrain.  (Fig. 
no,  A,w.)  These  lobes  are  solid  masses  of  cells  like  those  in  the  forebrain  and 
undoubtedly  arise  as  an  outgrowth  from  it.  In  the  later  stages,  therefore  long 
after  the  ocelli  are  fully  formed,  they  increase  greatly  in  size,  expanding  laterally 
and  forward,  thus  forming  two  wing-like  plates,  which  still  later  break  up  into 
many  scattered  sensory  buds  united  by  a  nerve  plexus,  B,  w. 

Each  sensory  bud  contains  several  radiating  cells;  the  latter  are  clear  on  the 
periphery,  and  their  pointed  inner  ends  are  granular  and  capped  by  refractive 
plates  or  rods,  like  those  on  the  retinal  cells.  (Fig.  no,  C.)  These  buds, 
therefore,  resemble  the  isolated  ommatidia  arising  from  the  lateral  olfactory 
nerves  in  Limulus. 

In  the  adult  Branchipus,  the  buds  are  united  with  the  brain  by  loose  nerve 
strands  containing  dark  colored  bipolar  cells,  the  remnants  of  the  stalk  by  which 
the  median  olfactory  lobes  were  connected  with  the  brain.  A  small  cluster  of 
nuclei,  g.,  at  the  base  of  the  median  nerve  represents  the  remnant  of  the  unpaired 
portion  of  the  lobes. 

Nowikoff,  '05,  also  recognizes  the  resemblance  of  these  cell  clusters  (in  Lim- 
nadia)  to  groups  of  retinal  cells,  as  I  had  previously  done  for  Limulus  in  1893. 
He  regards  them  as  detached  retinal  cells  belonging  to  the  median  ocellus.  But 
the  development  of  these  cells  in  Branchipus,  long  after  the  ocelli  are  formed,  and 
the  development  of  the  lateral  olfactory  organ  in  Limulus,  show  clearly  enough 
that  the  isolated  ommatidia  are  formed  from  the  breaking  up  of  independent  sense 
organs,  quite  distinct  from  the  median  eye. 

The  median  frontal  organ  of  Branchipus  clearly  corresponds  to  the  median 
olfactory  organ  of  Limulus,  not  only  in  its  position,  but  in  its  development  as  a 
ganglionic  outgrowth  of  the  forebrain.  There  is,  however,  this  difference,  that  in 
Branchipus  there  are  no  recognizable  hemispheres,  and  the  sensory  buds  are 
formed  from  the  median  olfactory  outgrowth,  while  in  Limulus  they  are  formed 
from  the  lateral  one. 

Apus. 

In  Apus  (Fig.  in),  the  frontal  organ  is  represented  by  a  thick  oval  sclerite 
behind  the  eyes.  Here  the  underlying  ectoderm  is  thickened  and  contains  ver- 
tical fibers  crossed  by  several  layers  of  horizontal  ones.  Between  these  coarse 
fibers  is  a  network  of  large  ganglion-like  cells,  that  appear  to  be  connected  with 
the  branches  of  two  large  nerves  l.f.n.  (lateral  olfactories),  containing  numerous 
scattered  ganglion  cells.  These  nerves  arise  from  the  base  of  the  lateral  eye  gan- 
glia and  are  distributed  over  a  wide  area,  behind  and  between  the  lateral  eyes, 
including  the  thickened  ectoderm  beneath  the  sclerite. 


i68 


THE  OLFACTORY  ORGANS  AND  THE  OLFACTORY  LOBES. 


Similar  conditions  to  those  in  Branchipus  and  Apus  prevail  in  other  phyllo- 
pods, but  we  need  not  consider  them  here.  It  is  enough  to  show  that  the  remark- 
able olfactory  organ  of  Limulus  becomes  more  intelligible  when  compared  with  the 
condition  of  the  frontal  organs  in  phyllopods.  In  both  cases  we  may  witness 
important  steps  in  the  transformation  of  primitive  segmental  sense  organs  into 
a  very  special  condition  preparatory  for,  and  in  part  realizing,  a  new  function. 

The  causes  lying  back  of  this  transformation  are  remote  and  probably  in- 
accessible. I  formerly  supposed  that  the  unfavorable  position  of  the  organs  in 


e. 


,  c.e.v 


m 


fo 


.-V 


FIG.  in. — Head  of  Apus,  showing  the   eye  chamber  c.e.v.  and  its  external  opening,  o,  the  median  frontal  organ, 
m.f.o,  and  the  course  of  the  lateral,  frontal  nerve,  l.f.n. 

Limulus  might  have  had  something  to  do  with  their  loss  of  visual  functions,  but  I 
now  regard  this  as  merely  a  coincidence,  since  their  position  in  the  free  swimming 
phyllopods  is  not  unfavorable  to  their  use  as  eyes,  and  yet  they  have  suffered  a 
similar  transformation. 


III.  COMPARISON  OF  OLFACTORY  ORGANS  IN  VERTEBRATES  AND  ARTHROPODS. 

i.  Number  of  Placodes. — In  arthropods  the  olfactory  organ  arises  from  two 
pairs  of  sensory  placodes  that  still  retain  structures  characteristic  of  visual  cells. 
According  to  the  amount  of  median  fusion  that  has  taken  place,  the  adult  organ 
may  be  regarded  as  a  single  unpaired  one,  (Limulus) ;  or  as  three,  a  paired  and 
unpaired  one,  (Apus);  or  as  two  pairs,  (Branchipus  and  other  phyllopods.) 

In  vertebrates  the  primitive  olfactory  organ  has  been  regarded  by  various 
authors  as  single,  paired,  or  multiple.  The  first  view  has  been  widely  entertained, 
especially  by  the  older  anatomists,  and  was  based  largely  on  the  condition  in 
the  cyclostomes  and  in  Amphioxus.  Of  more  recent  authors,  Burckhart,  1908,  is 
inclined  to  regard  the  vertebrate  olfactory  organ  as  formed  by  the  fusion  of  two 
pairs  of  placodes.  Kuppfer  distinguishes  three  parts  in  Petromyzon,  an  unpaired 
one  at  the  point  where  the  neuron  last  closes  and  one  on  either  side. 

These  conflicting  views  are  intelligible  on  the  assumption  that  the  vertebrate 
organ  is  derived  from  three  or  four  separated  anlagen,  as  it  is  in  Limulus  and  the 


COMPARISON    WITH   VERTEBRATES.  169 

phyllopods,  and  that  in  both  classes  it  may  undergo  varying  degrees  of  fusion,  or 
of  unequal  development  of  its  constituent  parts. 

2.  Number   of   Nerves. — In   the   arthropods,  the   olfactory  organ   always 
shows  traces  of  two  pairs  of  nerves,  even  when  the  organ  itself  is  practically 
unpaired.     I   pointed   out   in    1893    that   the   two   pairs   of   olfactory  nerves, 
then  known  in  but  a  few  vertebrates,  were  comparable  with  the  two  pairs  in 
Limulus,  but  not  with  any  other  cranial  nerves  known  elsewhere,  either  in  verte- 
brates or  invertebrates;  I  stated  that:     "It  is  now  known  that  each  olfactory 
nerve  of  the  higher  vertebrates  is  represented  in  amphibia  by  two  distinct  nerves, 
which  have  been  likened  to  the  dorsal  and  ventral  roots  of  a  spinal  nerve.     But  if 
this  were  so  they  would  differ  from  all  other  spinal  nerves  in  that  both  dorsal  and 
ventral  branches  supply  sense  organs.     Moreover,  on  any  supposition  they  are 
entirely  different  from  those  belonging  to  the  other  sense  organs  of  the  forebrain, 
such  as  the  lateral  and  parietal  eye,  and  the  auditory  organ.     This  condition  is 
quite  inexplicable  on  any  theory  founded  on  vertebrate  anatomy.     But  this  very 
thing  occurs  in  the  olfactory  organ  of  Limulus,  although  the  meaning  of  it  cannot 
be  explained  there  any  more  than  in  vertebrates." 

It  is  interesting  to  recall  the  statements  made  at  that  time,  since  they  have 
been  in  some  respects  so  fully  confirmed  by  the  subsequent  discovery  of  two  pairs 
of  olfactory  nerves  by  Pinkus  1894,  in  Protopterus;  by  Allis  1897,  in  Amia;  by 
Locy  1899,  in  the  elasmobranchs;  and  by  Zewertzoff  1902,  in  the  embryos  of  Cer- 
adotus.  If  the  condition  in  Limulus  had  received  more  serious  consideration,  it  is 
very  possible  that  the  little  "foot  note"  to  the  ancestral  history  of  the  vertebrate 
brain,  which  according  to  Locy,  is  furnished  by  the  development  of  the  nervus 
terminalis,  might  have  expanded  into  a  chapter. 

3.  Structure  and  Termination  of  the  Nerves. — Arthropods.     Both  pairs 
of  nerves,  while  supplying  the  same  organs,  are  widely  different  in  their  histo- 
logical  characters,  and  in  their  central  termination.     Both  pairs  are  ganglionated. 
The  lateral  nerves  contain  very  coarse  nerve  fibers  with  distinct  sheaths,  and 
scattered  clusters  of  gigantic  ganglion  cells;  they  terminate  in  the  base  of  the  brain, 
near  the  roots  of  the  optic  tracts.     The  median  nerve  contains  fine  sheathless 
fibers,  dense  masses  of  neuropile  and  small  ganglion  cells;  it  has  its  roots  in 
the  olfactory  lobes  and  in  the  hemispheres. 

The  olfactory  nerves  are  usui  generis"  and  are  only  remotely  comparable 
with  any  other  cranial  nerves,  such  as  the  optic  nerves,  the  segmental  gustatory 
nerves,  or  with  the  components  of  less  specialized  peripheral  nerves. 

Verebrates.  According  to  Locy,  both  pairs  of  olfactory  nerves  are  ganglion- 
ated, and  although  closely  associated  in  their  peripheral  termination,  have  sepa- 
rate central  origins,  hence  they  are  considered  to  be  separate  nerves,  not  as 
separate  parts  of  one  nerve. 

What  Pinkus  says  of  the  nervus  terminalis,  viz.,  "  Eine  kolbige  Anschwellung 
dieses  Nerven,  welche  durch  die  Einlagerung  grosskerniger,  von  alien  anderen 
nervosen  Zellen  des  Protopterus  anscheinend  verschiedenen  Zellen  bedingt  ist, 


1 70          THE  OLFACTORY  ORGANS  AND  THE  OLFACTORY  LOBES. 

macht  es  wahrscheinlich  dass  wir  es  hier  mit  einem  neuen  Organ  zu  thun  haben" 
applies  equally  well  to  the  lateral  olfactory  nerve  of  Limulus. 

The  nervus  terminalis  in  elasmobranchs  may  have  either  a  neural  or  a  haemal 
origin,  but  it  is  generally  closely  connected  with  the  lamina  terminalis  (Locy); 
or  according  to  Pinkus  in  Protopterus,  it  "Am  vorderende  des  recessus  praeopticus 
das  Zweischenhirn  verlast,"  thus  indicating  its  probable  origin  near  the  root  of 
the  lateral  eye  ganglion,  as  opposed  to  the  origin  of  the  main  olfactory  from  the 
dorsal  anterior  surface  of  the  hemispheres. 

4.  Origin  of  Olfactory  Ganglia. — Arthropods.     The  lateral  placode  is 
a    primitive    visual   organ   which   becomes   bodily   converted   into   the    giant 
ganglion  cells  of  the  lateral  nerves.     The  median  placode  is  retained  to  form 
the  epithelial  area  in  or  near  which  all  the  nerves  terminate.      Its  ganglion 
cells  are  very  minute  and  arise  as  outgrowths  of  the  hemispheres  and  of  the 
olfactory  lobes. 

Vertebrates.  The  difference  between  the  development  of  the  median  and 
general  placodes  is  unknown. 

5.  Position    of    Placode    Cells.— Arthropods.     The    olfactory    placodes 
arise  from  the  anterior  lateral  margin  of  the  open  medullary  plate,  but  unlike  the 
adjacent  visual  placodes  they  are  not  swept  into  the  neurocoele  by  the  overgrowth 
of  the  palial  fold;  consequently  the  sensory  epithelium  is  upright,  and  does  not 
form  the  wall  of  a  closed  sac. 

Vertebrates.     The  same. 

6.  Serial  Location  of  the  Placodes  and  their  Migration. — In  arthropods 
(Limulus),  the  lateral  olfactory  placodes  are  originally  located  on  the  margins  of 
the  medullary  plate  (procephalic  lobes) ,  between  the  median  ocelli  and  the  lateral 
eyes;  they  therefore  appear  to  form  the  second  set  of  cranial  sense  organs  and 
nerves;  the  median  ocelli  forming  the  first  set,  and  the  lateral  eyes,  the  third 
(Fig.    142).     The   lateral   olfactory   placodes   first   move   toward   the   anterior 
median  margin  of   the  palial  fold  (edge  of  the  neuropore)  and  then  forward, 
taking  up  a  position  in  the  adult  either  on  the  neural  surface  (Limulus),  the  apex 
(Branchipus),  or  the  haemal  surface  of  the  head  (many  phyllopods) ,  its  position  in 
each  case  being  determined  by  local  variations  in  the  growth  of  the  forebrain  and 
the  external  surface  of  the  forehead.     The  arrangement  of  ocelli,  olfactory  organs, 
and  lateral  eyes  in  the  fully  formed  head,  may,  or  may  not,  agree  with  their  primi- 
tive serial  arrangement  on  the  margins  of  the  cephalic  lobes.     The  olfactory 
organs  may  stand  alone  in  the  adult  (Limulus)  or  they  may  unite  with  the  ocelli 
and  lateral  eyes  to  form  a  compact  median  group  (Apus,  Limnadia  etc.).     (Figs. 
8  and  in.) 

In  vertebrates  the  same  conditions  are  indicated,  but  the  serial  order  of  the 
ocellar,  olfactory,  and  lateral  eye  placodes  cannot  be  certainly  determined  in 
vertebrates  without  locating  their  positions  on  the  margins  of  the  open  neural 
plate  more  accurately  than  has  yet  been  done.  Their  serial  order  on  the  surface 
of  the  head  in  the  later  stages  is  not  decisive. 


COMPARISON    WITH   VERTEBRATES.  17 1 

During  the  closing  of  the  medullary  plate,  the  olfactory  organs  may  or  may 
not  unite  in  the  median  line;  but  they  invariably  move  forward  either  to  the 
median  neural  surface  (cyclostomes),  or  still  farther  forward  onto  the  anterior 
haemal  side  of  the  forehead.  (Fig.  4.) 

In  most  ostracoderms  (Bothriolepis,  Tremataspis,  Cephalaspis),  the  olfactory 
organs  and  the  median  and  lateral  eyes  unite  to  form  a  very  compact  group  on 
the  neural  surface  of  the  head,  very  similar  to  the  grouping  in  Apus  and  other 
phyllopods,  where  they  may  be  located  on  either  the  neural  or  haemal  surface. 
(Figs.  5,  8,  12  and  in.) 

In  the  cyclostomes,  all  the  pro-cephalic  sense  organs  are  on  the  neural 
surface,  but  they  are  not  so  compactly  arranged  as  in  the  ostracoderms  or  in  the 
phyllopods. 

7.  The  Olfactory  Lobes. — Arthropods.     In  the  arachnids,  the  olfactory 
lobes  make  their  appearance  as  a  deep  transverse  infolding  on  the  very  an- 
terior margin  of  the  medullary  plate.     They  soon  sink  below  the  surface  and 
move  backward  onto  the  haemal  side  of  the  forebrain.     The  posterior  wall  of  the 
infolding  gives  rise  to  the  olfactory  neurones;  the  anterior  wall  is  membranous, 
and  later  disappears.     The  cavity  of  the  infolding,  as  long  as  it  can  be  recognized, 
communicates  with  the  spaces  between  the  hemispheres,  and  with  those  under 
the  palium,  i.e.,  with  the  potential  first  and  second  ventricles.     (Figs.  46  and  47.) 
The  roots  of  the  median  olfactory  nerve  and  the  parietal  eye  nerves  may  be 
located  in  the  olfactory  lobes. 

Vertebrates.  The  olfactory  lobes  arise  as  deep  transverse  infoldings  across 
the  anterior  margin  of  the  open  medullary  plate  (frog),  (Figs.  25  and  26), 
therefore  from  precisely  the  same  location  and  in  the  same  manner  as 
in  the  arachnids.  The  lobes  are  finally  located  on  the  anterior  haemal  margin  of 
the  forebrain,  and  their  cavities  communicate  as  in  Limulus.  They  are  the  only 
brain  lobes  that  have  a  conspicuous  connection  with  both  the  olfactory  organ  and 
with  the  parietal  eyes.  (Figs.  43,  44.) 

8.  Function. — The  olfactory  organ  of  fishes  is  recognized  to  be  an  olfactory 
organ  largely  on  morphological  evidence.     Whether  or  no  it  actually  has  what 
is  commonly  understood  to  be  an  olfactory  function,  whatever  that  may  be,  rests 
on  surmise.     Nevertheless,  it  would  still  be  proper  to  speak  of  it  as  an  olfactory 
organ,  even  if  it  were  experimentally  demonstrated  that  it  reacted  to  sound  or  to 
light,  because  we  know  that  it  is  the  true  homologue  of  the  olfactory  organ  in  the 
mammals. 

It  is  well  to  bear  this  in  mind  in  comparing  the  olfactory  organ  of  arthropods 
with  that  of  vertebrates.  Although  our  case  rests  primarily  on  morphological 
evidence,  the  evidence  afforded  by  function,  while  meager,  is  confirmatory. 
Stimulation  of  the  olfactory  organ  of  Limulus  with  various  kinds  of  food,  with  acids 
and  with  ammonia,  does  not  usually  produce  any  characteristic  reflexes.  Even 
drops  of  rather  strong  hydrochloric  acid,  or  ammonia,  have  no  more  effect  than 
when  applied  to  other  parts  of  the  body;  they  cause  a  slight  start,  nothing  more. 


172          THE  OLFACTORY  ORGANS  AND  THE  OLFACTORY  LOBES. 

In  order  to  test  its  glandular  nature,  the  olfactory  organ  was  cut  out,  its  outer 
surface  wiped  dry,  and  then  the  attached  nerves  stimulated  with  electricity; 
no  traces  of  a  secretion  appeared.  But  electrical  stimulation  of  the  olfactory 
region  in  uninjured  male  crabs  in  some  instances  at  once  produced  very  remark- 
able leg  movements,  rarely  seen  under  any  other  circumstances. 

When  the  electrodes  are  applied  to  the  olfactory  organs  of  the  male,  if  the 
experiment  is  successful,  rapid  chewing  movements  of  the  mandibles  are  produced, 
accompanied  by  vigorous  snapping  of  the  chelicerae,  which  may  finally  become 
rigid  and  stretched  out  backward  at  full  length.  At  the  same  time  the  second 
pair  of  legs  (the  ones  used  to  seize  the  females)  which  during  all  our  preceding 
experiments  on  the  gustatory  organs  have  remained  motionless,  are  now  quickly 
and  repeatedly  flexed,  as  though  trying  to  hug  or  grasp  some  object  and  force  it 
toward  the  r^outh;  all  the  other  legs  remain  motionless.  Stimulation  of  the  region 
about  the  olfactory  organ,  or  along  the  median  line  between  the  olfactory  organ 
and  the  brain,  or  above  the  brain,  may  produce  the  same  effect. 

These  experiments  indicate  that  the  olfactory  organ  is  a  chemotactic  organ, 
whose  activities  are  associated  with  the  process  of  eating,  although  it  is  difficult  to 
explain  why  the  chewing  movements  are  not  produced  by  direct  stimulation  of  the 
olfactory  organ  with  food.  On  the  other  hand,  the  extraordinary  hugging  and 
grasping  movements  aroused  in  the  second  pair  of  legs  of  the  males,  when  the 
organ  is  electrically  stimulated,  indicate  that  it  is  used  in  finding  the  females  during 
the  mating  season.  That  an  organ  for  this  purpose  must  be  present  seems  cer- 
tain, for  the  males  during  the  breeding  season  seek  out  the  females  and  attach 
themselves  to  them  with  great  precision.  In  confinement,  the  males  usually 
attach  themselves  to  the  abdomen  of  the  females,  but  males  whose  olfactory 
organ  had  been  cut  out  did  not  do  so.  Smearing  the  olfactory  organs  of  males 
with  the  ova  or  secretions  of  oviducts  produces  no  effect. 

In  primitive  vertebrates,  the  olfactory  organ  was  doubtless  of  great  importance 
in  mating,  as  indeed  it  is  through  the  whole  series  of  vertebrates.  It  is  of  special 
interest  that  they  were  intimately  associated  with  sexual  activities  in  such  remote 
ancestors  of  the  vertebrates  as  the  arachnids.  In  this  connection,  the  olfactory 
function  of  the  antennae  of  insects,  and  its  relation  to  sexual  reproduction  will  be 
recalled. 

Summary  and  Conclusion. 

The  gustatory  organs  play  an  important  part  throughout  the  entire  range  of 
arthropods,  and  they  have  done  so  ever  since  the  appendages  have  been  used  as 
aids  to  nutrition.  In  Limulus,  they  form  the  most  voluminous  nerve  tracts  and 
nerve  centers  of  any  single  set  of  organs,  and  the  great  size  of  the  hemispheres  is 
largely  due  to  important  gustatory  centers  that  are  located  in  them. 

The  olfactory  apparatus  and  the  olfactory  function  arose  in  the  higher 
arachnids  through  the  secondary  modifications  of  preexisting  organs  that  had 
some  other  function  or  meaning. 


SUMMARY  AND    CONCLUSION.  173 

The  primary  sensory  functions  of  the  marine  arachnids  were,  therefore, 
visual  and  gustatory,  and  the  main  centers  for  these  functions  lie  respectively  in 
the  optic  ganglia  and  the  primitive  cerebral  hemispheres. 

In  the  higher  marine  arachnids,  and  toward  the  beginning  of  the  ostracoderm, 
or  primitive  vertebrate  stage  in  phylogeny,  the  olfactory  function,  as  a  secondary 
aid  to  nutritive  and  sexual  activities,  became  definitely  localized,  and  the  most 
anterior  section  of  the  forebrain  was  successfully  preempted  as  the  main  olfactory 
center. 

The  auditory  function  was  definitely  localized  at  a  much  later  period  than 
any  of  the  three  preceding  ones,  and  it  has  probably  for  that  reason  never  suc- 
ceeded in  creating  for  itself  a  definite,  sharply  circumscribed,  brain  region. 


CHAPTER  XL 
FUNCTIONS  OF  THE  BRAIN. 

PART  I. 

Introduction.— In  the  preceding  chapters,  we  have  shown  that  there  is  a 
far  reaching  resemblance  in  structure  and  development  between  the  brains  of 
vertebrates  and  arachnids.  In  this  chapter,  we  shall  show  that  they  agree  in 
function,  and  in  their  physiological  relations  to  other  parts  of  the  body. 

In  the  arachnids,  the  location  of  several  important  cerebral  centers  is  already 
clearly  indicated  by  the  peripheral  termination  of  the  associated  nerves,  as  for 
example,  the  visual,  gustatory,  cardiac,  and  respiratory  centers.  Nevertheless, 
it  seemed  highly  desirable,  indeed  imperative,  that  there  should  be  some  experi- 
mental evidence  to  demonstrate  the  course  of  the  principal  nerve  impulses,  and  to 
locate  by  experiment  the  centers  that  control  a  group  of  similar  activities,  or  that 
bring  them  into  coordinate  relation  with  other  activities. 

Although  Limulus  has  the  largest  forebrain,  or  hemispheres,  of  any  inverte- 
brate known,  it  does  not  approach  such  animals  as  the  hymenoptera,  the  cephalo- 
pods,  the  crayfish,  or  the  lobsters,  in  alertness,  or  in  the  variety  of  its  responses  to 
visual,  tactile,  or  other  stimuli.  When  compared  with  the  members  of  its  own 
class,  such  as  the  spiders  and  scorpions,  it  appears  stupid  and  quite  unaffected 
by  the  events  going  on  in  the  world  about  it.  Limulus,  no  doubt,  appears  to  lead 
a  sluggish  life  in  the  muddy  bottoms  of  deep  waters;  but  we  should  be  greatly  in 
error  if  we  were  to  estimate  the  probable  volume  and  complexity  of  its  coordinating 
centers,  or  of  what  corresponds  to  the  hemispheres  of  vertebrates,  by  its  so-called 
'  'manif estation  of  intelligence. ' '  Indeed  Limulus  would  furnish  very  little  material 
that  could  be  used  for  experimentation  or  observation  along  these  lines.  But, 
for  the  study  of  some  of  the  simpler  reflexes,  Limulus  is  not  excelled  by  any  other 
animal. 

The  following  experiments  were  made  in  the  summer  of  1897,  at  Woods  Hole. 
In  the  summer  of  1898,  Mr.  Raymond  Pearl,  then  a  student  at  Dartmouth,  work- 
ing under  my  direction,  repeated  many  of  my  experiments,  and  the  following  year 
added  others  of  his  own.  More  than  seventy  different  operations  were  performed, 
mostly  on  adult  animals.  They  generally  involved  the  transecting,  or  the  remov- 
ing, of  various  parts  of  the  brain,  or  cord,  in  order  to  determine  the  path  of  nerve 
impulses,  or  to  locate  the  centers  of  control. 

174 


METHODS.  175 

We  shall  describe  a  few  of  the  more  important  experiments,  and  summarize 
the  results  of  the  others  that  bear  on  the  main  problems  here  under  discussion. 


The  principal  method  of  obtaining  the  normal  reflexes  \y^s  to  place  healthy 
crabs  on  their  backs  on  some  convenient  table,  allowing  the  posterior  end  of  the 
abdomen  to  hang  over  the  edge.  After  a  few  minutes  their  muscles  relax,  and 
unless  disturbed,  they  remain  perfectly  quiet  for  a  long  time.  Meantime,  local 
stimuli  may  be  applied  which,  if  a  little  care  is  exercised,  usually  produce  very 
definite  reflexes  without  arousing  the  animal  from  its  comatose  condition. 

The  usual  stimuli  for  the  chewing  reflexes,  were  drops  of  clam  juice,  or  pieces 
of  clam,  or  the  like,  of  the  same  temperature  as  the  air;  and  a  breath  of  warm  air, 
or  the  gentle  touch  of  the  finger  tips,  for  the  crossed  thoracic,  the  abdomino-thor- 
acic,  or  other  temperature  reflexes.  Various  other  stimuli  were  also  used  from 
time  to  time,  as  indicated  in  the  description  of  results.  Having  familiarized 
myself  with  the  normal  reflexes,  the  brain  or  cord  was  sectioned  in  various  ways. 
After  the  recovery  from  the  shock,  which  lasts  from  five  minutes  to  an  hour  or 
two,  the  crab  was  tested  as  before  and  the  difference  in  behavior  noted. 

The  operations  were  performed  in  various  ways,  the  principal  difficulty  being 
to  avoid  the  great  loss  of  blood  following  any  puncture  of  the  skin  near  the  brain 
or  cord. 

When  the  section  had  to  be  accurately  located,  there  was  no  way  but  to  thor- 
oughly bleed  the  animal,  expose  the  parts,  and  section  as  desired  at  leisure.  This 
was  the  method  followed  in  transecting  one-half  of  the  abdominal  cord  at  a  given 
point,  and  in  cutting  it  in  halves  lengthwise. 

In  transecting  the  collar,  the  animal  was  tied  down  and  the  legs  fixed  in  a 
convenient  position;  a  quick  cut  was  then  made  across  the  collar,  care  being  used 
to  keep  the  opening  in  the  skin  as  small  as  possible.  To  prevent  the  loss  of  blood, 
that  spurts  with  great  force  from  the  opening,  the  wound  was  instantly 
plugged  with  a  tight  fitting  wad  of  absorbent  cotton  smeared  with  vaseline.  If 
the  operation  is  successful,  very  little  blood  is  lost,  the  animal  quickly  recovers, 
and  may  live  for  six  or  eight  weeks,  or  longer. 

The  principal  errors  to  be  guarded  against  arise  from  the  difficulty  of 
making  the  sections  in  the  desired  place,  and  from  the  degeneration  of  the  wounded 
or  isolated  parts  of  the  brain.  In  some  cases,  an  isolated  segment  of  the  nerve 
collar  would  degenerate  and  completely  disappear  in  a  few  days  after  the  operation; 
or  the  degeneration  may  extend  into  other  parts  of  the  brain  and  vitiate  the  results. 
In  some  of  the  most  successful  cases,  the  cut  surfaces  of  the  brain,  after  a  lapse 
of  several  weeks,  were  covered  with  an  incrustation  which,  if  removed,  left  the 
surfaces  almost  as  clean  and  sharply  defined  as  when  the  wounds  were 
first  made. 

To  check  these  sources  of  error,  we  have  made  careful  post-mortem  examina- 
tions and  have  excluded  all  those  experiments  in  which  there  is  any  doubt  about 
the  location  of  the  wound,  or  the  extent  of  the  degeneration. 


i76 


FUNCTIONS    OF    THE    BRAIN. 


Experiment  I — A. 

August  16.  Sectioned  anterior  end  of  left  crus.  (Fig.  113,  A.I.)  Before  the  operation, 
it  was  found  that  if  the  crab  came  to  rest  on  its  back,  and  the  margin  of  the  abdomen  was  gently 
touched  with  the  fingers,  the  legs  on  the  opposite  side  would  always  be  raised  a  fraction  of  a  second 
before  the  legs  of  the  same  side.  When  the  movements  are  once  started,  they  become  general. 

After  the  operation,  the  left  chelicera  is  very  restless,  snapping  and  moving  aimlessly 
back  and  forth.  The  second  left  leg  is  also  restless,  and  usually  elevated  higher  than  the  others. 

I.  Thoracic  Reflexes. — Fifteen  minutes  later. 

a.  Hand,  placed  on  left  side  of  thorax,  causes  a  slight  start  of  left  legs  and  left  gills. 
Reverse  experiment  gives  corresponding  results. 


FIGS.  112-113. —  Brains  of  adult  Limuli,  that  have  been  cut  in  various  ways  in  order  to  determine  the  func- 
tion of  the  principal  brain  regions  and  the  course  of  the  nerve  impulses.  All  the  figures  show  the  brains  from 
the  neural  surface,  with  the  right  side  toward  the  reader's  right  hand.  The  roman  numerals  indicate  different 
operations  performed  at  different  times  on  the  same  brain. 

b.  A  little  later  the  results  are  somewhat  varied.     The  right  legs  are  less  readily  induced 
to  make  reflex  movements  by  stimulation  of  either  side  than  are  the  left. 

c.  Hand  placed  on  the  left  side  of  thorax  caused  a  slight  stirring  of  the  legs  of  both  sides, 
or  a  raising  of  the  right  or  left  legs,  but  no  purposeful  movements  on  either  side.     But  placing 
the  hand  on  the  right  margin  causes  well  marked,  thrusting  away  movements  of  the  right  legs. 

d.  Twenty-four  hours  later,  repeated  c.  with  same  results. 

II.  Abdomino -thoracic  Reflexes.— a.  Fifteen  minutes  after  the  operation.  Hand  placed 
on  left  margin  of  the  abdomen  causes  sudden  depression  of  the  gills  of  both  sides  and  raising  of 
the  right  legs.  Reverse  experiment  gives  corresponding  results. 

The  gills  on  the  side  stimulated  contract  a  little  before  those  of  the  opposite  side.  This 
is  in  marked  contrast  with  the  fact  that  the  thoracic  appendages  of  the  opposite  side  always  move 
first  when  one  side  of  tne  abdomen  is  stimulated,  showing  that  the  abdominal  reflexes 
are  mainly  uncrossed,  and  the  abdomino-thoracic  are  mainly  crossed. 


EXPERIMENTS.  177 

b.  Twenty-four  hours  later.     The  right  legs  frequently  perform  spontaneous  movements, 
as  in  a  normal  crab,  but  the  left  are  quiet  unless  stimulated. 

c.  Hand  placed  on  the  right  margin  of  the  abdomen  causes  very  vigorous  thrusting  away  move- 
ments of  left  legs,  more  vigorous  than  can  be  produced  in  any  other  way.     The  right  legs  may 
be  raised  and  flexed  in  a  sort  of  spasm,  or  may  not;  but  they  never  thrash  about  as  the  left  legs 
do.     The  movements  of  the  left  legs  are  governed  entirely  by  the  stimulation,  and  cease  when 
the  fingers  are  removed  from  the  margin  of  the  abdomen. 

d.  When  the  hand  is  placed  on  the  left  margin  of  the  abdomen  the  right  legs  immediately 
move  back  and  forth  in  the  usual  manner,  the  left  legs  either  remaining  quiet,  or  spasmodically 
flexed.  If  the  crossed  reflexes  of  the  right  legs  are  violent  they  may  not  cease  on  removing 
the  stimulus,  and  the  animal  may  attempt  to  regain  its  upright  position. 

There  is  a  marked  difference  between  the  movements  of  the  right  legs,  in  d,  and  of  the  left 
ones  in  c.  The  movements  of  the  right  legs  are  those  of  a  normal  crab  when  stimulated.  The 
left  legs  may  move  more  vigorously,  in  response  to  a  crossed  abdominal  impulse,  than  the  right, 
but  their  movements  are  aimless,  and  cease  with  the  cessation  of  the  stimulus. 

These  experiments  show  conclusively  the  controlling  and  directing  effect  of  the  cerebral 
hemispheres. 

Experiment  I — B. 

Same  animal.  Thirty-six  hours  later.  Cut  all  the  free,  post-oral  cross  commissures  of  the 
thorax,  leaving  the  vagus  commissures  intact.  (Fig.  113,  A.  II.} 

I.  Thoracic  Reflexes. — a.  Ten  minutes  after  the  operation.     Hand  on  either  margin  of 
the  thorax  causes  slight  movements  of  the  legs  on  the  same  side,  but  none  whatever  on  the  op- 
posite one;  except  that  when  the  right  side  was  stimulated,  the  second  leg  on  the  left  made  vigor- 
ous movements. 

These  experiments  were  repeated  at  intervals  of  one  or  two  hours,  with  the  same  results, 
except  that  the  uncrossed  reflexes  gradually  became  more  pronounced. 

The  experiment  shows  that  there  are  crossed  and  uncrossed  thoracic  reflexes,  and  that  the 
crossed  ones  pass  to  the  opposite  side  through  the  thoracic  and  the  forebrain  commissures. 

II.  Abdomino-thoracic  Reflexes. — a.  Fifteen  minutes  after  the  second  operation,  hand 
placed  on  the  margin  of  the  abdomen  produced  only  faint  movements  of  the  legs  of  the  same  side. 
On  repeating  the  experiment,  at  intervals  of  an  hour,  the  crossed,  abdomino-thoracic  reflexes 
gradually  appeared,  and  three  or  four  hours  later  became  well  marked. 

III.  Gustatory  Reflexes. — a.  After  cutting  the  left  crus,  the  normal  chewing  movements 
could  be  readily  produced,  except  that  the  second  left  leg  was  spasmodic  and  irregular  in  its 
movements. 

b.  After  cutting  the  cross  commissures,  the  chewing  movements  that  could  be  induced  on 
the  right  were  very  feeble;  none  at  all  could  be  induced  on  the  left.  These  negative  results  were 
probably  due  to  the  feeble  condition  of  the  animal. 

IV.  Olfactory  Reflexes. — On  stimulating  the  olfactory  organ  with  the  electrodes,  move- 
ments of  all  the  right  thoracic  appendages  and  the  first  two  on  the  left  are  produced. 

Experiment  II— A. 

August  6  Female.  Sectioned  thoracic  cross  commissures  and  the  right  crus  back  of 
sixth  leg.  (Fig.  113,  B,  I.) 

August  14.  Crab  is  very  restless  when  taken  from  the  water.  The  thoracic  appendages 
are  in  almost  constant  motion,  waving  about  in  an  aimless  manner. 

I.  Thoracic  Reflexes.— August  14.  Uncrossed  reflexes  well  marked;  crossed,  indistinct  or 
absent. 


178  FUNCTIONS    OF    THE    BRAIN. 

II.  Abdomino-thoracic. — August  14.  a.  Hand  placed  on  right  side  of  abdomen  causes 
raising  of  the  abdomen  and  flexing  of  left  legs.     Hand  placed  on  left  margin  of  abdomen,  no 
result;  or  if  the  fingers  cover  considerable  area,  a  slight  raising  of  abdomen  may  be  produced. 

b.  August  25.  Crab  is  vigorous.  Hand,  or  even  the  tip  of  a  finger,  placed  lightly  on  the 
right  margin  of  the  abdomen  causes  raising  of  the  left  legs,  followed  shortly  afterward  by  the 
right,  and  then  by  general  movements  of  both  sides.  Hand  placed  on  the  left  margin  of  the 
abdomen,  and  on  the  left  gills,  produces  at  first  no  effect;  but  if  the  stimulus  is  increased,  then 
the  left  legs  are  raised,  followed  by  general  movements,  including  movements  of  the  right 
legs.  Experiment  repeated  many  times,  with  same  results. 

III.  Gustatory  Reflexes. — -a.  August   7.   Stimulation  of  jawrs  caused  normal  chewing 
movements  on  either  side,  but  movements  of  one  side  do  not  harmonize  with  those  of  the  other. 
b.  August  25.     Same. 

IV.  Respiratory  Reflexes. — August  24.  a.  When  at  rest,  the  left  abdominal  appendages 
are  more  elevated  than  the  right. 

b.  Stimulation  of  the  gill  warts  with  clam  causes  twitching  of  the  stimulated  endopodites, 
then  several  lateral  movements,  the  members  of  each  pair  stimulated  alternately  crossing  and 
uncrossing  over  the  median  line,  and  finally  a  full,  rhythmical,  up  and  down,  respiratory  move- 
ment of  all  the  abdominal  appendages. 

Experiment  II — B. 

August  25.  Cut  both  crura  back  of  the  chelicerae.  Subsequent  examination  showed  that 
the  cut  was  made  in  front  of  the  second  neuromere  on  the  left,  and  behind  it  on  the  right. 
(Fig.  113,  B.II.)  General  movements  of  the  legs  and  respiratory  movements  of  the  gills 
followed,  but  they  lasted  only  a  short  time. 

I.  Gustatory  Reflexes. — a.  Immediately  after  operation,  washed  away  the  blood  and 
stimulated  the  jaws  with  clam,  producing  marked  leg  movement,  as  in  chewing,  but  very  feeble 
jaw  movement,     b.  Repeated  the  experiment  after  five  minutes  with  same  results,     c.  Again, 
two  days  later,  stimulation  of  jaws  with  food  produces  chewing  reflexes,  consisting  of  leg  move- 
ment only  on  the  left;  on  the  right,  no  reflexes. 

II.  Respiration. — a.  On  removing  the  crab  from  the  water,  all  the  left  gills  pulsate  a  few 
times,  the  right  remain  motionless. 

b.  When  returned  to  the  water  after  long  exposure  to  the  air,  the  respiration  becomes  nearly 
normal.     The  left  legs  are  very  restless  and  move  back  and  forth  in  a  lateral  direction.     The 
right  legs  are  relaxed  and  motionless,  except  the  sixth,  which  is  directed  backward  and  moving 
slightly. 

c.  Respiratory  movements  may  now  be  induced  by  rubbing  the  gills  with  clam.     The  same 
premonitory  twitching  and  lateral  movements  as  in  experiment  i.     Repeated  frequently  with 
same  results.    When  the  movements  are  well  under  way,  the  left  gills  are  raised  higher  than  the 
right. 

III.  Purposeful  Movements  of  Sixth  Leg.— When  the  fingers  were  placed  on  the  left 
abdominal  appendages,  the  left,  sixth  leg  was  thrust  repeatedly  backward  over  the  median  sur 
faces  of  the  gills,  with  the  very  evident  purpose  of  thrusting  away  the  stimulating  object.     The 
other  appendages  moved  very  slightly,  but  did  not  in  any  way  make  purposeful  movements 
on  stimulation  of  thorax. 

Experiment  III — A. 

July  29  Female.  Cut  the  nerves  to  the  endopodites  of  all  the  left  abdominal  appendages 
about  half  way  up  the  appendages.  No  other  effects  were  observed  than  the  loss  of  sensibility 
of  the  left  abdominal  endopodites. 


EXPERIMENTS.  179 

This  animal  was  then  used  for  the  following  successful  experiment.  At  the  time  the  second 
operation  was  made,  the  crab  was  in  such  good  condition  and  its  normal  action  had  been  so 
little  altered  that  it  was  not  felt  that  confusion  might  result  from  this  attempt  to  economize 
material. 

Experiment  III — B. 

The  second  operation  was  performed  August  6,  5  P.  M.  The  autopsy,  three  weeks  later, 
showed  that  the  left  crus  was  sectioned  close  to  the  spinal  cord,  and  all  the  thoracic  cross  com- 
missures severed.  (Fig.  113,  C.I.)  At  the  anterior  end,  the  left  crus  was  cut  so  that  a  small  piece 
of  the  left  cerebral  hemisphere  remained  attached  to  it.  But  only  a  very  few  cerebral  cells,  if 
any;  could  have  been  connected  with  the  crus. 

I.  Gustatory  Reflexes. — a.  Immediately  after  the  operation,  the  chewing  reflexes  were 
inhibited.     Five  minutes  later,  excellent  reflexes,  including  the  chelicerae,  were  obtained  on 
both  sides;  but  the  two  sides  were  not  coordinated. 

b.  August  7,  9  A.  M.  On  stimulating  right  jaws  with  clam,  obtain  prompt  and  vigorous 
chewing  movements  of  the  jaws,  but  with  moderate,  or  normal  chewing  movements  of  the  legs. 

On  stimulating  left  jaws,  obtain  at  first  the  same  results,  but  the  leg  movement  gradually 
grows  more  energetic  till  it  is  absurdly  exaggerated  in  rapidit^  and  range,  and  finally  becomes 
much  confused,  the  legs  moving  wildly  back  and  forth,  and  often  clashing  with  one  another. 
This  rapid  movement  may  be  followed  by  a  spasmodic  bending  of  the  tips  of  two  or  three  ap- 
pendages into  the  mouth,  where  they  are  held  in  a  trembling  tetanus  or  rigor.  All  the  left  legs 
are  involved  in  this  movement,  except  the  left  chelicera,  whose  nerve  was  cut  a  short  distance 
from  the  brain. 

Repeated  these  experiments  several  times  on  August  10,  14,  and  26,  obtaining  in  each  case 
essentially  the  same  results. 

II.  Thoracic  Reflexes. — a.  August  7.     On  placing  the  fingers  on  the  left  side  of  the 
thorax,  there  is  no  response,  and  if  the  left  legs  happen  to  be  making  the  chewing  movements, 
the  latter  are  not  in  the  least  disturbed. 

b.  Later.     Fingers  placed  on  the  posterior,  ventral  surface  of  the  left  side  cause  no  move- 
ment beyond  a  slight  start  when  the  contact  is  made,  and  uneasy  opening  of  the  chelae.     But 
on  touching  the  anterior  quarter  of  the  ventral  surface,  rapid  movements  of  the  second  and 
third  left  legs  are  produced.     These  movements  at  first  do  not  last  long,  and  are  inconspicuous 
when  compared  with  the  movements  that  may  be  produced  on  the  opposite  side.     But  on  the 
following  days  they  became  distinctly  purposeful,  repelling  movements. 

c.  August  7.  On  placing  the  hand  on  the  right  ventral  side  of  the  thorax,  all  the  right  legs 
move  furiously  back  and  forth  in  unison,  while  the  left  continue  their  chewing  movements  as 
before. 

d.  August  10.     Hand  placed  on  the  right  side  of  thorax  causes  active,  purposeful  thrusting 
away  movements  of  the  right  legs. 

e.  August  25  and  27.     Repeated  a,  b,  and  d,  with  same  results. 

III.  Abdomino-thoracic  Reflexes. — a.  Placing  the  fingers  on  right  margin  of  abdomen 
caused  back  and  forward  movements  of  all  the  right  legs  (except  chelicera)  and  with  very 
marked  thrusting  away  movements  of  the  sixth  leg.     No  movements  of  the  left  legs. 

b.  Hand  placed  on  the  left  margin  caused  obscure  movements  of  the  right  legs. 

c.  Hand  placed  on  either  side  of  the  margin  of  the  abdomen  caused  faint,  rhythmical  con- 
tractions of  the  gills  of  both  sides,  but  movements  of  the  right  gills  are  the  strongest. 

d.  August  26.     Repeated  a,  b,  and  c,  with  same  results. 

IV.  Temperature  Reflexes.— a.  August  25.     On  breathing  gently  on  the  ventral  surface 
of  the  quiescent  crab,  that  had  been  lying  on  its  back  in  the  air  for  some  time,  a  general  muscular 
spasm  is  instantly  produced.     All  the  legs  are  waved  about,  but  the  left  legs  are  thrown  into 


l8o  FUNCTIONS    OF    THE    BRAIN. 

prolonged  violent  movements  during  which  they  are  convulsively  flexed,  and  the  tips  thrown 
repeatedly  toward  the  mouth.     The  right  legs,  meantime,  becoming  quiet. 

b.  As  soon  as  the  crab  had  quieted  down,  the  experiment  was  repeated,  but  with  the  ut- 
most care  not  to  produce  too  violent  a  stimulus.  The  little  puffs  of  warm  air  could  be  so  regu- 
lated as  to  cause  the  left  legs  to  move,  while  the  right  remained  motionless.  The  experiment 
was  repeated  many  times  with  the  same  results,  showing  that  the  left  side  reacted  much  more 
readily  than  the  right. 

V.  Respiration. — a.  August  7.     When  at  rest  in  the  air,  the  gills  are  twisted  toward  the 
left,  the  left  gills  tightly  compressed,  the  right  ones  slightly  elevated.     At  first  there  was  a  tend- 
ency for  the  left  gills  to  move  spontaneously  in  rhythmical  respiratory  movements.    A  week  or 
two  later,  the  right  gills  frequently  performed  the  normal  yawning  movements,  or  the  normal 
respiratory  movements,  the  left  gills  remaining  motionless. 

b.  Placed  in  water,  normal  respiratory  movements  begin  at  once,  except  that  the  left  gills 
are  raised  higher  than  the  right. 

c.  August  26.  Crab  still  vigorous.     Repeat  a  and  b  with  same  results. 

d.  August  27.  Stimulation  of  gills  with  clam,  or  finger  tips,  does  not  induce  respiratory 
movements. 

VI.  Equilibrium;  Locomotion,  —a.  August  7.  Crab  rights  itself  repeatedly  when  placed  on 
its  back  in  the  aquarium.     When  righted,  it  constantly  moves  in  a  circle  toward  the  left  with 
right  side  raised  high  on   the  legs,  the  left  side  depressed.     The  caudal  spine  turned  to  the 
right,  at  an  angle  of  about  20°.     The  crab,  when  righted,  circulates  to  the  left,  because  the 
right  legs  alone  make  the  motor  movements.     The  circular  movement  continues  for  hours  at  a 
time.     August  27,  condition  same  as  a. 

b.  August  7.  On  removing  the  crab  from  the  water  and  placing  it  on  its  back,  the  left 
legs  move  restlessly  and  aimlessly,  often  bending  the  tips  into  or  toward  the  mouth.     Movement 
continues  for  more  than  an  hour.     Right  legs  remain  quiet,  but  may  move  vigorously  if  properly 
stimulated. 

c.  At  certain  intervals,  when  in  the  air,  all  the  right  legs  swing  in  unison  forward,  and  then 
with  a  vigorous  stroke  backward.     The  forward  and  backward  movements  are  repeated  many 
times  with  great  regularity,  precisely  as  in  swimming,  except  that  the  gills  did  not  join  in  the  move- 
ment.    The  left  legs  never  made  these  characteristic  movements. 

d.  August  19.      In  water  the  crab  sometimes  fails  to  right  itself.     In  such  cases,  the  swim- 
ming movement  of  the  right  legs  may  continue  for  hours  with  great  regularity,  but  without 
sufficient  force  to  move  the  animal  about.     The  left  legs  are  meantime  passive. 

e.  August  25,  same  conditions  described  in  c  and  d  are  retained. 

VII.  Autopsy. — August  27.  Three  weeks  after  the  hemisection  of  the  brain  the  crab  was 
alive  and  vigorous.     On  removing  the  brain,  it  was  found  that  all  the  parts  about  the  wound 
were  thickly  incrusted  with  a  granulated  matter,  which  when  removed  showed  the  cut  surfaces 
of  the  commissures  and  of  the  crura  to  be  very  little  changed.    There  was  no  indication  of  degen- 
eration, or  regeneration  of  the  surrounding  parts.     There  was  a  thick  deposit  of  sepia  colored 
pigment  about  the  wound  in  the  forebrain.     The  cotton  plugs  were  incrusted  with  a  granular 
matter,  and  they  apparently  had  not  interfered  by  pressure  or  otherwise  with  the  action  of  the 
nervous  system. 

Experiment  IV — A. 

August  25.  Large  female.  Cut  both  crura  in  front  of  the  second  thoracic  appendage. 
(Fig.  113,  D.I.}  Immediately  after  the  operation,  the  left  legs  moved  restlessly,  the  right 
remained  quiet. 

I.  Thoracic  Reflexes. — a.  Immediately  after  the  operation,  no  definite  crossed  or  un- 
crossed thoracic  reflexes  could  be  obtained  by  warming  the  sides  of  the  thorax  in  the  usual  way. 


EXPERIMENTS.  l8l 

b.  Several  hours   later.     There    was,  in    no  case,  any  raising  of  the  legs,  or  purposeful 
thrusting  away  movements  of  the  legs  on  either  side,  although  faint  movements  of  the  opposite 
legs  followed  a  stimulation  of  the  sides  of  the  thorax. 

c.  If  the  hands  were  placed  on  the  margin  of  the  thorax  while  the  chewing  movements 
were  going  on,  the  movements  on  the  stimulated  side  were  inhibited. 

II.  Abdomino-thoracic  Reflexes. — The  following  results  are  in  marked  contrast  with  the 
above: 

a.  Hand  placed  on  the  margin  of  the  abdomen  causes  raising  of  the  legs  of  the  opposite 
side,  where  they  are  held  in  a  sort  of  tetanus,  as  long  as  the  stimulus  is  applied.     When  the 
fingers  are  removed,  they  instantly  drop  back  on  the  carapace.     There  is  no  purpose  in  the 
movements  and  it  is  difficult  to  understand  their  meaning.     The  legs  of  both  sides  may  be 
affected,  but  those  on  the  side  opposite  to  the  stimulus  are  flexed  first  and  to  the  greater  extent. 

III.  Leg-gill  Reflexes. — If  the  surface  of  the  gills  and  operculum  is  touched,  the  sixth 
legs  at  once  make  vigorous  and  well  directed  movements  to  rub  the  stimulated  spot.     If  the 
stimulation  is  on  one  side  only,  the  sixth  leg  of  the  opposite  side  moves  first  and  most  vigorously 
The  fifth  and  fourth  legs  may  be,  to  some  extent,  involved  in  the  movements. 

The  movements  of  the  sixth  legs,  under  these  conditions,  are  remarkable.  The  ends  of 
the  legs  are  rubbed  against  each  other,  and  over  the  surface  of  the  gills,  something  like  the 
"washing"  movements  of  the  posterior  pair  of  legs  in  a  common  house  fly. 

IV.  Gustatory  Reflexes.— a.  Immediately  after  the  operation,  stimulation  of  the  jaws 
with  food  caused  chewing  movements  of  both  legs  and  jaws  on  the  left  side,  but  none  on  the  right 
Three  hours  later,  however,  vigorous  chewing  movements  were  produced  on  both  sides.     The 
coxal  chewing  movements  were  vigorous  and  normal,  the  leg  movements  greatly  exaggerated. 

V.  Respiratory  Reflexes. — Stimulating  the  gills  with  clam  produced  no  effect. 

Experiment  IV — B. 

Twenty-four  hours  later.  Crab  still  shows  remarkable  vitality  and  spontaneity.  Made 
a  sagittal  cut  through  the  cross  commissures  of  the  thorax  and  right  crus,  D,II.  For  a  few 
minutes  after  the  operation,  the  left  legs  are  very  restless  and  the  right  are  quiet. 

I.  Thoracic  Reflexes. — a.  Hand  placed  on  the  right  margin  of  thorax  produces  no 
reflexes.     On  left  margin,  retraction  of  the  left  legs. 

II.  Abdomino-thoracic  Reflexes. — a.  Hand  placed  on  right  margin  of  abdomen  causes 
slight  movements  of  left  legs,  none  of  right. 

b.  Hand   placed  on  left  margin  causes  stronger  movements  than  before  of  the  left  legs, 
none  of  right. 

c.  Hand  placed  on  the  gills  causes  violent  movement  of  left  legs,  none  of  right. 

III.  Gustatory  Reflexes. — a.  Immediately  after  the  operation,  no  chewing  movements 
could  be  induced  by  stimulating  the  jaws  with  food.     Applying  the  electrodes  to  the  left  jaws 
caused  movements  of  the  corresponding  legs,  but  no  movements  followed  when  they  were  ap- 
plied to  the  right  jaws.     The  right  legs  would  not  respond  to  any  change  of  temperature, 
whether  applied  directly  or  indirectly. 

b.  One  hour  later,  obtained  a  faint  chewing  movement  on  the  left  side,  on  stimulating  the 
jaws  with  clam;  none  on  the  right. 

IV.  Respiratory  Reflexes. — After  the  second  operation,  respiratory  movements  of  the 
gills  ceased,  and  for  twenty-four  hours  no  rhythmical  movements  of  the  gills  could  be  produced, 
either  by  stimulation  with  clam,  or  by  placing  the  crab  in  sea  water.     But  the  next  morning  the 
following  curious  facts  were  observed.     The  crab  was  found  on  its  back  in  the  aquarium, 
just  as  it  had  been  left  the  previous  day.     No  respiration  had  apparently  taken  place  during  the 
night,  as  the  gills  were  covered  with  a  light  sediment. 

a.  On  removing  the  apparently  dead  crab  from  the  water  and  stimulating  the  gills  with 


182  FUNCTIONS    OF    THE    BRAIN. 

the  electrodes,  peculiar  rhythmical  contractions  followed.     The  movements  stopped  in  a  few 
moments. 

b.  On  rubbing  very  gently  a  small  piece  of  clam  on  the  gills,  the  movements  began  again, 
but  more  vigorous  than  before.     On  returning  the  crab  to  the  water,  the  movements,  which 
had  meantime  ceased,  began  again  and  continued  with  some  interruption,  for  purposes  of 
experimentation,  for  several  hours.     These  movements  were  as  follows:     i.  The  abdominal 
appendages  remained  well  elevated,  pulsating  in  short  strokes,  once  about  every  one  and  one- 
half  second,  for  twenty-five  seconds.     2.  Two  full  vertical  pulsations  then  follow,  each  one  being 
a  vigorous  flattening  of  the  appendages  against  the  abdomen,  followed  by  an  elevation  of  the 
same  to  their  full  height.     3.  They  move  repeatedly  back  and  forth  across  the  median  line, 
rubbing  the  posterior  surface  of  one  appendage  over  the  anterior  surface  of  its  mate,  as  though 
rubbing  or  washing  the  gills.     They  may  remain  crossed  and  motionless  for  several  seconds, 
but  finally  return  to  their  first  position.     4.  One  full  pulsation  followed  by  5.  a  long  pause  of 
twenty-five  seconds,  and  then  the  whole  begins  again.     One  whole  series  of  movements  takes 
place  in  about  seventy  seconds,  but  this  period  may  be  gradually  prolonged  till  all  the  move- 
ments cease. 

c.  When  the  crab  was  taken  from  the  water  and  placed  on  its  back,  the  movements  grad- 
ually ceased. 

d.  Breathing  on  the  gills  produced  only  a  slight  contraction  of  the  same.     A  drop  of  water 
produced  two  or  three  faint  pulsations.     Scratching  the  abdominal  appendages  with  the  fingers, 
or  with  a  stick  produced  no  result,  but  on  rubbing  them  with  a  small  bit  of  clam,  the  compli- 
cated series  of  rhythmical  movements  described  in  b  began  at  once. 

Experiment  V — A. 

10.30  A.  M.  Large  female.  Sectioned  both  crura  back  of  chelicerae.  All  spontaneous 
movements  cease.  (Fig.  113,  £.7.) 

I.  Thoracic  Reflexes. — a.  Hand  placed  on  either  margin  of  the  thorax  causes  contraction 
of  the  gills  of  the  same  side,  and  later  the  raising  of  the  legs  of  the  opposite  side.    In  both  cases, 
the  legs  on  the  side  stimulated  start  slightly,  but  are  not  raised.     No  purposeful  movements  of 
the  legs  are  made  to  remove  the  irritation. 

II.  Gustatory  Reflexes. — Three  hours  after  the  operation  no  chewing  movements  could 
be  produced  by  stimulating  the  jaws. 

Experiment  V — B. 

3  p.  M.  Sectioned  the  right  half  of  the  ventral  cord,  in  front  of  the  first  abdominal  ganglion, 
E.II. 

I.  Abdominal- thoracic  Reflexes.— a.  Hand  placed  on  the  left  margin  of  the  abdomen 
caused  raising  of  the  right  legs,  first  to  sixth.  Hand  placed  on  the  right  margin,  caused  move- 
ment of  the  fifth  and  sixth  legs  on  the  opposite  side. 

These  two  experiments  show  that  impulses  cross  both  above,  and  below,  the  cut,  //. 

Experiment  V— C. 

3.30  P.  M.  Made  a  sagittal  cut  through  the  vagus  neuro meres,  but  without  cutting  the 
free,  thoracic  cross  commissures,  E.III. 

I.  Abdomino-thoracic  Reflexes. — At  first  all  crossed  abdomino-thoracic  reflexes  ceased. 

a.  A  few  minutes  later,  placing  hand  on  right  margin  of  abdomen  causes  the  fourth,  fifth, 
and  sixth  legs  of  left  side  to  be  raised. 

b.  Hand  placed  on  left  side  of  abdomen  causes  fifth  and  sixth  legs  of  same  side  to  be  raised, 
but  these  movements  are  not  so  strong  as  when  the  right  side  of  the  abdomen  is  stimulated. 

Crab  died  after  about  twenty-four  hours. 


EXPERIMENTS.  183 

Experiment  VI — A. 

August  n,  ii  A.  M.  Exposed  the  spinal  cord,  and  made  a  longitudinal  median  section 
through  the  first  abdominal  ganglion.  F.I.  Rhythmical  contractions  of  the  gills  followed,  lasting 
a  short  time. 

Experiment  VI — B. 

The  right  cord  was  then  cut  across  about  midway  between  the  brain  and  the  first  abdominal 
ganglion.  F.II. 

I.  Respiratory  Reflexes.— a.  At  first  no  reflexes  followed  the  section,  but  after  one  or 
two  minutes  respiratory  movements  began.     The  left  gills  were  raised  much  higher  than  the 
right,  the  latter  being  apparently  dragged  up  by  the  left  gills,  rather  than  by  their  own  action. 
When  respiratory  movements  ceased,  the  left  gills  remained  in  a  higher  position  than  the  right. 

b.  When  placed  in  the  water,  the  respiration  was  at  first  about  normal,  but  in  a  few  min- 
utes it  almost  ceased,  leaving  the  left  gills  moving  slowly,  the  right  motionless.  After  about  an 
hour,  respiration  ceased,  leaving  the  left  gills  raised,  and  the  right  closely  pressed  against  the 
abdomen. 

II.  Abdomino-thoracic  Reflexes. — a.  On  placing  the  hand  on  the  right  margin  of  the 
abdomen,  all  the  left  legs  are  promptly  raised,  remaining  in  that  position  till  the  hand  is  removed, 
when  they  again  fall  back  slowly  into  the  thorax.     The  right  legs  are  also  raised,  but  after  the 
left. 

b.  Hand  placed  on  the  left  margin  of  the  abdomen  causes  raising  of  the  right  legs  (but  the 
response  is  not  as  prompt  and  vigorous  as  that  of  the  left  legs  in  the  previous  experiment).     The 
left  legs  are  not  raised,  but  their  chelae  stir  uneasily. 

c.  Repeated  a  and  b  two  hours  later  with  the  same  results. 

d.  Stimulating  with  a  weak  electric  current  at  b,  causes  contractions  of  the  right  gills,  also 
slight  movements  of  the  left  gills  (see  below) ;  at  c,  causes  contractions  of  the  left  legs  and  the 
gills  on  both  sides;  at  a,  causes  contractions  of  the  right  legs. 

e.  2.30  P.  M.     Applying  electrodes  at  a,  the  right  gills  are  slightly  raised,  and  then  moved 
back  and  forth  faintly  as  in  respiration.     Left  gills  move  much  less  than  right. 

/.  Stimulating  at  c,  obtain  movements  of  left  gills  and  partial  ones  of  the  right,  and  with 
vigorous  and  immediate  movements  of  the  left  legs. 

g.  Stimulation  at  a,  causes  immediate  movements  of  the  right  legs  only. 

We  thus  see  that  stimuli  applied  directly  to  the  spinal  cord  produce  uncrossed  reflexes  of 
the  appendages  anterior  to  the  cut,  and  both  crossed  and  uncrossed  below  the  cut. 

But  temperature  impulses,  starting  on  the  sides  of  the  abdomen  and  traveling  centripetally, 
produce  both  crossed  and  uncrossed  reflexes  in  the  thorax.  It  is  not  clear  why  direct  stimulation 
of  the  spinal  cord  should  produce  only  uncrossed  impulses  above  the  point  of  stimulation. 

Experiment  VI — C. 

3.30  P.  M.  Made  an  accurate  sagittal  cut  through  the  vagus  neuro meres,  care  being  taken 
not  to  cut  the  first  three  or  four  post-oral  commissures.  F.III 

a.  Stimulating  at  a  or  c,  produced  leg  movements  of  the  corresponding  side,  as  before. 

b.  But  stimulation  of  the  margin  of  the  abdomen  and  gills,  with  the  hand,  produced  no 
leg  reflexes,  although  breathing  on  the  legs,  or  dropping  tepid  water  on  them,  produced  prompt 
movements  of  the  same. 

It  would  thus  appear  that  all  the  uncrossed  temperature  impulses  started  in  the  abdomen 
cross  in  the  vagus  neuromeres. 


1 84  FUNCTIONS    OF    THE    BRAIN. 

Experiment  VII — A. 

Adult  male.  Sectioned  right  cord  (Fig.  113,  G.7).  Twitching  of  the  right  operculum  and 
first  right  gill  followed  the  operation. 

Experiment  VII— B. 

Cut  the  whole  ventral  cord  in  halves,  lengthwise,  G.I  I. 

a.  The  caudal  spine  is  thrown  toward  the  left  and  remains  so  permanently. 

b.  On  breathing  on  the  abdomen  and  gills,  the  gills  are  retracted  and  the  right  legs  alone 
are  raised. 

c.  Placing  the  fingers  on  the  left  margin  of  the  abdomen  causes  the  raising  of  all  the  right 
legs;  the  legs  are  strongly  flexed  and  the  points  thrown  medianly  and  backward. 

d.  Hand  on  the  right  side  produces  no  results,  except  that  in  some  cases  there  is  a  slight 
start  of  the  left  legs  and  a  faint  gaping  of  the  chela?,  but  no  movement  at  all  resembling  those  seen 
on  the  opposite  side  of  the  body.     This  impulse  probably  reaches  the  forebrain  through  the 
longitudinal  integumentary  nerve.     (Fig.  70.) 

e.  One  hour  later      Repeated  several  times  d,  b,  and  c,  with  same  results. 

Experiment  VIII. 

July  29.  Male.  The  right  and  left  halves  of  the  cord  were  separated  behind  the  opercular 
neuromere,  by  a  median,  longitudinal  cut  extending  through  all  the  abdominal  neuromeres. 
All  the  free,  post-oral  commissures  of  the  brain  were  also  cut,  leaving  the  vagus  commissures 
intact. 

I.  Abdominal  and  Abdomino-thoracic  Reflexes. — July  30.     Stimulation  of  the  left 
side  of  the  abdomen  caused  a  drawing  down  of  the  left  gills  and  an  immediate  and  strong  upward 
movement  of  the  right  legs,  followed  by  a  slight  raising  of  the  legs  on  the  left  side.     Stimulation 
of  the  right  side  of  the  abdomen  in  the  same  way,  produced  the  same  result,  and  vice  versa. 
In  both  cases  there  was  a  movement  of  the  tail  toward  the  stimulated  side. 

The  experiment  shows  unimpaired  crossing  of  impulses  in  the  vagus  neuromeres. 

II.  Respiration. — July  29.     The  right  and  left  halves  of  the  gills  were  breathing  in  alter- 
nation, the  gills  on  the  right  side  being  raised  while  those  on  the  left  were  depressed.     The 
coordination  of  the  respiratory  movements  was  perfect  longitudinally,  but  the  two  sides  were 
beating  independently. 

Experiment  IX. 

I.  Purposeful  Reflexes  of  the  Sixth  Legs.— July  15.     Male.     Transected  the  right  half 
of  the  cord  between  the  first  and  second  gill  neuromeres.     (Fig.  113,  H.g.} 

July  18  and  19.  a.  Stimulation  of  the  left  side  of  the  abdomen.  Caused  a  drawing  down 
of  the  gills  on  the  same  side,  together  with  a  spasmodic,  upward,  non-purposeful  movement  of 
the  legs  on  the  right  side,  followed  by  a  purposeful  movement  of  the  sixth  leg  on  the  left  side. 

b.  Stimulation  of  the  right  side  of  the  abdomen  gave  same  results  vice  versa,  except  that 
in  this  case  the  sixth  left  leg  (as  before)  performed  the  purposeful  movement,  the  sixth  on  the 
right  being  merely  raised. 

Abdominal  stimulation  produced  no  purposeful  movements  of  the  sixth  leg  on  the  cut  side. 

II.  Yawning. — After  the  crab  had  been  out  of  water  for  about  an  hour,  it  would  yawn, 
the  operculum  and  the  first  gill,  both  right  and  left,  moving  in  the  usual  way,  while  the  gills 
behind  the  cut  behaved  as  follows:     The  left  gills  were  raised  in  time  with  the  operculum  and 
first  gill,  although  they  were  not  raised  as  high  as  in  the  normal  animal.     The  right  gills  did  not 
move  at  all,  except  as  they  were  slightly  dragged  upward  by  the  left. 


EXPERIMENTS.  185 

III.  Respiration. — Eight  hours  after  operation.     In  air.     As  the  gills  behind  the  cut 
began  their  opening  phase,  the  left  gills  started  first,  and  apparently  dragged  the  right  ones  up 
with  them.     When  the  gills  closed,  the  left  started  down  first  and  dragged  the  right  after.     The 
operculum  and  the  first  gill  behaved  normally. 

When  the  crab  was  put  back  in  water,  the  respiratory  movements  would  begin  as  described 
above,  but  after  a  while  they  would  become  more  nearly  normal.  From  this  it  would  appear 
that  the  coordination  of  respiratory  movements  is  brought  about  in  some  part  of  the  nervous 
system  anterior  to  the  abdominal  neuromeres.  Same  results  were  obtained  on  seven  succes- 
sive days,  the  abnormality  of  the  respiration  in  air  being  greater  than  in  water. 

IV.  Swimming  Movements. — July  23.  The  right  and  left  operculum,  the  right  and  left 
first  gill  and  the  four  posterior  left  gills  performed  normal  swimming  movements  in  unison. 
All  the  right  gills  behind  the  cut  were  either  quiet,  or  respiring,  while  the  others  were  swim- 
ming.    This  indicates  a  center  for  the  swimming  movement  of  the  gills  in  front  of  the  free 
abdominal  neuromeres. 

Experiment  X. 

July  19,  1898.  Male.  Both  cords  were  cut  between  the  neuromeres  of  the  first  and  second 
gills,  H.  10. 

I.  Respiration. — a.  July  29.  In  water.     All  the  gills  were  breathing,  but  out  of  rhythm 
The  operculum  and  first  gill  moved  together,  but  out  of  time  with  the  posterior  gills.     In  the 
anterior  group,  the  operculum  was  the  first  to  start  each  inspiratory  movement,  and  was  followed 
up  by  the  first  gill.    The  four  gills  behind  the  cut  beat  fairly  well  together,  but  the  rhythm  within 
the  group  is  imperfect  and  all  are  out  of  time  with  the  anterior  group.     In  the  respiratory  move- 
ment of  these  four  posterior  gills,  the  most  posterior  one  was  the  first  to  start  the  upward  move- 
ment.    Results  indicate  that  the  gills  behind  the  cut  have  been  separated  from  their  center  of 
coordination. 

b.  July  23  to  28.  The  four  gills  behind  the  cut  frequently  performed  the  "cross  rubbing" 
or  ''scraping"  movement,  the  first  gill  not  participating.  This  "cross  rubbing"  movement  of 
the  posterior  gills  was  always  followed  by  strong  swimming  movements  of  the  operculum,  first 
gill,  and  thoracic  appendages.  At  times,  the  operculum  and  first  gill  would  stop,  while  the 
appendages  behind  the  cut  kept  on  with  unbroken  rhythm;  or  the  appendages  in  front  of  the 
cut  would  be  performing  the  swimming  motions,  while  those  behind  the  cut  were  breathing  as 
usual.  This  indicates  a  separate  nerve  mechanism  for  respiratory  and  locomotor  activities  of 
the  gills. 

Experiment  XI. 

In  another  experiment,  the  cord  was  cut  behind  the  second  gill  neuromere,  H.I2.  Results: 
i.  the  three  gills  behind  the  cut  made  the  respiratory  movements  more  vigorously  and  frequently 
than  the  two  gills  in  front  of  it;  and  2.  the  operculum  and  the  gills  in  front  of  the  cut  "yawned" 
frequently,  while  those  behind  the  cut  were  motionless. 

II.  Swimming  Movements. — At  no  time  after  the  operation  did  the  gills  behind  the  cut, 
in  experiments  X  and  XI,     perform  swimming  movements.     The  abdominal  appendages  in 
front  of  the  cut  made  the  swimming  movements  often  and  in  a  normal  manner. 

Miscellaneous  Experiments. 

The  following  results  were  obtained  at  various  times  by  stimulating  the  cord  and  the  peri- 
pheral nerves  with  an  induced  electric  current. 

a.  On  stimulating  any  one  of  the  posterior  thoracic  haemal  nerves  on  the  left  side,  the  right 
legs  and  the  right  halves  of  the  operculum  and  gills  were  raised,  the  legs  pointing  toward  the 


l86  FUNCTIONS    OF    THE    BRAIN. 

region  stimulated.  The  left  legs  make  purposeful  movements.  The  effect  is  not  confined  to 
the  appendage  corresponding  to  the  nerve  stimulated,  since  after  stimulating  one  nerve  all  the 
legs  of  the  opposite  side  are  raised. 

b.  Stimulation  of  the  longitudinal  integumentary  nerve  gave  no  results. 

c.  The  abdominal  cord  was  transected  between  the  neuromeres  of  the  first  and  second  gills. 
Stimulating  the  proximal  end  of  a  haemal  nerve  in  the  isolated  segment  of  the  cord  caused  a 
raising  of  the  abdomen,  followed  by  a  very  slight  rhythmical  movement  of  the  gills. 

d.  Stimulation  of  a  branchial  nerve  causes  a  contraction  of  the  legs  and  gills  of  the  same 
side,  but  no  rhythmical  movement. 

e.  Stimulation  of  the  right  or  left  cord  in  front  of  the  first  abdominal  ganglion  causes  con- 
traction of  legs  and  gills  of  the  same  side,  but  no  rhythmical  gill  movements. 

PART  II. 
SUMMARY  OF  EXPERIMENTAL  AND  ANATOMICAL  RESULTS. 

I.  GUSTATORY  REFLEXES. 

a.  Normal  Action. — i.  Each  leg  of  the  second  to  fifth  pairs,  when  its  taste  organs  are 
stimulated,  performs  the  chewing  movements  alone,  without  starting  the  action  in  the  adjacent 
legs  of  the  same,  or  of  the  opposite  side.  2.  Stimulating  the  chilaria  of  one  side  may  induce 
chewing  movements  in  all  the  legs  of  that  side.  3.  The  chelicerse  have  no  " taste  spines." 
They  are  brought  into  action  by  stimulating  the  taste  organs  on  one  or  more  of  the  other  legs 
of  the  same  side.  The  action  may  then  be  transferred  to  its  mate,  if  the  stimulus  is  strong 
enough.  4.  When  the  taste  organs  of  several  legs  on  the  same  side  are  stimulated,  all  the  legs 
of  that  side  work  in  a  harmonious  rhythm.  5.  When  both  sides  are  stimulated,  the  rhythmic 
movements  of  the  right  legs  harmonize  with  that  of  the  left.  6.  There  are  two  independent 
movements  in  chewing,  the  lateral,  or  in  and  out  movement  of  the  jaw-like  coxae,  and  the  thrust- 
ing of  the  tip  of  the  legs  in  and  out  of  the  mouth. 

Structure  of  the  Gustatory  Apparatus.— The  principal  reflexes  described 
above  can  be  explained  by  the  structure  of  the  parts  involved.  The  condi- 
tions, so  far  as  we  have  been  able  to  analyze  them  by  the  anatomical  and  exper- 
imental methods,  are  shown  in  a  diagrammatic  form  in  Fig.  114.  We  have 
shown,  for  example,  by  the  anatomical  analysis,  that  :  i.  The  taste  organ  nerves 
of  the  jaws,  flabellum,  and  chilaria  form  distinct  fascicles,  whose  inner  ends 
(after  giving  off  local  dendrites,  g.c.1)  unite  to  form  an  immense,  longitudinal 
tract,  g.tr.y  terminating  in  a  voluminous  mass  of  neuropile,  or  secondary  taste 
center,  on  the  neuro-lateral  surface  of  the  cheliceral  neuromere  (diencephalon) 
near  the  base  of  the  cerebral  peduncles,  g.c.2  2.  A  tract  extends  beyond  this 
center  along  the  cerebral  peduncles  to  a  tertiary  center  that  forms  a  large  lobe  on  the 
median  face  of  each  hemisphere,  g.c.3  At  the  base  of  the  hemispheres,  underneath 
the  posterior  median  lobes  and  on  the  anterior  neural  surface  of  the  cheliceral 
neuromere,  is  a  group  of  large  nerve  cells,  ch.Hc.,  which  by  one  set  of  dendrites 
bring  the  secondary  taste  center  into  relation  with  the  cerebral  cortex  of  the  same 
side,  and  by  another  set  with  the  opposite  side  of  the  collar,  through  the  forebrain 
commissure.  4.  Finally  a  cluster  of  about  fifteen  large  nerve  cells,  H.as., 
lying  a  little  above  the  taste  lobe,  on  the  median  surface  of  the  hemispheres, 
sends  one  set  of  extensive  dendrites  to  the  entire  cerebral  cortex,  another  to  the 


THE  GUSTATORY  APPARATUS. 


i87 


secondary  taste  center,  and  a  third  along  the  surface  of  the  mid-  and  hindbrain 
neuromeres,  probably  terminating  around  the  motor  neurites  at  the  base  of  each 
pedal  ganglion. 


Olfactory 
Coordination 

feeo.orter.  Ceii 
\        lolf.  vis.  gust, 
locom-  swim. 


FIG.  114. — Diagram  of  the  brain  of  Limulus,  showing  the  course  of  the  principal  nerve  impulses,  and  the  loca- 
tion of  the  principal  centers  of  control.  The  figure  is  constructed  from  the  data  obtained  by  experimental  and  ana- 
tomical methods.  Only  the  first  and  fourth  thoracic,  and  the  first  branchial  appendage  on  the  right  side  of  the 
animal  are  shown.  The  most  important  points  illustrated  are  (a)  the  primary,  g.c1.,  secondary,  g.c.2,  and  tertiary, 
g.c.3,  gustatory  centers;  (b)  the  relation  of  the  motor,  sensory,  and  association  neurones  to  the  muscles  used  in  the 
chewing  reflexes;  (c)  the  crossed  thoracic  temperature  impulses;  and  (d)  the  crossing  of  the  branchio-thoracic  im- 
pulses in  the  vagus  neuromeres.  The  lettering  of  the  neurones  is  the  same  as  that  in  Figs.  56  to  66. 

Closely  associated  with  the  secondary  taste  center  is  the  "  swallowing  center," 
or  the  lateral  stomodaeal  ganglion,  sl.g.,  lying  on  the  median  margins  of  the  cheli- 
ceral  neuromere,  next  to  the  walls  of  the  stomodaeum.  It  is  connected  with  its 
mate  by  a  special  pre-oral  commissure,  st.c. 


1 88  FUNCTIONS    OF    THE    BRAIN. 

The  Nerve-muscle  Chewing  Apparatus. — The  leg  movement  in  chewing 
is  produced  by  two  muscles,  a  flexor  and  an  extensor,  both  innervated  by  a  branch 
of  the  pedal  nerve.  (Fig.  114,  ex2  andyP.)  The  motor  nerve-cells  of  these  mus- 
cles while  not  certainly  located  probably  lie  in,  or  close  to,  the  pedal  ganglion,  h2. 

The  jaw  movement  is  controlled  by  nine  muscles.  The  "in"  movement  is 
produced  by  four  plastro  coxals,  pl.cx.,  two  in  front,  and  two  behind,  going  from 
the  edge  of  the  plastron  to  the  sides  of  the  coxae;  the  "out"  movement  by  the  five 
coxo-tergals,  cx.t.,  two  in  front  and  three  behind,  extending  from  the  base  of  the 
coxa  to  the  dorsal  shield.  These  nine  muscles  are  innervated  by  the  anterior 
and  posterior  ento-coxal  nerves,  a.en.cx.  and  p.  en.cx.  that  spring  from  the  clusters 
of  motor  cells  lying  on  the  haemal  side  of  the  brain,  one  on  each  side  of  the  pedal 
ganglion.  (Fig.  66,  H.)  Each  of  these  motor  cells  gives  off  numerous  fibers  to  the 
ento-coxal  muscles;  to  the  crus  of  the  same  side,  and  to  the  opposite  crus,  through 
the  corresponding  commissure.  (Fig.  114,  h.) 

Experimental  Results. — The  results  obtained  by  cutting  the  collar  at  various 
places  are  naturally  not  always  intelligible,  but  when  they  are  they  appear  to  be 
in  harmony  with  the  anatomical  relations  just  described.  These  results  are  as 
follows: 

i.  Cutting  the  posterior  end  of  one  crus,  or  of  both  crura,  behind,  or  close 
to  the  sixth  thoracic  neuromere  does  not  materially  affect  the  chewing  reflexes. 
2.  Cutting  across  one  crus  close  to  the  hemispheres,  produces  increased  vigor,  and 
a  diminished  coordination  in  the  chewing  movements  of  the  legs  on  the  cut  side. 
The  jaw  movement  is  either  unmodified  or  slightly  diminished.  If  both  crura 
are  cut,  the  above  results  are  obtained  on  both  sides.  These  operations  not  only 
separate  the  hemispheres,  but  the  main  gustatory  and  swallowing  centers  from 
the  thoracic  neuromeres.  The  results  point  to  the  presence  of  separate  control- 
ling centers  on  each  side  of  the  forebrain.  3.  An  isolated  segment  of  the  collar, 
containing  one  or  two  neuromeres,  lying  between  the  second  and  sixth  neuromeres, 
and  separated  from  the  opposite  side  by  cutting  its  cross  commissures,  may  give 
feeble,  uncoordinated,  gustatory  reflexes,  but  such  an  isolated  portion  of  the  collar 
generally  degenerated  and  soon  failed  to  give  further  response.  4.  However,  in 
experiment  III — B,  the  left  side  of  the  collar  was  completely  isolated  and  lived  with- 
out perceptible  degeneration  for  three  weeks.  But  this  segment  undoubtedly  in- 
cluded that  part  of  the  cheliceral  neuromere  containing  the  secondary  gusta- 
tory center  and  the  swallowing  center,  and  only  a  small  part,  if  any,  of  the 
hemisphere.  This  segment  readily  produced  the  chewing  reflexes;  the  reflexes 
were  normal  except  for  the  exaggerated  and  uncoordinated  leg  movements  on 
the  isolated  side,  and  the  lack  of  harmony  between  the  chewing  movements  on 
the  one  side  with  those  on  the  other.  The  results  show:  a.  that  the  coordination 
and  inhibition  of  the  leg  movements  on  the  one  side  lie  in  the  hemisphere  of  the 
same  side,  probably  in  the  large  median  lobe,  or  tertiary  gustatory  center;  b.  that 
the  rhythmic  control  of  the  chewing  movements  of  one  side  is  located  in  the  second- 
ary gustatory  center  in  the  cheliceral  neuromere;  and  c.  that  the  coordination  of 


COURSE  OF  NERVE  IMPULSES.  189 

the  movements  of  the  right  and  left  sides  is  controlled  by  means  of  fibers  in  the 
thoracic  cross  commissures,  probably  by  the  crossed  collaterals  of  the  motor 
neurones. 

The  Swallowing  Reflexes. — There  are  no  nerves  or  sense  organs  visible 
near  the  lips,  or  in  the  soft  skin  about  the  mouth;  and  no  reflexes  could  be  pro- 
duced by  touching  these  parts  with  food.  "  Swallowing"  apparently  depends  on 
preliminary  stimulation  of  the  coxal  taste  organs.  The  chewing  movements  are 
often  interrupted  by  a  tetanic  spasm  of  the  legs  and  a  prolonged  "bite"  of  the 
great  mandibles  on  the  sixth  pair.  During  this  period,  a  muscular  spasm  of  the 
stomodaeum  appears  to  take  place,  by  means  of  which  the  materials  that  have 
been  tucked  into  the  mouth  by  the  coxal  spurs  are  swallowed.  The  reflex  is 
probably  initiated  in  the  stomodaeal  ganglion  by  stimuli  received  from  the  anterior 
end  of  the  gustatory  tract. 

Course  of  Nerve  Impulses  in  the  Gustatory,  Chewing,  and  Swallowing 
Reflexes.  —We  may  picture  the  probable  course  of  the  nerve  impulses  in  the 
chewing  reflexes  as  follows :  Stimulation  of  the  gustatory  cell,  g.o,  may  i .  discharge 
an  impulse  by  the  first  set  of  collaterals  directly  to  the  motor  neurones  of  its  own 
segment.  2.  To  produce  the  continued  rhythmic  discharge,  it  is  apparently 
necessary  for  the  impulse  to  be  conveyed  to  the  secondary,  or  cheliceral  center, 
and  then  back  to  the  motor  neurones  of  the  chewing  muscles.  3.  Impulses  may 
be  carried  from  the  secondary  center  to  the  tertiary  center  in  the  hemispheres; 
this  center  appears  to  exercise  a  depressing,  or  inhibitory,  control  of  the  motor 
neurones,  especially  of  those  supplying  the  flexors  and  extensors  of  the  leg.  4. 
The  only  way  to  arouse  the  motor  neurones  of  a  given  leg  or  jaw  to  normal  action 
is  via  its  own  sensory  fibers,  either  directly  through  the  primary  center,  or  indirectly 
through  the  secondary  center,  or  both.  5.  The  linear  coordination  of  gustatory 
movements  on  one  side  is  affected  by  the  cheliceral  center  of  the  same  side.  6. 
Bilateral  coordination  is  affected  via  the  thoracic  cross  commissures. 

The  remarkable  difference  in  the  action  of  the  same  leg  muscles,  when  stimu- 
lated via  different  sensory  channels,  i.e.,  gustatory  and  general  cutaneous,  maybe 
due  to  several  causes  the  nature  of  which  is  very  obscure.  They  are  indicated 
to  some  extent  by  the  known  structure,  and  to  some  extent  by  the  nature  of  the 
reaction.  The  general  cutaneous  tracts,  for  example,  are  not  so  sharply  defined 
and  are  not  composed  of  such  distinct  segmental  fascicles  as  the  gustatory  tracts, 
which  may  account  for  the  fact  that  stimulation  of  a  definite  group  of  temperature 
organs  on  one  side  of  the  shield  usually  produces  a  reaction  in  several  legs  of  the 
opposite  side,  not  in  one.  Moreover  stimulation  of  the  temperature  organs  pro- 
duces a  rather  ill  defined  leg  movement,  that  at  once  ceases  when  the  stimulus  is 
removed.  If  the  taste  organs  are  stimulated,  a  definite  rhythmic  action  follows 
that  does  not  cease  at  once  when  the  stimulus  is  removed.  This  may  be  due  in 
part  to  the  fact  that  the  stimulating  action  of  the  substance  may  in  this  case  con- 
tinue after  the  source  of  it  has  been  removed,  or  in  part  to  the  presence  of  a  "center" 
which  continues  to  act  after  the  peripheral  stimulus  ceases.  But  even  these  con- 


190  FUNCTIONS    OF    THE    BRAIN. 

ditions  do  not  explain  the  rhythmic  repetition  of  the  reactions  in  one  case  and  the 
absence  of  rhythm  in  the  other.  It  indicates  that  there  is  some  "open  and  shut" 
mechanism  that  can  be  reached  and  set  into  action  via  g.o.,  gc1,  h;  or  via  g.,  gc.2, 
gc1,  h.;  but  not  via  th.t.  gc.1,  h.  or  via  any  other  way. 

II.  THE  CROSSED  THORACIC  REFLEXES. 

The  experiments  show  that : 

1.  Gentle  stimulation  (temperature)  on  one  side  of  the  thorax  first  causes 
aimless  movements  of  the  opposite  legs,  followed  by  aimless  movements  of  the 
legs  on  the  same  side.     If  the  stimulation  is  increased,  the  legs  of  the  same  side 
make  coordinated  movements  that  tend  to  thrust  the  stimulating  object  away; 
and  finally  the  opposite  side  may  join  in  the  coordinated  movements. 

2.  If  the  nerve  collar  is  cut  on  one  side  between,  say,  the  second  and  third 
neuromeres,  stimulation  of  the  cut  side  produces  the  same  results  as  before, 
except  that  the  legs  back  of  the  cut,  on  the  same  side,  do  not  make  purposeful 
"thrusting  away"  movements,  while  those  cephalad  to  the  cut  continue  to  do  so, 
if  the  sides  of  the  thorax  cephalad  to  the  cut  are  stimulated. 

3.  If  all  the  free  thoracic  commissures  are  cut,  the  crossed  reflexes  cease. 
These  results  show :  a.  that  certain  coordinated  purposeful  movements  on  one 

side  of  the  thorax  are  controlled  by  the  corresponding  side  of  the  forebrain,  in 
all  probability  by  the  corresponding  hemisphere;  b.  that  the  path  of  the  direct 
crossed  impulses  is  through  the  thoracic  commissures,  and  c.  that  the  coordination 
of  purposeful  movements  on  one  side  of  the  thorax  with  those  on  the  other  is 
accomplished  via  the  commissures  at  the  base  of  the  forebrain. 

The  simpler  relations  of  these  reflex  paths  are  shown  in  connection  with  the 
gustatory  tracts  in  Fig.  114.  It  will  be  noted  that  while  stimulation  of  the 
temperature  organs  at  th.t.  produce  a  few  simple  movements  of  several  legs  of  the 
opposite  side,  stimulation  of  the  taste  cell  at  g.o.  produces  a  continued  rhythmic 
discharge  into  the  muscles  at  the  base  of  one  leg  on  the  same  side,  with  the  result 
that  first  a  leg  flexor,  then  a  leg  extensor  muscle  contracts,  followed  by  the  con- 
traction of  the  four  plastro  coxals,  pi. ex.,  and  then  by  the  five  coxo-tergals,  cx.t. 

III.  THE  CROSSED  AND  UNCROSSED  ABDOMINO-THORACIC  REFLEXES. 

When  the  ventral  margin  of  the  abdomen  is  gently  stimulated,  the  legs  on 
the  opposite  side  of  the  thorax  are  aimlessly  raised,  followed  by  a  start  or  spasm 
of  the  legs  on  the  same  side. 

Numerous  experiments  show  that  a.  the  abdominal  temperature  impulses 
may  cross  on  entering  the  cord,  passing  cephalad  to  the  opposite  crus.  (Fig.  114, 
ab.  t2) ;  b.  that  the  greater  number  of  impulses  pass  up  the  same  side  of  the  cord 
they  enter,  and  that  all  these  impulses  cross  to  the  opposite  side  through  the  com- 
missures of  the  vagus  neuromeres,  ab.t3',  c.  that  they  do  not  cross  in  the  free  tho- 
racic commissures. 


SUMMARY   OF   RESULTS.  IQI 

The  vagus  neuromeres  are  therefore  the  centers  for  important  decussations 
of  impulses  passing  cephalad  on  their  way  from  the  cord  to  the  anterior  brain 
neuromeres,  v.dec. 

IV.  LOCOMOTION. 

Locomotion  is  normally  accomplished  by  coordinated  walking  movements 
of  the  legs,  or  by  a  rhythmic  beating  of  the  legs  and  gills  in  unison,  as  in 
swimming. 

i.  Cutting  the  collar  on  one  side,  behind  the  hemispheres,  diminishes  or 
inhibits  the  walking  or  swimming  movements  on  the  cut  side.  Such  animals 
walk  or  swim  in  circles,  turning  toward  the  cut  side  because  the  legs  on  the  uncut 
side  are  the  most  active,  or  they  are  the  only  ones  that  make  any  walking  or  swim- 
ming movements.  In  water,  as  in  air,  the  legs  on  the  uncut  side  frequently  per- 
form the  normal  swimming  movement,  while  those  on  the  cut  side  are  quiescent, 
or  are  performing  some  other  reflexes,  as,  for  example,  chewing.  We  therefore 
conclude  that  the  primary  reflex  centers  for  locomotion  lie  in  the  last  five  thoracic 
neuromeres,  and  for  the  "gill  swimming"  in  the  anterior  neuromeres  of  the  cord. 
The  secondary  control  centers  lie  in  the  forebrain,  one  on  each  side. 

V.  EQUILIBRIUM. 

The  nature  of  the  apparatus  by  means  of  which  the  crab  tends  to  right  itself 
is  unknown,  but  apparently  the  part  of  the  brain  in  which  this  function  is  centered 
is  near  the  first  two  vagus  neuromeres.  This  is  shown  by  the  fact  that  cutting 
across  the  anterior  part  of  the  collar,  on  one  or  both  sides,  or  destroying  the  hemi- 
spheres, or  cutting  the  ventral  cord  behind  the  vagus  neuromeres,  does  not  destroy 
the  tendency,  or  the  power,  to  turn  the  neural  side  down,  when  free  to  move; 
while  the  cutting,  or  removal,  of  the  vagus  neuromeres  does  destroy  this  tendency. 

VI.  RESPIRATION. 

The  Respiratory  Mechanism. — There  are  two  distinct  sets  of  respiratory 
muscles,  an  adductor  and  an  abductor  for  each  abdominal  appendage  a.bm. 
and  e.bm.-,  and  a  large  compound  muscle,  the  branchio-thoracic,  or  hypobranchial, 
attached  by  separate  slips  to  the  bases  of  all  the  branchial  appendages.  (Fig. 
77,  B,  b.th.m.  and  Fig.  114.) 

The  abductors  extend  from  the  haemal  entapophyses  to  the  anterior  wall, 
and  the  adductors  from  the  entapophyses  to  the  posterior  wall  of  the  branchial 
appendages.  The  branchial  muscles  are  supplied  by  motor  branches  from  the 
branchial  nerve;  the  sensory  branches  supply  an  elaborate  system  of  free  nerve- 
ends, /.e.,  temperature,  b.t.,  and  other  sense  organs,  distributed  over  the  surface 
of  the  appendage. 

The  motor  neuromeres  for  the  branchial  muscles  form  three  groups,  two 
on  the  anterior  haemal,  and  one  on  the  posterior  haemal  side  of  the  corresponding 
ganglion  to  the  branchial  nerve.  (Fig.  62,  H1'2'3') 


I Q2  FUNCTIONS    OF    THE    BRAIN. 

Each  neurone  sends  an  enormous  number  of  dendrites  into  the  ganglion, 
mingling  with  the  central  ends  of  the  sensory  fibers,  and  a  large  number  of  fibers 
through  the  branchial  nerve  to  the  anterior  and  posterior  branchial  muscles. 
Isolation  of  these  centers,  by  cutting  the  cord  on  both  the  anterior  and  pos- 
terior sides  of  the  ganglion,  does  not  destroy  the  action  of  the  corresponding 
appendage. 

The  hypobranchial  muscle  extends  diagonally  forward  from  the  tendinous 
stigmata,  or  hollow  infoldings  at  the  base  of  the  appendages,  to  the  haemal  surface 
of  the  carapace.  (Fig.  77.)  It  serves  to  flex  the  abdomen,  but  primarily  to  draw 
the  bases  of  the  appendages  forward  and  haemally,  thus  expanding  the  chamber 
between  the  roots  of  the  appendages,  and  drawing  the  water  from  the  sides  through 
the  gill  leaves. 

It  is  innervated  by  a  large,  longitudinal  nerve,  formed  by  the  union  of  seven 
segmental  nerves,  one  from  the  haemal  nerve  of  the  opercular  segment  and  one 
from  each  of  the  six  following  haemal  nerves.  Each  segmental  bundle  of  nerve 
fibers  takes  its  origin  from  a  cluster  of  neurones  located  on  the  opposite  side  of 
the  next  preceding  ganglion,  close  to  the  reflex  center  for  that  appendage.  (Figs. 
59  and  60.) 

Respiratory  Reflexes. — No  final  conclusion,  as  to  the  sources  of  the  respi- 
ratory impulses  can  be  reached  till  the  action  of  this  nerve  and  muscle  has  been 
experimentally  demonstrated.  It  is  clear  that  sectioning  the  cord  at  one  or  more 
points  would  not  be  likely  to  greatly  modify  the  action  of  the  hypobranchial 
muscle,  as  it  would  still  receive  nerves  from  the  ganglia  in  front  of  and  behind 
the  cuts.  This  point  has  been  overlooked  by  Miss  Hyde  and  has  not  been  suffi- 
ciently covered  by  our  own  experiments. 

It  seems  probable  that  the  contraction  of  the  muscle  as  a  whole  may  be 
induced  by  impulses  coming  from  one  or  more  neuromeres  through  the  roots  of 
the  anterior,  or  haemal,  nerves,  and  as  such  a  contraction  would  affect  all  the  gills 
at  the  same  time  it  would  tend  to  unify  their  action  and  thus  materially  aid  the 
linear  coordination  of  the  respiratory  rhythm. 

The  location  of  the  common  center  controlling  the  whole  series  of  branchial  re- 
flexes could  not  be  determined,  but  judging  from  the  forward  displacement  of  the 
motor  cells  and  of  the  central  ends  of  the  accompanying  sensory  fibers,  it  probably 
lies,  in  part,  at  any  rate,  in  the  vagus  neuromeres.  This  conclusion  is  strengthened 
by  the  fact  that  the  destruction  of  the  vagus  neuromeres  materially  modifies  the 
respiratory  activities.  It  apparently  lowers  the  threshold  that  inhibits  the  "cross 
rubbing"  or  the  normal  respiratory  movements,  for  if  the  vagus  neuromeres  are 
destroyed,  or  separated  from  the  cord,  gentle  stimulation  of  the  isolated  gills 
with  tactile,  temperature,  or  chemical  agents,  starts  the  respiratory  reflexes  in  them 
much  more  readily  than  in  those  not  so  isolated.  Moreover,  the  forced  "yawn- 
ing" of  the  gills  and  the  swimming  movements,  which  represent  a  modified  re- 
spiratory movement,  disappear  in  those  gills  that  are  not  directly  connected  with 
the  vagus  region.  There  is  also  a  striking  difference  in  the  rhythm,  range,  rate, 


COMPARISON    WITH    VERTEBRATES.  1  93 

and  spontaneity  of  movements  between  the  gills  cut  off  from  the  hindbrain  and 
those  that  are  united  directly  with  it  by  one  or  both  cords. 

We  may  therefore  conclude  that  each  branchial  neuromere  contains  only  a 
part  of  a  respiratory  reflex  center;  and  that  the  inhibitory  control  and  the  coordi- 
nation, or  unification,  of  the  respiratory  and  other  related  gill  movements  is 
produced,  in  part,  by  the  action  of  a  special  respiratory  center  located  in  the 
vagus  neuromeres,  and  in  part  by  the  hypobranchial  nerve  and  muscle. 

The  coordination  of  right  and  left  sides  is  affected  via  the  cross  commissures. 

Comparison  with  Vertebrates.  —  The  results  obtained  from  an  experi- 
mental study  of  the  respiratory  centers  of  Limulus  by  Miss  Hyde,  Mr.  Pearl  and 
myself  are  in  essential  agreement,  and  they  harmonize  with  Miss  Hyde's  work 
on  other  invertebrates  and  on  the  skate.  In  her  admirable  paper  on  the  "  Localiza- 
tion of  the  Respiratory  Center  in  the  Skate,"1  she  makes  the  following  statements: 

1.  "  Students  working  in  my  laboratory  have  proved  that  the  relative  position 
of  the  respiratory  center  in  the  central  nervous  system  of  the  acrididae  is  practically 
the  same  as  in  Limulus. 

2.  "The  respiratory  movements  of  the  skate  are  segmental  processes.     The 
relationship  of  the  respiratory  organs  and  their  segmental  centers  is  not  so  obvious 
as  it  is  in  the  lower  forms  (i.e.,  Limulus).     The  developmental  changes  of  shifting 
and  consolidation  have  begun  to  mask  the  segmental  connections  of  the  different 
parts  of  the  brain. 

3.  "Each  ganglion,  through  special  fibers  and  cells,  controls  the  activity  of 
the  respiratory  muscles  with  which  it  is  segmentally  related  and  is  capable 
of    initiating     impulses     that     produce     coordinated    rhythmical    respiratory 
movements. 

4.  "  The  medulla  may  be  severed  both  from  the  cord  and  the  regions  of  the 
brain  anterior  to  it,  or  divided  along  its  median  suture,  into  two  bilateral  halves 
without  impairing  the  functions  of  the  respiratory  center.     Each  half  is  capable 
of  sustaining  coordinated  respiratory  movements  which  part  of  the  time  may  be 
different  in  rhythm  on  the  two  sides. 

5.  "Not  only  may  either  the  spiracle  and  first  gill  arch,  innervated  by  the 
seventh  and  ninth  nerves,  or  the  last  four  gill  arches,  innervated  by  the  tenth, 
when  isolated  from  the  rest  of  the  respiratory  mechanism  by  a  median  and  trans- 
verse section  continue  their  movements,  but  all  other  than  the  special  part  of 
the  respiratory  center  that  controls  these  divisions  may  be  destroyed,  and  either 
the  four  gill  arches  or  the  spiracle  and  first  gill  arch  will  still  pursue  their  coordi- 
nated respiratory  activity. 

6.  "The  skate  illustrates,  in  its  type  of  respiratory  center,  an  intermediate 
stage,  between  the  simple  segmental  arrangement  of  the  neurons  presiding  over 
the  coordinated  respiratory  movements  found  among  invertebrates,  and  the  com- 
plex, modified,  and  specialized  centers  existing  in  higher  vertebrates." 


Ant.  Journ.  Physiol.,  1904. 
13 


194  FUNCTIONS    OF    THE    BRAIN. 

Thus  the  structural  and  physiological  evidence  indicates,  beyond  reasonable 
doubt,  that  the  vagus  neuromeres  (opercular  and  chilarial)  and  the  five  branchial 
neuromeres  of  the  marine  arachnids  have  become  consolidated  into  a  single,  compact 
group,  which  in  the  vertebrates  unites  with  the  hindbrain  to  form  the  posterior 
part  of  the  medulla. 

VII.  THE  CEREBRAL  HEMISPHERES. 

We  have  shown  that  in  Limulus  the  hemispheres  are  primarily  connected 
with  the  sensory  nerves  of  but  one  sense  organ,  the  olfactory.  They  contain, 
however,  important  secondary  centers  belonging  to  the  visual  and  to  the  gustatory 
organs.  They  are  true  cerebral  centers,  both  in  structure  and  function,  and 
are  similar  to  the  primitive  hemispheres  of  vertebrates,  in  that  they  regulate  or 
control  a  large  number  of  complex  activities  of  which  the  several  primary  reflex 
centers  lie  in  the  more  remote  parts  of  the  central  nervous  system.  They,  for 
example,  exercise  a  tonic,  or  inhibitory,  influence  over  the  posterior  part  of  the 
brain  and  the  cord,  and  they  are  the  source  of  impulses  that  check,  or  maintain,  or 
coordinate,  the  walking  and  swimming  movements,  the  leg  movements  in  chewing, 
and  the  purposeful  movements  of  the  legs  in  removing  local  irritants. 


CHAPTER  XII. 
THE  HEART. 

I.  LOCATION  OF  THE  HEART. 

In  the  annelids  and  in  some  primitive  arthropods,  the  heart  is  a  straight 
tube  lying  on  the  opposite  side  of  the  body  from  the  nerve  cord,  and  extending 
practically  from  one  end  of  the  body  to  the  other. 

In  the  typical  arachnids  and  in  many  of  the  higher  arthropods  (insects  and 
Crustacea),  the  primitive  heart  tube  is  shortened,  in  part  by  the  conversion  of  the 
anterior  end  into  a  non-contractile  aorta,  and  in  part  by  the  absence  of  the  more 
posterior  portion.  The  part  that  persists  as  a  true  pulsating  heart  is  located,  as 
a  rule,  in  the  first  eight  post- thoracic  segments,  that  is  in  the  vagal  and  branchial 
segments  (Limulus  and  scorpion).  (Fig.  3.)  The  heart  may,  in  the  earlier  em- 
bryonic stages,  extend  into  the  sixth  (scorpion)  (Figs.  15  and  16,  A1),  or  into  the 
fifth  and  sixth  thoracic  segments  (Limulus).  (Figs.  141  to  151.)  But  these  more 
anterior  heart  segments  are  less  highly  developed,  and  may  be  reduced  to  a  non- 
pulsating  chamber  that  forms  the  proximal  end  of  the  aorta.  The  most  volum- 
inous part  of  the  heart  in  the  adult  Limulus  is  its  posterior  part,  opposite  the  middle 
branchial  appendages,  i.e.,  between  the  third  and  seventh  pair  of  ostia.  (Fig.  i,  B.) 

The  location  of  the  heart  is  greatly  influenced  by,  or  itself  controls,  the  loca- 
tion of  the  tracheal  stigmata,  the  lung  books,  and  the  gills,  since  all  these  organs 
retreat  from  the  anterior  head  region  in  nearly  the  same  order,  and  usually  occupy 
about  the  same  post-thoracic  segments.  (Fig.  3.) 

The  gradual  retreat  of  the  heart  and  the  respiratory  organs  from  the  head  and 
thorax,  and  their  concentration  into  a  special  group  of  post-thoracic  segments 
may  be  readily  followed  to  its  culmination  in  such  forms  as  Limulus,  scorpion, 
and  spiders.  This  striking  process  becomes  especially  significant  when  it  is  seen 
that  in  the  vertebrates  also  these  organs  occupy,  as  nearly  as  one  may  determine, 
the  same  metameres.  (Fig.  3,  D,  308.) 

II.  DEVELOPMENT  OF  THE  HEART. 

The  location  of  the  heart  is  determined  by  very  remote  but  persistent  condi- 
tions that  affect  the  form  and  structure  of  the  whole  anterior  part  of  the  head  and 
trunk.  The  principal  event  in  these  changes,  which  have  been  and  are  pro- 
gressive, affecting  the  embryos  of  all  segmented  animals  alike  but  in  a  varying 
degree,  is  the  gradual  disappearance  from  before  backward  of  the  segmented 
lateral  plates  of  mesoderm  belonging  to  the  head  and  thoracic  metameres. 

In  Limulus,  in  the  scorpion,  and  in  spiders,  the  surviving  mesoderm  of  the 
cephalothorax  consists  almost  exclusively  of  the  six  pairs  of  thoracic  somites  (head 


196  THE   HEART. 

cavities)  that  give  rise  to  the  endocranium,  the  jaw  and  leg  muscles,  and  the  coxal 
glands  (head  kidney).  (Fig.  138,  c.so.) 

There  is  a  conspicuous  territory,  lateral  to  the  cephalic  lobes  and  thoracic 
appendages,  that  presents  little  or  no  indication  of  segmentation.  (Figs.  15,  16, 
19,  21,  31-33,  141-156.)  It  is  covered  by  a  thin  layer  of  ectoderm  with  numerous 
underlying,  oval  cells,  or  fiber  cells,  that  are  eventually  converted  into  muscles, 
or  peculiar  bodies  resembling  blood  corpuscles.  See  vascular  area  (Chapter  XIII, 
page  232.) 

Back  of  this  region  the  lateral  walls  of  the  embryo  are  divided  into  distinct 
segments.  The  peripheral  margin  of  these  segments,  up  to  a  comparatively  late 
embryonic  period,  ends  in  a  kind  of  germ  wall,  where  the  advancing  sheets  of 
ectoderm,  mesoderm,  and  endoderm,  or  "yolk  cells,"  merge  into  a  common 
primitive-streak-like  thickening.  (Fig.  134,  g.w.) 

In  the  vagal  and  branchial  metameres,  the  lateral  plates  of  mesoderm  con- 
sist of  definite  somatic  and  splanchnic  layers,  which  enclose  separate  coelomic 
chambers.  In  these  regions,  the  fiber  cells  are  absent,  but  numerous  blood  cor- 
puscles are  present,  formed  from  liberated  germ  wall  cells. 

As  the  lateral  ends  of  the  mesodermic  segments  approach  the  haemal  sur- 
face, they  separate  from  the  germ  wall,  take  on  a  crescentic  form  and,  uniting  with 
their  mates  of  the  opposite  side,  form  the  walls  of  a  heart  segment,  or  cardiomere. 
The  cardiac  ostia  represent  the  spaces  between  the  anterior  and  posterior  walls 
of  the  adjacent  segments.  (Figs.  136,  137,  h.) 

Only  the  lateral  plates  of  the  sixth  thoracic,  the  chelarial,  opercular,  and 
five  branchial  segments  form  definite,  or  permanent,  cardiomeres. 

The  coelomic  cavities,  at  the  haemal  ends  of  the  segments  that  form  cardio- 
meres, become  partly  shut  off  to  form  the  pericardial  chamber.  The  neural 
portion  of  the  coelom,  belonging  to  the  five  branchial  segments,  forms  the  five 
great  veno-pericardiac  canals. 

As  the  margins  of  the  lateral  plates  advance,  the  cardiac  nerves  follow  after, 
keeping  close  to  the  intersegmental  thickenings  of  the  ectoderm,  thus  reaching 
the  heart  tube  opposite  the  intersegmental  ostia,  a  position  which  they  retain 
throughout  life.  (Figs.  115-151,  s.c.n.) 

As  the  lateral  plates  of  the  cardiac  segments  advance  over  the  yolk,  they 
expand,  fan-like,  into  the  unoccupied  yolk  surface  in  front  and  behind  more 
rapidly  than  along  the  true  parallels  of  the  yolk  sphere.  The  result  is  that  when 
they  unite  on  the  haemal  surface,  the  anterior  heart  segments  lie  farther  forward 
in  the  thoracic  territory  than  their  neural  ends.  This  unequal  displacement 
gradually  carries  the  anterior  end  of  the  heart  tube  forward  till  it  almost  meets 
the  anterior  end  of  the  forebrain,  which  is  being  crowded  backward  in  the  opposite 
direction.  (Figs.  17,  26,  31,  138,  157.) 

Thus  in  the  later  embryonic  periods  and  in  the  adult,  the  original  relation  of 
the  cardiomeres  to  the  neuromeres  is  greatly  disguised,  except  in  so  far  as  it  is 
shown  by  the  terminals  of  the  segmental  cardiac  nerves. 


DEVELOPMENT.  197 

Owing  to  the  rapid  concrescence  of  the  more  posterior  lateral  plates,  the  yolk 
territory  behind  the  tail  end  of  the  embryo  is  covered  at  an  early  period.  Hence 
further  apical  growth  must  take  place  in  a  vertical  direction,  or  in  such  a  manner 
as  to  raise  the  apex  of  the  tail  off  the  surface  of  the  egg.  (Fig.  157,  D.)  Under 
these  new  conditions,  the  formation  of  both  neural  and  haemal  surfaces  takes 
place  at  very  nearly  the  same  time,  and  under  similar  conditions.  The  heart 
does  not  extend  into  this  region  of  the  trunk. 

It  is  thus  seen  that  there  are  three  natural  divisions  of  the  haemal  surface, 
where  the  physical  conditions  are,  necessarily,  fundamentally  different;  namely, 
in  the  cephalothoracic,  in  the  abdominal,  and  in  the  caudal.  The  factors  that 
produce  or  control  these  conditions  are  the  relations  that  exist  between  the  rate  of 
apical  and  bilateral  growth  and  the  volume  of  the  yolk  sphere  over  which  this 
growth  is  obliged  to  take  place.  (Figs.  17,  23,  34.) 

Other  Arachnids. — In  the  scorpion  and  in  spiders  (Epeira),  the  heart  develops 
in  essentially  the  same  manner  as  in  Limulus.  (Figs.  15,  16,  17,  20,  22.) 

The  details  of  the  process  of  heart  formation,  as  seen  in  sections,  especially 
the  stages  immediately  preceding,  and  during  the  concrescence  of  the  cardio- 
meres,  have  not  been  worked  out.  They  should  receive  more  a  careful  study  than 
we  have  been  able  to  give  them. 

Comparison  of  Vertebrate  and  Arachnid  Heart. — A  study  of  the  early 
.stages  in  the  development  of  the  heart  in  arachnids  and  primitive  vertebrates 
shows  that  in  both  classes  we  are  dealing  with  different  phases  of  the  same  process. 
In  both  classes,  the  heart  is  formed  from  the  peripheral  ends  of  lateral  plates 
belonging  to  a  variable  number  of  branchial  metameres.  (Fig.  32-33.)  These 
ends  grow  in  an  anterior  haemal  direction  and  concresce  in  the  median  haemal 
surface,  behind  the  anterior  end  of  the  forebrain  and  the  cephalic  navel,  or 
neostoma. 

In  the  arachnids,  the  heart  is  composed  of  a  single  layer  of  loose,  striated 
anastomosing  muscle  cells  covered  by  a  fibrous  membrane.  (Fig.  2.)  The 
heart  tube  is  enclosed  in  a  distinct  pericardial  chamber,  the  pericardial  walls  and 
the  walls  of  the  heart  being  continuous  at  the  anterior  and  the  posterior  ends, 
and  at  the  points  where  the  aortic  trunks  arise.  (Fig.  118.)  The  heart  is  also 
.attached  to  the  pericardial  walls  throughout  the  entire  length  of  the  neural  and 
haemal  surfaces  by  numerous  connective  tissue  fibers  and  muscular  strands. 

The  pericardium  on  the  neural  side  forms  a  tough  fibrous  membrane  of  con- 
siderable thickness,  but  it  does  not  appear  to  contain  muscular  bands. 

In  the  vertebrates,  we  may  recognize  the  same  kind  of  growth  from  the  same 
region  to  the  same  region.  In  its  earliest  condition,  the  vertebrate  heart  is  an 
axial  cord  composed  of  a  syncytial  meshwork  with  irregular,  interstitial  spaces. 
(S.  Mollier.)  This  is  the  so-called  mesenchematous  stage  of  the  amphibian  and 
selachian  heart.  At  a  later  period,  the  primary  cord  is  metamorphosed  into  a 
thin-walled  tube,  the  endocardial  tube,  and  a  muscular  layer,  or  myocardium, 
forms  around  it.  The  two  layers  are  separated  by  an  extraordinarily  wide  space, 


198  THE   HEART. 

bridged  by  fine  fibers.  This  space  ultimately  disappears  and  the  inner  and 
outer  tubes  unite  to  form  the  definitive  walls  of  the  heart. 

The  volume  and  complexity  of  this  primordial  heart  tube,  its  early  appear- 
ance, and  its  wide  separation  from  the  myocardium  are  most  remarkable.  These 
conditions  are  not  satisfactorily  explained  by  the  assumption  that  the  cardiac 
endothelium  is  a  secondarily  acquired  investment  of  a  primitive  muscular  heart 
tube.  Limulus  has  the  largest  and  most  complex  heart  of  any  living  arthropod, 
and  if  an  endothelium  layer  is  present  in  any  invertebrate,  it  should  be  present 
there,  but  a  careful  search  in  both  adult  and  embryonic  hearts  failed  to  reveal  any 
trace  of  such  a  layer.  It  is  possible  that  in  vertebrates,  the  cardiac  and  peri- 
cardiac  walls  of  the  arachnids  have  united  to  form  a  two-layered  heart.  But  this 
would  not  account  for  the  presence  of  the  cardiac  ganglia  on  the  outer  surface  of 
the  myocardium  in  vertebrates. 

An  alternative  and  preferable  hypothesis  would  be  to  assume  that  the  arach- 
nid heart  represents  the  ventricular  portion  in  vertebrates,  and  that  the  posterior 
posterior  portion  of  the  pericardia!  chamber  represents  the  thinwalled  auricu- 
lar or  atrial  portion  and  the  venous  sinus.  (Fig.  44.) 

III.  CIRCULATION. 

Arachnids. — The  circulation  in  Limulus  has  reached  a  very  high  stage  of  devel- 
opment. Milne  Edwards,  in  his  classic  work  on  the  anatomy  of  Limulus,  says, 
"The  circulatory  system  of  Limulus  is  more  perfect  and  more  complicated  than 
in  any  other  arthropod.  The  venous  blood,  instead  of  being  distributed  in  inter- 
organic  lacunae,  as  in  the  Crustacea,  is,  through  a  great  part  of  its  course,  en- 
closed in  special  vessels,  having  walls  distinct  from  the  adjacent  organs,  and 
often  rising  by  branches  of  remarkable  delicacy  and  passing  into  chambers,  well 
circumscribed  for  the  most  part.  The  nourishing  fluid  passes  from  these  reservoirs 
into  the  gills,  and  hence,  by  a  system  of  branchio-cardiac  canals,  to  the  peri- 
cardial  chamber  and  the  heart,  which  is  very  large.  It  is  then  forced  into  the 
tubular,  resisting  arteries,  which  have  a  most  complex  arrangement,  with 
frequent  anastomoses  and  with  terminal  ramifications  of  marvellous  tenuity  and 
richness,  and  which  can  be  followed  even  in  the  most  delicate  membranes." 

The  heart  of  Limulus  is  a  voluminous  muscular  tube,  ending  blindly  behind. 
It  is  provided  with  eight  pairs  of  slit-like  openings,  or  ostia,  each  opening  guarded 
by  two  semilunar  valves  through  which  the  blood  enters  the  heart  from  the  peri- 
cardial  chamber.  (Figs.  115-118.)  The  blood  is  pumped  forward,  and  escapes 
through  three  pairs  of  aortae,  one  pair  of  cerebral  arteries,  and  a  frontal  artery. 
The  three  terminal  arteries  are  guarded  by  one  very  large  valve  on  the  haemal 
wall  of  the  heart.  The  walls  of  the  heart  consist  of  a  loosely  felted  mass  of  inter- 
woven muscle  bands,  without  any  recognizable  endothelium.  (Fig.  2.) 

Vertebrates. — Comparison  of  the  circulation  in  Limulus  with  that  in  verte- 
brates is  difficult.  There  are  some  striking  resemblances  and  some  equally 
striking  differences. 


CIRCULATION.  199 

The  more  general  resemblance  between  the  arachnid  and  vertebrate  circu- 
lation is  shown  by  the  direction  of  the  blood  currents  and  by  the  distribution  of 
the  main  arterial  and  venous  trunks.  (Fig.  118.)  In  Limulus,  the  blood  flows 
laterally  and  neurally  through  five  pairs  of  aortic  trunks.  The  anterior  pair 
i.e.  (internal  carotids)  go  to  the  base  of  the  brain,  where  they  form  a  closed  circle 
around  the  oesophagus  c.w.  (circle  of  Willis  around  the  infundibulum,  Figs.  43, 
44)  and  then  backward  along  the  brain  and  spinal  cord.  The  following  four 
pairs  are  short  trunks  opening  directly  into  two  longitudinal  channels  in  which 
the  blood  flows  forward  eoc.c.  (external  carotids)  and  backward  (radices  aortae) 
into  the  unpaired  aorta  ao.  Two  large  venous  trunks  (cardinals)  collect  the 
blood  from  the  anterior,  lateral,  and  posterior  parts  of  the  body  and  conduct  it  to 
the  gills. 

Important  changes,  however,  have  taken  place  that  we  cannot  explain  satis- 
factorily. In  vertebrates,  the  ostia  have  evidently  closed  without  leaving  any 
trace  behind;  and  apparently  one  of  the  posterior  pairs  of  the  venous  channels, 
br.c.  now  opens  directly  into  the  posterior  end  of  the  heart,  instead  of  into  the 
pericardial  chamber  (Cuvierian  ducts).  The  relation  of  the  gill  chamber  to  the 
aorta  has  also  been  radically  changed. 

The  curvature  of  the  vertebrate  heart,  its  splitting  at  the  posterior  end 
(vitelline  veins),  and  its  elimination  from  the  trunk  segments  are  more  readily 
understood.  These  conditions  are  undoubtedly  produced  by  the  "yolk  navel," 
which  is  in  turn  produced  by  the  increasing  size  of  the  yolk  sphere;  that  is,  the 
cardiac  ends  of  the  lateral  plates  belonging  to  the  branchial  segments  are  forced 
by  the  increasing  size  of  the  yolk  sphere  to  reach  the  haemal  surface  of  the  egg  in 
the  gradually  shortening  area  between  the  overgrowing,  precocious  forebrain  and 
the  anterior  margin  of  the  uncovered  yolk  surface  (yolk  navel).  (Fig.  17.)  As 
this  cardiac  area  is  being  continually  shortened  by  the  increasing  precocity  of  the 
forebrain,  and  by  the  increasing  size  of  the  yolk  sphere,  and  as  the  heart  itself  is 
meantime  increasing  in  volume,  it  is  forced  to  assume  the  S-shaped  loops  so 
characteristic  of  vertebrates,  in  order  to  occupy  the  only  space  that  is  left  open 
for  it.  (Fig.  44.)  These  loops,  once  initiated,  are  accentuated  by  the  unequal 
mechanical  stress  of  the  enclosed  blood  current,  which  continues  to  sculpture  and 
mould  the  heart  walls,  as  a  river  its  banks,  till  organic  equilibrium  is  again 
reached  in  the  four  chambered  heart  of  mammals. 

The  splitting  of  the  posterior  end  of  the  heart  in  vertebrate  embryos  is  the 
direct  result  of  the  increasing  size  of  the  yolk  sphere,  which  favors  the  early 
concrescence  on  the  haemal  side  of  the  head  of  the  anterior  cardiomeres,  but  delays 
the  concrescence  of  the  more  posterior  ones,  because  they  necessarily  appear  later 
than  the  anterior  ones,  and  have  to  travel  over  the  arcs  of  larger  circles.  Thus 
the  ununited  posterior  cardiomeres  may  form  two  divergent,  pulsating  vessels 
(vitelline  veins)  along  the  sides  of  the  yolk  navel,  long  after  the  anterior  ones  have 
united  to  form  a  single  tube.  (Figs.  17,  23.) 


200  -    THE   HEART. 

IV  .  THE  CARDIAC  NERVES  AND  GANGLIA. 

Limulus  gives  us  the  most  detailed  available  picture  of  the  structure  and 
relations  of  the  cardiac  nerves  in  invertebrates. 

We  recognize  five  longitudinal  cardiac  nerves  extending  either  over  the  sur- 
face of  the  heart,  or  close  to  it;  and  seven  or  eight  pairs  of  segmental  or  trans- 
verse ones,  which  connect  the  longitudinal  nerves  with  the  vagus  or  branchial 
neuromeres. 

The  Median  Cord  or  Ganglion. — This  is  the  primary  nerve  center  for  the 
heart.  It  is  a  median  cord  of  ganglion  cells,  readily  visible  to  the  naked  eye, 
lying  on  the  haemal  surface  of  the  heart  and  extending  from  one  end  of  the 
heart  to  the  other.  (Fig.  115,  m.c.n.) 

It  arises  at  an  early  embryonic  period  from  a  thickening  of  the  overlying 
ectoderm.  It  probably  extended,  in  primitive  arthropods,  the  whole  length  of 
the  body.  In  the  forms  I  have  studied  (Acilius,  Limulus,  and  scorpion),  it  extends, 
during  the  embryonic  period,  over  a  longer  territory  than  the  heart  and  appears 
to  stand  in  the  same  relation  to  the  haemal  surface  of  the  body  that  the  middle 
cord  does  to  the  neural  surface.  The  median  cord,  therefore,  belongs  primarily 
to  the  overlying  ectoderm.  It  lies  on  the  outer  surface  of  the  myocardium,  and 
is  not  at  any  point  actually  imbedded  in  it. 

All  the  ganglion  cells  of  the  heart  lie  in  the  median  cord,  or  at  the  roots  of 
the  strands  that  arise  from  it. 

The  cardiac  plexus  is  an  irregular  meshwork  of  nerve  fibers,  arising  from 
the  median  cord  and  spreading  over  the  haemal  surface  of  the  heart.  The  strands 
diminish  in  caliber  and  finally  form  slender  bundles  that  penetrate  the  walls 
of  the  heart,  where  most  of  them  appear  to  end  in  minute  end  plates,  on  the  surface 
of  the  muscle  strands. 

The  Lateral  Cardiacs. — Many  of  the  larger  strands  of  the  plexus  unite  on  the 
sides  of  the  heart  to  form  distinct  lateral  nerves,  easily  visible  to  the  naked  eye, 
l.c.n. 

The  median  cord,  the  plexus,  and  the  lateral  nerves  stand  out  with  great  dis- 
tinctness when  treated  with  methylene  blue,  and  they  may  be  easily  studied  under 
high  powers,  in  preparations  of  the  whole  heart  of  either  the  young  or  the  adult 
animal.  The  lateral  cardiacs  never  contain  ganglion  cells. 

The  pericardial  nerves  extend  lengthwise  on  the  side  walls  of  the  peri- 
cardial  chamber,  breaking  up,  at  either  end,  into  minute  fibers,  the  terminations 
of  which  could  not  be  certainly  ascertained,  p.n. 

The  Segmental  Cardiacs. — There  are  seven  or  eight  pairs  of  segmental 
cardiac  nerves,  s.c.n.  7~13'  They  arise  from  the  seventh  to  the  thirteenth  haemal 
nerves  inclusive,  in  close  connection  with  the  rami  communicantes  of  the  hypo- 
branchial  nerves.  (Fig.  59.)  They  extend  to  the  haemal  side  of  the  body,  giving 
off  numerous  small  branches  to  the  neighboring  muscles  and  integument.  The 
five  branchial  ones  turn  inward,  giving  off  rami  communicantes  to  the  peri- 


THE    CARDIAC   NERVES  AND    GANGLIA. 


201 


FIG.  115. — The  heart  and  adjacent  organs  of  an  adult  Limulus,  showing  the  ostia;  principal  blood-vessels; 
ganglionated  median  nerve,  m.c.n.;  the  cardiac  plexus;  the  pericardial  nerves,  P.M.;  the  lateral,  l.c.n.,  and  seg- 
mental  cardiacs,  s.c.n.  6-13.  From  Patten  and  Redenbaugh;  slightly  modified. 


2O2  THE   HEART. 

cardial  nerves;  some  of  them  unite  with  the  median  cardiac,  in  the  region 
of  the  last  five  pairs  of  ostia.  (Figs.  115-117.)  The  segmental  cardiacs  of  the 
seventh  and  eighth,  or  vagus  neuromeres,  unite  to  form  one  large  nerve  which 
anastomoses  with  the  pericardial  trunks,  but  neither  it,  nor  the  sixth,  could  be 
traced  directly  to  the  median  cardiac. 

###*#*#** 

The  entire  system  of  cardiac  nerves  probably  represents  a  modification  of  a 
primitive  system  of  longitudinal  and  circular  integumentary  nerves  distributed 
to  the  skin,  muscles,  and  other  organs  on  the  haemal  surface  of  the  body.  With 
the  reduction  of  the  primitive  heart  to  a  shorter,  more  compact  organ,  lying  in  the 
posterior  thoracic  and  branchial  regions,  there  was  a  corresponding  reduction 
in  the  length  of  the  several  longitudinal  nerves  and  in  the  number  of  pairs  of  seg- 
mental cardiac  nerves  uniting  the  heart  with  the  nerve  cords. 

In  some  arthropods  it  is  probable  that  there  is  some  connection  between  the 
cardiac  and  the  stomodaeal  nerves.  (See  Polici,  Naples  Mittheilung,  1908,  and 
other  papers  by  the  same  author.)  Such  a  connection,  if  it  still  exists  in  Limulus, 
must  be  very  minute,  and  can  only  be  detected  by  a  special  application  of  the 
methylene  blue  method. 

V.    THE   MINUTE   STRUCTURE  OF  THE  CARDIAC  GANGLION. 

Nerve  Cells. — We  may  recognize  in  the  median  cardiac  nerve  three  different 
kinds  of  ganglion  cells:  a.  Small,  multipolar  cells  that  stain  very  quickly  and 
deeply  in  methylene  blue,  and  that  form  a  thick,  irregular  covering,  several  layers 
deep,  over  the  outer  surface  of  the  cord.  (Fig.  1 16,  gn.c'.)  In  exceptional  cases,  they 
extend  for  some  distance  on  to  the  larger,  lateral  branches  of  the  plexus,  where  they 
form  irregular  flakes  or  clusters;  but  I  have  never  seen  any  isolated  ganglion  cells 
on  the  lateral,  or  on  the  pericardial  trunks,  or  on  any  of  the  smaller  strands  of 
the  cardiac  plexus.  I  doubt  very  much  whether  any  ganglion  cells  exist  in  the 
heart  outside  the  median  cord,  or  the  roots  of  the  larger  strands  near  where  they 
leave  the  cord. 

In  the  adult,  these  cells  are  about  32/4.  in  diameter.  They  are  pear- 
shaped,  the  cell  body  giving  rise  to  many  fine  dendrites  which  form  dense  felted 
masses  of  varicose  fibers.  One  can  usually  distinguish  among  them  one  long 
fiber,  extending  inward,  diagonally  across  the  inner  surface  of  the  median  cord, 
and  out  of  it,  through  one  of  the  branches  of  the  other  side. 

These  cells  are  very  numerous  in  the  heart  segments  belonging  to  the  mid- 
dle branchial  neuromeres,  where  they  form  a  thick  and  continuous  but  irregular 
coating  to  the  median  cord. 

Toward  the  anterior  end  of  the  heart,  the  cord  becomes  much  smaller, 
and  in  the  first  three  or  four  segments,  these  cells  are  either  absent  or  reduced 
to  a  few,  scattering  clusters,  usually  located  opposite  the  ostia.  b.  The  second 
kind,  Gn.c.,  consists  of  giant  bipolar  cells,  which  in  the  adult  are  about  140  x  IOOM. 


THE   CARDIAC    GANGLION. 


203 


They  occupy  the  axial  portion  of  the  median  cord,  and  are  confined  to  the  middle 
and  the  posterior  segments,  none  having  been  observed  in  the  first  three,  or  in  the 
last  segment. 

The  body  of  these  cells  is  seldom  fully  impregnated  with  methylene  blue. 
The  nucleus,  central  protoplasm,  and  spiral  fibers  may  be  faintly  outlined,  in 


FIG.  116. — The  lateral  margin  of  the  ganglionated  median  nerve  of  the  heart  of  an  adult  Limulus,  from  the 
region  of  the  sixth  cardiomere ;  it  shows  the  two  kinds  of  ganglion  cells,  G.ti.c.  and  gn.c.;  the  clusters  of  neuropile, 
n.p.;  and  the  paccinian-like  terminals,  P.c.,  imbedded  in  the  muscular  substances  of  the  heart.  Methylene-blue 
preparation. 

sharp  contrast  with  the  small,  almost  black,  multipolar  cells.  Their  enormous 
axis  cylinders,  however,  are  usually  deeply  colored.  They  extend  forward  and 
backward,  apparently  the  whole  length  of  the  cord,  bending  from  side  to  side,  and 


204 


THE   HEART. 


giving  off,  from  time  to  time,  branching  collaterals.     Both  collaterals  and  axones 
appear  to  pass  out  of  the  cord,  into  the  lateral  plexus. 

The  axial  portion  of  the  cord  also  contains  irregular  masses  of  neuropile, 
but  whether  they  are  derived  from  the  axones  of  the  giant  cells,  or  from  those  of 
the  multipolar  ones,  or  from  both,  could  not  be  certainly  determined.  When  too 


FIG.  117. — Diagram  illustrating  the  distribution  of  the  cardiac  neurones  and  their  probable  relations  to  the 
branchial  and  vagus  neuromeres.  The  heart  and  the  nerve  cord  are  shown  projected  onto  the  same  plane 
Vag.  D.,  vagus  division;  BY.  D.,  branchial  division  of  the  heart. 


many  fibers  are  not  colored,  one  may  see  irregular  baskets  of  intertwining  fibrils 
at  frequent  intervals  all  through  the  thicker  parts  of  the  cord,  and  in  the  larger 
strands  of  the  plexus,  n.p.  Some  are  apparently  free  terminal  arborescences, 
others  form  a  basket  work  around  the  body  of  the  giant  cells;  or  several  multi- 


THE    CARDIAC    GANGLION. 


205 


polar  cells,  together  with  their  dendrites,  form  enveloping  baskets  around  the 
body  of  the  giant  cells. 

c.  The  third  kind  of  cells  consists  of  small,  bipolar  neurones,  found  in  the 
first  three  or  four  segments  of  the  heart.  They  are  not  numerous,  and  in  some 
cases  they  appear  to  be  absent.  They  resemble  the  giant  cells  in  shape,  but  are 
smaller  and  are  deeply  stained  in  methylene  blue. 

In  the  young  Limuli,  two  or  three  inches  long,  the  small  cells  are  more  clearly 
pear-shaped  and  have  fewer  dendrites,  the  rounded  body  projecting  freely  from 
the  sides  of  the  cord,  to  which  it  is  attached  by  one  or  two  branching  processes. 

Motor  Terminals. — The  larger  strands  forming  the  cardiac  plexus  lie  on 
the  outer  surface  of  the  heart;  the  smaller  branches  gradually  penetrate  between 
the  muscle  bundles  to  the  deeper  layers,  where  one  may  frequently  see  them 
terminate  in  the  characteristic  motor  end  plates. 

Sensory  Terminals. — Toward  the  anterior  end  of  the  heart,  on  either  side 
of  the  median  nerve,  there  are  peculiar,  spherical  masses  dimly  visible,  imbedded 
in  the  muscle  layers,  that  probably  represent  free  sensory  terminals.  Two  or 
three  nerve  fibers  approach  these  bodies  and  form  there  concentric  coils  of  fibrillae, 
with  two  or  three  thicker  vertical  fibers  in  the  center  of  the  coils,  P.c. 

It  is  not  possible  to  distinguish  the  fibers  that  enter  the  heart  from  the  seg- 
mental  cardiacs  from  those  that  arise  in  the  median  nerve  cord.  All  the  ganglion 
cells  increase  greatly  in  numbers  with  age. 

Cardiac  Ganglia  in  Vertebrates. — The  great  size  of  the  median  cardiac 
nerve  of  arthropods  and  its  conspicuous  origin  from  the  overlying  ectoderm  led 
me  to  look  for  a  similar  origin  of  the  cardiac  ganglia  in  vertebrates. 

In  trout  embryos,  in  frog  embryos  (Rana  septemtrionalis)  of  seven  millimeters, 
and  in  chick  embryos,  from  thirty  to  forty-eight  hours,  there  is  a  thin,  longitudinal 
cord  lying  on  the  haemal  surface  of  the  heart,  which  resembles  the  median  nerve 
in  the  heart  of  Limulus,  and  which  appears  to  be  the  anlage  of  the  cardiac  ganglion. 
In  the  30-36  hour  stages  of  the  chick,  it  extends  along  the  surface  of  the  auricles 
and  ventricles  for  about  150  mm.,  and  is  connected,  here  and  there,  by  swollen 
strands,  with  the  overlying  ectoderm. 

I  was  not  able  to  follow  in  a  satisfactory  manner  the  history  of  this  structure, 
but  it  seems  to  me  probable,  from  its  general  appearance  and  location,  that  under 
suitable  conditions  it  will  be  possible  to  trace  its  development  into  the  cardiac 
ganglia  of  the  adult. 

VI.  EXPERIMENTS  ON  THE  HEART. 

In  my  notes  from  the  summer  of  1897,  I  find  records  of  observations  to  the 
effect  that  the  isolated  heart  continued  to  beat  for  many  hours,  either  in  salt  solu- 
tion, or  in  a  moist  chamber;  that  small  segments  of  the  heart  would  continue  to 
beat,  provided  a  piece  from  the  posterior  part  of  the  median  ganglion  was  at- 
tached; and  that  separate  pieces  that  did  not  contain  this  part  of  the  median  nerve, 


2O6 


THE   HEART. 


or  from  which  it  had  been  removed,  ceased  to  beat;  and  finally  that  the  anterior 
end  of  the  median  nerve  (from  the  first  three  segments)  did  not  control  the  heart 
beat  of  its  territory,  since,  in  its  absence,  the  first  three  segments  would  continue 
to  beat  provided  there  was  a  nerve-fiber  bridge  to  the  posterior  part  of  the  median 
ganglion. 


FIG.  1 1 8. — Diagram  of  the  heart,  pericardium,  and  principal  blood-vessels  of  Limulus. 
veins  in  half  tone.     Seen  from  the  haemal  surface. 


Arterial  trunks  in  black; 


The  following  is  a  record  of  some  experiments  made  three  years  later,  by 
R.  Pearl,  a  student  in  Dartmouth  College  working  under  my  direction: 

A.  July  27,  1900.     The  heart  was  exposed  and  cleaned  of  connective  tissue. 

a.  Stimulation  of  the  median  longitudinal  nerve  with  a  weak  current 
caused  one  long  continuous  systole,  b.  Stimulation  of  the  lateral  cardiac  nerves 
caused  an  increase  in  the  strength  of  the  systole  and  in  the  rapidity  of  the  beat. 
The  rhythmical  beat  continued  during  the  stimulation.  On  removing  the  elec- 
trodes a  complete  diastole  followed.  During  the  stimulation  there  was  no  com- 
plete relaxation  of  the  heart  muscles  between  systoles,  c.  Stimulation  of  the 


EXPERIMENTS.  207 

segmental  and  the  pericardial  nerves  gave  no  results,  d.  The  median  cardiac 
nerve  was  now  cut  (transected)  about  one-third  the  distance  from  the  anterior 
end  of  the  heart  about  in  the  middle  of  the  third  cardiomere.  Rhythmic  pulsation 
of  that  part  of  the  heart  in  front  of  the  cut  stopped  for  half  an  hour.  At  the  end 
of  this  time  the  anterior  part  began  to  beat  again,  e.  Dissected  out,  and  removed, 
a  piece  of  the  median  cardiac  nerve,  about  three-fourths  of  an  inch  long,  from  the 
anterior  end  of  the  posterior  part  left  after  the  first  operation.  All  pulsations  of 
the  section  of  the  heart  without  any  median -nerve  (i.e.y  the  fourth  cardiomere, 
approximately)  immediately  ceased,  while  the  parts  in  front  and  behind  con- 
tinued to  beat  rhythmically.  After  an  hour,  the  fourth  cardiomere  recovered 
and  began  beating  with  a  rhythm  of  its  own,  distinct  from  that  of  the  posterior 
part  of  the  heart.  /.  Stimulation  of  the  lateral  nerve  of  the  fourth  cardiomere, 
about  one  hour  after  operation  e,  caused  one  strong  contraction  of  that  part  of  the 
heart  instead  of  the  increased  beat,  as  before.  Stimulation  of  the  lateral  nerve 
of  the  posterior  cardiomeres  gave  the  same  results  as  before,  i.e.,  acceleration  and 
augmentation  of  the  beat.  g.  Stimulation  of  the  haemal  nerves  of  the  branchial 
neuromeres,  or  of  the  branchial  nerves,  or  of  the  ventral  cord  itself,  produced  no 
effect  on  the  heart  beat.  Stimulation  of  a  lateral  cardiac  nerve,  after  removal  of 
the  median  nerve,  caused  a  contraction  on  the  stimulated  side  only. 

B.  Rhythm  of  Heart  Beat  after  Stimulation. — Counts  were  made  of  the 
number  of  beats  per  minute  of  the  normal  unstimulated  heart,  and  also  during 
stimulation  of  the  various  nerves.  The  results  will  be  presented  in  tabular  form. 

a.  Unstimulated,  32  beats  per  minute. 
Electrodes  on  the  median  nerve,  24  beats  per  minute. 

Electrodes  on  a  lateral  branch  of  the  median  nerve,  12  beats  per  minute. 
Electrodes  on  a  lateral  branch  of  the  median  nerve,  14  beats  per  minute. 
Stimulation  of  the  lateral  cardiac  nerve  caused  a  contraction  of  the  side  of  the 
heart  stimulated,  but  did  not  affect  the  rhythm  of  the  opposite  side. 

b.  Unstimulated,  24  beats  per  minute. 

Electrodes  on  median  nerve  (near  middle  of  heart)  8  beats  per  minute. 
Electrodes  on  median  nerve  (posterior  end) ,  20  beats  per  minute. 
Electrodes  on  lateral  branch  of  median  nerve,  6  beats  per  minute. 

c.  Placing  the  electrodes  underneath  the  median  nerve  near  the  middle  of 
the  heart,  inhibits  the  beat  of  the  whole  heart. 

d.  Placing  the  electrodes  so  far  as  possible  on  the  muscles  only  of  the  heart, 
does  not  perceptibly  affect  the  beat.     Stimulation  of  the  anterior  abdominal 
nerves,  or  the  abdominal  neuromeres  does  not  cause  any  perceptible  change  in  the 
rhythm. 

Carlson  (Am.  Journ.  Physiol.,  1894  and  1905)  has  made  more  elaborate  ex- 
periments on  the  heart  of  Limulus  than  we  have,  but  so  far  as  our  experiments  and 
his  overlap,  they  are  in  agreement  on  the  most  important  points.  Carlson,  how- 
ever, was  not  aware  of  certain  details  in  the  histological  structure  of  the  median 
nerve,  as  indeed  we  were  not  at  that  time,  which  are  essential  to  the  interpretation 


208  THE   HEART. 

of  the  experiments.  Although  he  appears  to  have  been  influenced  only  by  his 
experiments  on  the  living  heart  of  Limulus,  he  has  come  to  conclusions  similar 
to  our  own.  He  says  (Am.  Journ.  Phys.,  1905,  p.  472) : 

"We  find  in  the  heart  of  Limulus  a  condition  similar  to  that  in  the  vertebrate 
heart,  the  venous  end  of  the  heart  exhibiting  the  greatest  automatism,  the  aortic 
end,  the  least,  or  no,  automatism."  There  is  another  similarity  between  them, 
in  that  "the  regions  of  the  heart  exhibiting  the  greatest  automatism  have  the 
greatest  number  of  ganglion  cells."  "And  still  another  similarity  in  the  dis- 
tribution of  the  ganglion  cells  with  reference  to  the  myocard."  "  In  the  vertebrate 
heart,  they  are  situated,  in  the  main,  on  the  surface  of  the  myocard,  and  this  is  also 
their  position  in  the  Limulus  heart." 

VII.  THE   HEART.     SUMMARY  AND    CONCLUSION. 

Interpreting  the  preceding  data  on  the  morphology,  minute  structure,  and 
the  activities  of  the  heart,  we  may  draw  the  following  conclusions : 

1.  The  heart  of  Limulus  may  be  divided  into  two  parts:     a.  An  anterior,  or 
vagus  division,  consisting  of  three  cardiomeres  derived  from  the  mesodermic 
segments  of  the  sixth,  the  seventh,  and  the  eighth  metameres,  i.e.,  from  the  sixth 
leg,  chelarial,  and  opercular  metameres.     The  segmental  cardiacs  of  this  division 
are  united  with  the  pericardial  nerves  but  not  directly  with  the  cardiac  ganglion. 
The  vagus  division   of  the  heart  has  a  greatly  diminished  ganglionic  center  and 
is  devoid  of  the  giant  ganglion  cells.     It  exhibits  little  or  no  automatism,     b. 
The  posterior,  or  branchial  division  of  the  heart  is  derived  from  the  mesoderm 
of   the  five  gill-bearing  metameres.     Each  of   these  cardiomeres  is  connected 
with  its  corresponding  neuromere  by  a  segmental  cardiac  nerve,  from  which 
branches  go  to  the  cardiac  ganglion  and  to  the  pericardial  nerves.     It  contains 
the  greater  part  of  the  small  ganglion  cells  and  all  of  the  largest  bipolar  cells. 
It  exhibits  marked  automatism.     In  the  dead  heart  there  is  a  perceptible  con- 
striction between  the  vagal  and  the  branchial  divisions. 

2.  The  vagal  and  the  branchial  divisions  of  the  heart  in  Limulus  are  compar- 
able with  the  bulbar  and  ventricular  divisions  of  the  heart  in  vertebrates.     That  the 
heart,  as  a  whole,  is  comparable  with  that  of  the  vertebrates  is  shown  by  the  fact  that 
both  organs  arise  on  the  haemal  side  of  the  head  by  the  concrescence  of  mesoblastic 
segments  derived  approximately  from  the  same  metameres.     In  both  cases,  the 
absence  of  the  cardiomeres  in  the  forebrain  region,  and  in  the  greater  part  of  the 
hindbrain  region,  may  be  traced  to  the  absence  there  of  the  segmental  lateral 
plates  of  mesoderm. 

3.  The  giant  bipolar  cells  of  the  branchial  division  of  the  heart  of  Limulus  are 
the  primary  agents  in  producing  the  rhythmic  beat  of  the  entire  heart.     Any 
part  of  the  heart  separated  from  these  cells  ceases  to  beat.     When  the  heart  is 
stimulated  by  laying  the  electrodes  on  the  under  side  of  the  median  nerve,  the 
only  place  where  the  giant  nerve  cells  are  fully  exposed,  the  heart  at  once  ceases 
to  beat. 


THE   HEART.       SUMMARY.  2OQ 

4.  We  may  picture  to  ourselves,  for  the  sake  of  an  initial  working  hypothesis, 
that  the  elements  have  some  such  arrangement  as  shown  in  Fig  117.  It  is 
assumed;  a.  that  the  giant  bipolar  cells  are  the  inhibitory  agents  and  the  centers 
from  which  the  rhythmic  impulses  radiate  to  all  parts  of  the  heart;  b.  they  are 
probably  connected  with  the  adjacent  dorsal  and  lateral  integument  through 
branches  of  the  segmental  nerves,  and  with  the  ventral  cord,  via  the  five  or  more 
segmental  cardiacs.  The  latter  probably  consist,  in  the  main,  of  axones  .from 
the  giant  cells,  and  possibly  of  axones  from  cells  located  in  the  branchial  neuro- 
meres;  c.  the  dendrites  of  the  giant  cells  are  probably  confined  to  the  median  cord; 
they  do  not  appear  to  extend  into  the  plexus  on  the  haemal  surface  of  the  heart, 
or  into  the  lateral  nerves;  d.  the  small  multipolar  cells  are  probably  motor  cells, 
distributing  their  fibers  through  the  main  branches  of  the  plexus  to  the  lateral 
nerves,  and  then  forward  (and  backward  ?)  to  their  terminals  in  the  muscle  cells 
of  the  heart;  e.  afferent  sensory  fibers  probably  run  from  their  paccinian  corpus- 
cle-like terminals  either  directly  to  the  median  cord,  or  to  it  via  the  lateral  nerves. 

On  these  assumptions,  we  may  explain  the  experimental  results  as  follows: 

Placing  the  electrodes  on  the  lateral  nerves,  or  on  the  median  surface  of  the 
median  one,  or  on  the  main  transverse  branches  of  the  plexus,  inhibits  the  heart- 
beat to  a  varying  extent  because  of  the  variable  number  of  accelerator  or  inhibitory 
fibers  that  are  contained  in  a  given  nerve  branch,  or  of  the  number  of  small 
ganglion  cells  overlying  the  larger  ones  in  the  median  cord.  The  automatism  of 
the  branchial  section  of  the  heart  is  due  to  the  presence  of  the  giant  bipolar  cells, 
from  which  the  rhythmic  impulses  pass  in  an  axial  direction  along  the  giant  nerve 
tubes  the  whole  length  of  the  heart.  From  these  tubes  numerous  collaterals  are 
given  off  right  and  left,  apparently  connecting  with  the  motor  cells  which  are  found 
along  the  entire  length  of  the  heart,  but  less  abundantly  at  the  anterior  than  at 
the  posterior  end.  The  giant  cells  are  covered  on  the  haemal  and  lateral  sides  by 
thick  layers  of  motor  neurones,  hence  they  can  be  reached  only  from  the  under 
side  of  the  median  cord.  When  the  cord  is  stimulated  from  that  side  complete 
inhibition  of  the  heart-beat  follows. 

It  is  difficult  to  understand  why  stimulation  of  the  vagus  and  branchial 
neuromeres  and  of  the  segmental  cardiacs  has  so  far  produced  no  noticeable  effects. 
It  is  probable  that  a  more  careful  investigation  of  this  point  will  furnish  important 
results. 


THE  NERVOUS  SYSTEM  AND  SENSE  ORGANS  OF  VERTEBRATES  AND  ARACHNIDS. 

General  Summary  of  Chapters  I-XII. 

i.  The  foundations  of  the  nervous  system  of  vertebrates  are  laid  in  the 
trochosphere  or  ccelenterate  stages,  and  the  most  important  steps  in  its  early 
evolution  were  made  in  the  rotifers,  phyllopods,  marine  arachnids,  and  ostraco- 
derms. 

14 


2IO  GENERAL    SUMMARY. 

2.  In  all  segmented  animals  the  central  nervous  system  is  morphologically 
identical. 

3.  In  all  segmented  animals  the  three  primary  axes  of  bodily  growth  have 
the  same  relation  to  the  neural  axis,  i.e.,  apical  growth  extends  in  a  cephalo-caudal 
direction  parallel  with  the  neural  axis;  transverse  growth  extends  right  and  left 
at  right  angles  with  the  neural  axis;  and  radial  growth  extends  in  an  ovocentric 
direction  approximately  at  right  angles  to  the  other  two. 

4.  The  three  axes  of  morphological  and  functional  differentiation  are  co- 
incident with,  and  follow  the  same  direction  as  the  axes  of  growth. 

5.  The  axes  of  growth  and  differentiation  lead  in  directions  of  diminishing 
returns. 

6.  In  embryonic  growth  the  ratio  between  the  relative  rate  of  apical  and 
bilateral  growth,  and  the  radius  of  the  yolk  sphere  determines  the  relative  time, 
place,  and  conditions  for  the  formation  of  the  organs  on  the  haemal  or  aboral 
surface  of  the  body. 

7.  The  volume  of  the  yolk  sphere  has  been  the  most  important  variable 
factor  in  modifying  the  mode  of  growth  and  the  form  of  the  body  in  the  segmented 
animals.     Its  increasing  volume  in   the  rotiferphyllopod-arachnid  phylum  has 
in  the  higher  arachnids  and  in  the  vertebrates  greatly  exaggerated  the  differ- 
ences between  the  neural  and  haemal  surfaces,  and  has  been  the  chief  cause 
of  the  linear  distortion  and  profound  modification  of  the  haemal  portions  of  the 
cephalic  metameres. 

8.  In   the   phyllopod-arachnid-vertebrate   phylum  the  body  grows  by  the 
spasmodic  generation  of  new  groups  of  metameres  at  the  caudal  end  of  the  body. 
The  new  metameres  of  each  generation  are  at  the  outset  unlike  those  of  the 
preceding  generation.     Each  generation  of  metameres  gives  rise  to  one  of  the 
primary  functional  and  morphological  subdivisions  of  the  body. 

9.  Cephalic  organs  and  cerebral  centers  are  laid  down  at  the  same  time  and 
in  the  same  order,  each  reflecting  the  condition  of  the  other.     The  linear  arrange- 
ment of  the  principal  functional  centers  in  the  arachnid  brain  was  determined 
by  the  historic  order  in  which  the  principal  subdivisions  of  the  body  were  generated 
during    the  phylogeny  of  the  phyllopod-arachnid  stock.     The  cephalo-caudal 
order  in  which  the  principal  functional  centers  are  arranged  in  the  vertebrate 
brain  indicates  the  historic  order  in  which  the  corresponding  peripheral  organs 
were  evolved  in  the  arachnids. 

10.  The  distribution  of  functions  and  organs  is  determined  primarily  by 
the  nature  of  apical  growth  and  by  the  conditions  under  which  it  takes  place.     It 
is  subject  to  a  progressive  readjustment  due  to  a  give-and-take  exchange  between 
the  new  and  the  old  organs  in  response  to  the  physical  demands  incident  to  in- 
creased size,  and  to  other  conditions  created  by  growth.     The  actual  arrangement 
has  been  worked  out  subject  to  the  following  factors:     i.  The  necessary  preced- 
ence in  functional  activity;  2.  priority  in  origin  and  the  consequent  preemption  of 
territory;  3.  the  demand  for  a  location  essential  to  effective  action. 


GENERAL    SUMMARY.  211 

11.  The  transfer  of  functional  centers  always  takes  place  in  a  cephalo-caudal 
direction.     The    process    consists  in  the  progressive  elimination  of  muscular, 
excretory,  nutritive,  and  structural  tissues  from  the  anterior  end  of  the  head,  and 
the  corresponding  increase  of  the  same  structures  at  some  point  farther  back,  the 
degree  of  elimination  varying,  in  the  main,  with  the  linear  and  lateral  location 
of  the  parts  concerned.     On  the  other  hand,  the  primary  sense  organs,  visual, 
gustatory,  and  olfactory,  and  their  cerebral  centers,  never  shift  their  relative 
positions,  and  steadily  inqrease  in  volume  and  structural  detail. 

Hence  there  are  three  factors  that  determine  the  character  of  the  anterior 
end  of  the  body;  i.  the  progressive  elimination  of  the  more  lateral,  non-sensory 
parts;  2.  the  increasing  development  of  the  more  axial  sensory  and  nervous  ones; 
and  3.  the  establishment  of  nervous  continuity  between  the  old  nerve  centers  at 
the  anterior  end  of  the  body  and  the  new  ones  at  the  posterior  end  as  fast  as  the 
latter  are  formed.  Hence  the  anterior  end  of  the  body  throughout  the  arachnid- 
vertebrate  stock  tends  to  become  more  and  more  sensory,  coordinating,  and  ad- 
ministrative in  character,  while  the  posterior  portion  serves  as  the  site  for  the  more 
modern  and  the  more  highly  specialized  functions. 

12.  From  the  very  earliest  stages  in  the  evolution  of  metamerism,  the  fore- 
brain  region  has  been  devoted  to  vision  and  coordination.     The  first  group  of 
metameres  to  appear  behind  or  around  the  mouth  (diencephalic  and  mesen- 
cephalic)  were  devoted  to  locomotion  and  to  tasting,  seizing,  chewing,  and  other 
ingestive  functions.     With  the  increased  size  due  to  the  addition  of  a  new  group 
of  metameres,  respiratory  and  circulatory  organs  became  essential  and  they  made 
their  appearance  behind  the  ingestive  region.     Thus  the  three  main  functional 
divisions  of  the  head,  the  visual  and  coordinating,  the  gustatory  and  ingestive 
(including  the  primordial  endocranium  for  the  attachment  of  chewing  muscles) 
and  the  cardiac  and  respiratory  regions  were  established  according  to  the  historic 
and  inherently  necessary  order  of  their  evolution.     They  were  elaborated  and 
still  further  emphasized  by  the  elimination  of  all  other  tissues  and  organs  foreign 
to  these  functions. 

13.  All  the  segmental  sense  organs  had  primarily  some  of  the  characteristics  of 
visual  organs.     They  were  located  along  the  lateral  margins  of  the  medullary 
plate   in    the  fore-  and  midbrain  regions.     Throughout  the  entire  phyllopod- 
crustacean-arachnid-vertebrate  stock  two  pairs  of  ocellar  placodes  are'  united  to 
form  a  true  parietal  eye.     The  retinal  placodes  of  the  parietal  eye  are  located  at 
the  dilated  distal  end  of  the  vesicle;  the  proximal  end  is  usually  tubular  and  opens 
either  on  the  outer  surface  of  the  head,  or  into  the  forebrain  vesicle. 

Two  other  pairs  of  placodes  form  the  stemmata,  or  frontal  ocelli  of  insects, 
or  the  frontal  organs  of  phyllopods  and  various  Crustacea.  In  Limulus  (probably 
also  in  trilobites  and  merostommata)  they  become  metamorphosed  into  true 
olfactory  organs  and  represent  the  preliminary  stage  of  the  olfactory  organs  of 
vertebrates. 

The  lateral  or  compound  eyes  of  arthropods  belong  to  the  most  posterior 


212  GENERAL    SUMMARY. 

procephalic,  or  first  post-oral  segment.  They  become  involved  during  the  em- 
bryonic stages  in  the  palial  fold  that  grows  over  the  forebrain,  giving  rise,  in  the 
vertebrates,  to  the  lateral  eye  retinas. 

The  auditory  organs  arose  from  a  large  midbrain  placode  near  the  base  of 
the  fourth  pair  of  thoracic  appendages.  (Limulus.) 

14.  The  parietal  eye  is  the  most  ancient  of  all  cerebral  sense  organs  and 
attains  its  characteristic  structure  and  location  at  a  very  early  period  in  the  history 
of  the  arachnid  phylum.     It  is  usually  functional  before  the  lateral  eyes  have 
made  their  appearance. 

.The  site  of  the  locomotor  organs  follows  the  center  of  gravity  backward, 
those  in  the  oral  region  giving  place  to  those  of  the  mesocepTialon ;  the  latter  to 
those  in  the  postcephalic  and  caudal  regions.  The-  excretory,  digestive,  and 
reproductive  organs  follow  in  the  same  direction.  (Fig.  308.)  Their  nerve 
centers  are  of  small  size  and  their  change  of  location  does  not  visibly  modify  the 
character  of  the  nerve  cords. 

The  lateral  eyes  are  the  next  oldest.  They  are  highly  developed  and  impor- 
tant functionally  in  the  adult  stages  of  nearly  all  the  higher  arthropods.  During 
the  ostracoderm  stage  they  were  involved  in  the  cerebral  vesicle,  and  for  a  time 
became  practically  useless,  the  parietal  eye  being  the  only  one  that  was  in  a  posi- 
tion to  serve  as  a  visual  organ.  In  the  true  vertebrates  the  lateral  eyes  regained 
their  function,  and  that  of  the  parietal  eye  gradually  disappeared. 

The  olfactory  organ,  while  derived  from  organs  probably  as  old  as  the  parietal 
eye,  did  not  take  on  the  morphological  characters  or  the  function  of  an  olfactory 
organ  till  the  highest  stages  in  the  evolution  of  the  marine  arachnids  were 
reached. 

The  auditory  organ  is  the  most  recently  acquired.  It  is  dormant  in  the 
arachnids,  and  apparently  begins  its  period  of  growth  and  functional  activity 
in  the  lower  vertebrates.  It  is  the  only  one  of  the  primary  sense  organs  that  shows 
a  notable  increase  in  morphological  complexity  and  in  the  range  of  its  functional 
activities  during  the  evolution  of  the  vertebrates. 

15.  The  special  cutaneous  organs  of  arthropods  include  two  distinct  kinds, 
the  taste  organs  and  the  slime  buds.     Both  sets  of  organs  perform  various  func- 
tions and  initiate  various  reactions,  but  all  of  them  may  be  properly  called  taste 
organs,  since  they  react  to  a  variety  of  chemical  substances,  either  in  the  food 
or  in  the  surrounding  media.     Both  kinds  may  be  widely  distributed  in  various 
parts  of  the  body,  but  those  belonging  to  this  category  are  sharply  localized,  and 
are  included,  either  separately  or  combined,  in  segmentally  arranged  fields  that 
are  provided  with  special  nerves  and  special  tracts  and  centers  in  the  brain. 

In  Limulus,  taste  organs  are  found  in  the  mandibles  of  the  thoracic  append- 
ages, except  the  first  and  last  pair,  in  the  chilaria,  and  the  largest  field  of  all  in  the 
flabellum.  In  the  scorpion  they  are  found  in  the  maxillaria,  the  genital  papillae, 
and  the  pectines.  In  the  mandibles  of  Limulus,  the  taste  organs  and  slime  buds 
are  located  in  separate  fields,  and  both  sets  on  stimulation  produce  chewing  reac- 


GENERAL    SUMMARY.  213 

tions.  In  the  scorpion  the  genital  papillae  and  the  pectines  illustrate  the  con- 
version of  entire  appendages  into  complex  sense  organs.  The  taste  organs  and 
slime  buds  increase  by  division;  the  former  may  thus  give  rise  to  numerous 
sensory  cells  arranged  in  long,  straight  lines. 

The  segmental  taste  organs  and  slime  buds  of  arachnids  are  the  forerunners 
of  the  special  cutaneous  organs  of  vertebrates;  the  taste  organs  of  the  arachnids 
corresponding  with  the  taste  buds  of  the  vertebrates,  and  the  slime  buds  probably 
in  part  with  the  neuromasts  or  lateral  line  organs.  The  fields  of  taste  organs 
and  slime  buds  of  the  arachnids  are  represented  in  vertebrates  by  the  placodes 
which  initiate  a  line  of  taste  organs,  or  of  neuromasts.  The  structure  of  the  organs, 
the  number  and  location  of  the  principal  lines  of  organs  in  the  embryo,  their 
relation  to  peripheral  nerves,  and  to  the  tracts  in  the  brain,  are  in  the  main  very 
similar  in  both  vertebrates  and  arachnids.  (Figs.  58  and  65,  g.n.r.) 

In  Limulus  the  enormous  integumentary  nerve  of  the  cheliceral  segment,  Fig. 
70,  deserves  special  attention.  It  resembles  the  ramus  lateralis  accessorius, 
which  in  ganoids  and  bony  fishes  is  distributed  to  the  back,  tail,  and  fins,  wher- 
ever taste  buds  are  found  (Johnston).  Unfortunately  the  character  of  the 
terminals  to  this  remarkable  nerve  in  Limulus  was  not  determined;  but  it  is  un- 
questionably a  purely  sensory  nerve,  supplying  the  neural  surface  of  the  entire 
posterior  part  of  the  body.  Its  ramification  is  specially  rich  around  the 
entrance  to  the  gill  chamber. 

1 6.  The  general  cutaneous  sense  organs  in  Limulus  are  scattered  over  all 
parts  of  the  body.  They  represent  various  minor  modifications  of  the  slime  buds, 
taste  buds,  and  of  free  nerve  endings,  and  serve  either  as  tactile,  temperature,  or 
chemotactic  organs.  They  are  connected  with  a  loose  subdermal  nerve  plexus 
which,  in  the  thoracic  and  abdominal  shields,  is  derived  from  the  ramifications 
of  the  haemal  nerves;  that  on  the  surface  of  the  gills,  operculum,  and  terminal 
joints  of  the  leg  is  derived  from  the  ramifications  of  the  neural  nerves. 

The  central  terminals  of  the  general  cutaneous  components  of  the  haemal 
nerves  end  in  a  large  tract  on  the  median  side  of  each  crus,  haemal  to  the  gustatory 
tract.  (Fig.  56,  G.c.tr.) 


17.  The  peripheral  nervous  system  of  arachnids  attains  a  condition  similar 
to  that  in  vertebrates.     There  are  two  main  systems  of  mixed  nerves,  the  neural 
and  haemal.     The  neural  nerves  have  enormous  ganglia  which  develop  independ- 
ently of  the  neural  axis;  the  haemal  nerves  are  without  ganglia.     Both  sets  remain 
separate  in  the  anterior  head  region,  but  in  the  posterior  head  region,  and  in  the 
trunk,  they  may  unite  to  form  nerves  of  the  spinal  cord  type,  that  is,  single  nerves 
with  separate  roots,  ganglionated  neural  ones  and  non-ganglionated  haemal  ones. 

1 8.  Further  specialization  takes  place  through  the  separation  from  the  primi- 
tive segmental  nerves  of  those  components  that  have  similar  peripheral  and  central 
terminals,  and  their  union  to  form  a  new  system  of  nerves  with  a  common  central 


214  GENERAL    SUMMARY. 

tract  and  a  common  centre,  e.g. ,  the  segmental  gustatory  nerves.  Or  the  nerves  from 
a  special  group  of  neuromeres  may  break  up  into  several  sets  of  components  which 
then  reunite  to  form  new  groups.  For  example,  in  the  vagal  and  branchial  complex 
of  arachnids,  which  represents  seven  neuromeres,  we  may  recognize  the  following 
groups  of  more  or  less  independent  components:  a.  the  combined,  almost  exclu- 
sively sensory  nerves  of  the  first  two  or  three  vagal  appendages  (scorpion) ;  b.  and  c. 
the  cardiac  and  intestinal  components;  d.  the  mixed  nerves  supplying  the  append- 
ages,  gills,  lung  books,  or  operculum;  and  e.  the  combined  motor  components 
that  constitute  the  hypobranchial  nerve  which  supplies  the  compound  hypo- 
branchial  muscle  derived  from  all  the  branchial  and  vagal  segments. 

19.  The  extent  to  which  segmental  nerves  have  been  reduced  to  a  single  set 
of  highly  specialized  components,  or  broken  up  into  several  independent  sets  of 
components,  decreases  in  a  cephalo-caudal  direction.     For  example,  each  of  the 
three  forebrain  neuromeres  in  arachnids  contains  only  the  purely  sensory  nerves 
of  the  olfactory  organ  and  of  the  parietal  eye  and  lateral  eyes.     In  the  six  follow- 
ing thoracic  segments  the  nerves  are  largely  sensory,  the  motor  components  being 
reduced  to  the  relatively  small  nerves  supplying  the  muscles  of  the  legs  and  those 
passing  from  one  side  of  the  carapace  to  the  other,  or  to  those  holding  the  endo- 
cranium  in  place.     All  the  procephalic  and  mesocephalic  haemal  muscles  have 
disappeared. 

20.  A  still  further  reduction  of  motor  components  took  place  in  the  verte- 
brate brain  with  the  fusion  of  the  anterior  thoracic  appendages  to  form  the  im- 
movable anterior  arch  of  the  mouth  (pre-maxillae  and  maxillae) ;  with  the  reduction 
of  the  free  thoracic  appendages  to  external  gills;  with  the  fixation  of  the  endo- 
cranium  by  its  fusion  with  the  exoskeleton;  and  finally  with  the  atrophy  of  nearly 
all  the  branchial  musculature  in  the  air-breathing  vertebrates. 

21.  The  most  important  events  in  the  conversion  of  the  arachnid  type  of 
brain  into  the  vertebrate  type  were  the  transfer  of  the  lateral  eye  placodes  to  the 
interior  of  the  cerebral  vesicle;  the  closing  of  the  neurostoma  by  the  closure  of  the 
medullary  plate;  the  transfer  of  the  optic  ganglia  to  the  roof  of  the  midbrain;  and 
the  union  of  the  branchial  neuromeres  with  those  of  the  vagus  region. 


CHAPTER  XIII. 
EARLY  STAGES  OF  ARTHROPOD  AND  VERTEBRATE  EMBRYOS. 

I.  PRIMARY  CAUSES  OF  DIFFERENTIAL  GROWTH. 

Before  we  attempt  to  explain  the  meaning  of  the  various  embryonic  stages  in 
arthropods  and  vertebrates,  it  is  desirable  to  reach  some  conclusion,  if  possible, 
as  to  the  general  nature  of  the  causes,  or  conditions  that  are  likely  to  control  the 
initial  stages  of  growth.  Even  if  it  is  quite  impossible  to  detect  the  real  causes, 
it  is  well  to  make  perfectly  clear,  merely  as  an  aid  to  exposition,  the  mental  attitude 
in  which  the  writer  approaches  his  problem. 

It  is  apparently  assumed  by  some  authors  that  differential  growth  takes  place 
in  developing  eggs  because  in  some  predetermined  manner  certain  agents  dis- 
tribute to  their  proper  places  preformed  materials,  which  then  develop  into 
definite  organs  because  they  are  made  of  the  same  material  from  which  those 
organs  arose. 

Such  artificial  explanations  are  now  found  only  in  the  biological  sciences  and 
are  to  be  regarded  as  the  decadent  offspring  of  the  doctrine  of  special  creation. 
They  are  pernicious,  because  the  clever  juggling  of  words  and  images  fixes  the 
attention  solely  on  the  imaginary  structure  of  imaginary  things,  thus  leading  one 
to  overlook  the  sequence  of  form  and  of  physical  and  chemical  conditions  that 
constitute  the  only  measurable  or  accessible  properties  of  living  things. 

An  erratic  boulder  does  not  grow  into  a  mountain  like  the  one  from  which 
it  came,  even  if  it  is  made  of  precisely  the  same  kind  of  materials.  And  even  if 
some  metaphysical  geologist  should  succeed  in  picturing  a  planatasmal  geo- 
phore  that  was  a  mountain  in  miniature,  or  that  stood  for  one,  or  represented  one, 
or  was  capable  of  becoming  one,  it  makes  little  difference  what  expression  one 
uses,  the  real  problem,  and  the  only  one  of  interest  to  a  matter  of  fact  geologist, 
would  be:  What  was  the  sequence  of  events  in  the  evolution  of  that  mountain? 
What  were  the  conditions  immediately  preceding  each  step  in  the  process  ? 

The  biologist  should  approach  the  problems  of  growth  and  morphology  in 
the  same  spirit.  Let  him  study  the  changes  of  form  and  action  as  they  occur, 
with  the  hope  of  discovering  a  sufficient  cause  for  each  one.  He  should  not  use 
omnipotence,  either  of  a  creator,  or  of  heredity,  or  of  chromosomes,  to  short 
circuit  the  process  of  evolution. 

I  shall  therefore  throw  aside,  for  the  time  being,  all  preconceived  ideas  as  to 
the  ultimate  composition  of  the  ovum,  or  of  the  growing  embryo,  and  shall  consider, 
as  they  appear  at  successive  periods,  some  of  the  simpler  physical  and  chemical 
conditions  likely  to  be  determining  factors  in  differential  growth.  It  is  assumed 

215 


2l6  EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 

that  under  certain  fixed  uniform  conditions  protoplasm,  or  some  of  its  constitu- 
ents, has  the  property  of  growing,  that  is  of  producing  more  material  like  itself; 
the  growth  taking  place  in  the  three  planes  of  space  at  a  uniform  rate  and  to  an 
unlimited  extent.  If  any  deviations  from  these  results  occur,  it  must  be  due  to 
new  conditions  that  arise  either  outside  the  growing  mass,  or  which  are  locally 
created  within  the  mass  by  growth  itself. 

It  is  evident  that  a  single  cell,  or  a  group  of  like  cells,  endowed  with  this 
initial  power  of  self  increase,  in  the  very  act  of  growing  necessarily  creates  internal, 
locally  diverse  physical  and  chemical  conditions;  and  that  these  unlike  conditions 
will  be  arranged  in  regular  graded  zones.  As  it  appears  that  these  zones  of  unlike 
conditions,  in  the  main,  coincide  with  the  zones  of  histological  and  morphological 
differentiation,  they  may  be  fairly  assumed  to  be  the  principal  causes  of  that 
differentiation.  That  is,  it  appears  that  progressive  differential  growth  is  self- 
creating,  and  that  homogeneous,  or  undifferentiating  growth  is  an  impossibility. 

The  details  in  the  end  results  may  be  colored  or  modified  by  the  presence 
at  the  outset  of  foreign  materials,  or  by  changes  in  the  external  environment, 
but  they  appear  to  play  such  a  subordinate  part,  compared  to  the  internal  condi- 
tions created  by  growth  itself,  that  for  the  present  they  may  be  neglected. 
The  medium  in  which  growth  takes  place  can  never  be  homogeneous  as  regards 
the  intensity,  or  the  location  of  the  sources  of  light,  heat,  gravity,  and  chemical 
agents,  so  that  each  part  of  a  growing  mass  will  have  its  own  particular  relations 
to  its  surrounding  medium.  The  rate  of  growth,  and  its  character  will  be  variously 
modified  by  these  local  conditions,  so  that  neither  a  homogeneous  body,  nor  a 
spherical  body  could  be  produced,  and,  so  far  as  we  know,  never  is  produced.  The 
inevitable  result  is  some  modification  of  a  sphere  consisting  of  concentric  shells 
or  strata,  each  stratum  having  various  local  modifications  of  its  surface. 

If  our  initial  mass  of  protoplasm  begins  its  growth  laden  with  dead  or  inert 
materials,  or  on  the  surface  of  a  yolk  sphere,  diversified  local  conditions  are  created 
at  each  stage  of  growth  that  have  a  very  definite  directive  influence  on  each  sub- 
sequent stage  of  growth. 

It  has  long  been  recognized  that  the  presence  of  yolk  modifies -the  rate  of 
cell  division  and  the  character  of  the  cells  produced.  But  it  has  not,  I  believe, 
been  heretofore  recognized  that:  i.  Radial  symmetry  is  an  inevitable  result  of  the 
physical  conditions  created  by  growth;  2.  that  the  morphological  structure  of  a 
large  class  of  animals  is  profoundly  modified  by  the  prevailing  volume  of  the  yolk 
sphere  over  which  the  initial  growth  takes  place;  3.  that  the  gradual  increase  in 
volume  of  the  nutrient  surface,  accompanied  by  apical  growth,  necessarily  results 
in  an  unlike  bipolar  concrescence,  bilateral  symmetry,  and  a  linear  sequence  of 
unlike  physical  conditions,  that  in  turn  produce  a  linear  series  of  unlike  organs; 
4.  that  the  overlying  strata  formed  in  a  spherical  mass  of  cells,  or  in  a  disc  growing 
on  a  nutrient  surface,  such  as  ectoderm,  somatic  and  splanchnic  mesoderm  and 
endoderm,  are  the  expression  of  the  various  physical  conditions  successively 
created  by  growth.  In  other  words,  the  whole  triaxial  framework  of  any  animal 


PRIMARY   CAUSES   OF   DIFFERENTIAL  GROWTH.  2 17 

may  be  regarded  as  the  inevitable  expression  of  conditions  successively  created 
by  growth,  or  by  the  conditions  under  which  growth  takes  place.  External 
environment  and  natural  selection  have  nothing  to  do  with  these  conditions, 
neither  does  "heredity"  initiate,  or  control,  or  create  them;  it  merely  imitates  or  re- 
peats them.  Hence,  as  a  creator  of  the  foundations  of  organic  structure,  we  may 
eliminate  natural  selection,  and  external  environment,  together  with  heredity  and 
all  its  ministering  tribe  of  "  corpuscles,"  as  we  have  eliminated  the  Gods  of  Love, 
of  War,  and  of  Peace. 

Let  us  illustrate  our  meaning  more  specifically.  If  cell  division  takes  place 
in  two  superficial  planes  at  right  angles  to  each  other,  a  regularly  expanding 
polygon  is  formed.  With  its  increasing  area,  the  two  division  planes  will  tend  to 
fall  into  radial  and  tangential  planes,  and  the  polygon  will  become  a  flat,  circular 
disc,  provided  the  rate  of  division  in  these  planes  coincides  with  the  ratio  between 
the  radii  and  the  circumference  of  expanding  circles.  But  as  soon  as  such  a  disc 
is  formed,  the  central  cells  are  placed  under  different  conditions  from  the  marginal 
ones,  as  to  nutrition,  respiration,  and  tension;  and  in  addition,  the  latter  are 
advancing  into  new  territory  by  their  own  growth,  and  at  the  same  time  they  are 
crowded  over  it  at  a  constantly  increasing  rate,  by  the  division  of  those  cells  that 
lie  nearer  the  center  than  they  do. 

Thus  a  difference  between  the  local  rate  of  cell  division,  the  local  rate  of 
cell  displacement,  and  the  local  rate  of  histological  differentiation  is  established. 
As  the  local  augmentation  or  diminution  of  growth  produced  by  these  conditions 
has  no  constant  relation  to  the  increasing  radii  and  circumference,  it  follows  that 
the  actual  form  of  the  disc  will  be  a  resultant  of  the  various  sets  of  forces.  The 
surface  layers  will  not  fit  the  deeper  ones,  or  the  central  portions  fit  the  periphery. 
The  disc  must,  therefore,  either  change  its  marginal  contour,  or  its  surface  con- 
tours, or  both.  That  is,  the  areas  of  unequal  growth  stresses  must  express  them- 
selves either  in  a  disc  with  a  broken  contour  (a  symmetrical  polygon,  hexagon?), 
or  in  a  circular  disc  with  symmetrically  placed  infoldings,  or  eruptions  of  its  surface, 
or  both.  (Fig.  119,  A.B.) 

If  we  now  consider  the  vertical  increment  of  our  hypothetical  group  of  cells, 
it  is  clear  that  the  conditions  change  more  rapidly  in  a  vertical  direction,  with 
increasing  thickness,  than  in  a  superficial  horizontal  one,  with  increasing  width. 
There  is  consequently  a  greater  modification  of  the  rate  of  growth  and  of  special- 
ization in  a  vertical  direction  than  in  a  horizontal  one.  The  inevitable  result  is, 
therefore,  not  a  homogeneous  sphere,  but  a  lens-shaped  disc  composed  of  unlike 
concentric  strata,  i.e.,  ectoderm,  mesoderm  and  endoderm,  each  stratum  com- 
posed of  zones  of  unlike  organs,  concentric  with  the  point  of  initial  growth. 

Thus  a  plan  of  an  adult  radiate,  for  example,  seen  from  its  oral  surface  in 
mercator  projection,  presents  a  succession  of  ring-like  zones,  intersected  by  radii, 
marking  the  distribution  of  like  parts.  The  central  area  represents  the  point  of 
origin  of  the  endoderm,  or  the  blastopore,  or  the  opening  to  the  primitive  gut. 
Around  this  opening  the  various  organs  are  arranged  in  concentric  circles  that  are 


2l8 


EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 


formed  at  successive  periods,  the  oldest  and  most  highly  specialized  around  the 
central  infolding,  the  newest  and  least  specialized  on  the  periphery. 

The  bilateral  type  appears  to  have  arisen  from  the  radiate  by  a  local 
outgrowth  from  it,  not  by  a  transformation  of  its  entire  body.  The  outgrowth 
gave  rise  to  the  new  body,  and  the  old  body  became  the  head  of  the  new  animal. 

A  premonitory  stage  of  this  transformation  may  be  seen  in  the  one  tentacled 
coelenterate  larvae,  as  I  pointed  out  in  1889,  and  the  same  type  may  be  again  seen 
in  the  trochosphere,  and  in  the  early  embryonic  stages  of  all  segmented  animals 
We  may  represent  such  forms,  laid  down  in  mercator  projection,  by  a  racquet- 
shaped  plate,  the  large  anterior  end  representing  the  body  of  the  ccelenterate 
that  is  to  become  the  head,  the  handle  representing  the  outgrowth  from  it  that  is 
to  form  the  new  body.  (Fig.  120.) 


FIGS.  119-120—121. — -Diagrams,  in  mercator  projection,  illustrating  the  three  principal  types  of  growth, 
and  the  coincident  morphological  and  physiological  differentiation.  FIG.  119. — Radial  type,  showing  the  lines 
of  unequal  physical  and  chemical  stress  created  by  radial  growth,  and  their  coincidence  with  the  lines  of 
morphological  differentiation.  FIG.  120.- — Apical  bilateral  type,  showing  the  origin  of  bilateral  symmetry, 
apical  growth,  and  metamerism,  as  a  result  of  unequal  radial  growth.  FIG.  121. — -Apical  asymmetrical  type, 
or  false  radial  type,  derived  from  the  apical  bilateral  type,  by  the  suppression  of  growth  on  one  side. 

Whatever  may  be  the  cause  of  the  unequal  radial  growth,  once  established, 
it  automatically  creates  bilateral  symmetry  and  metamerism.  To  illustrate:  If 
in  a  growing  circular  disc,  there  is,  for  any  reason,  a  local  increase  of  radial  growth, 
the  resulting  form  will  be  oval,  or  triangular,  or  banjo-shaped.  (Fig.  120.) 
The  isogeminal  lines  will  then  form  two  similar,  converging  series  on  either  side 
of  the  enlarged  radius.  At  any  time  in  the  history  of  this  organism,  there  will 
be  a  graded  linear  series  of  cells,  from  the  oldest  at  #,  to  the  youngest  at  an,  and  a 
double  graded  series  from  any  point  in  a-an  to  the  right,  or  left. 

There  can  be  but  two  points  in  the  entire  mass  at  any  time  that  are  alike 
as  to  age,  an  environment,  each  one  lying  in  a  corresponding  position  to  the  other 
on  opposite  sides  of  the  principal  axis  of  growth.  As  a  result  of  apical  growth, 


INTERPRETATION    OF    THE    EARLY    STAGES.  2IQ 

a  graded,  tri-axial  series  of  unlike  environments,  and  three  similar  graded  series  of 
cells,  unlike  as  to  age  is  established.  This  dual  series  of  unlike  conditions,  and 
cells  of  unlike  lineage,  coincides,  as  nearly  as  may  be  determined,  with  a  third  and 
fourth  series,  namely  the  lines  of  morphological  and  physiological  differentiation; 
hence,  the  conclusion  is  justified  that  the  two  latter  are  the  formal,  or  kinetic  ex- 
pression of  the  two  former.  That  is,  metamerism,  or  the  succession  of  unlike 
parts  in  a  cephalo-caudal  direction,  bilateral  symmetry,  or  the  succession  of 
unlike  parts  in  a  bilateral  direction,  and  the  formation  of  superimposed  germ  layers, 
are  the  inevitable  results  of  the  locally  diverse  physical  and  chemical  conditions, 
and  the  locally  diverse  cell  lineages  created  by  apical  growth.  They  cannot 
therefore  be  the  result  of  the  unfolding,  or  distribution,  of  diverse  specific  forma- 
tive materials. 

It  would  therefore  appear  that  there  is  a  definite  order  in  which  various  tissues 
are  automatically  created  by  their  individual  environments,  the  degree  of  histo- 
logical  specialization  having  a  constant  time  and  space  relation  to  the  germinal 
axis.  That  is,  if  we  assume  that  the  nervous  tissue  is  the  most  highly  specialized, 
then  it  is  clear  that  at  any  period  of  development,  the  most  highly  specialized  tis- 
sues predominate  in  the  germinal  axis,  and  that:  a.  the  grade  of  specialization 
diminishes  from  any  point  in  that  axis  right  and  left  to  the  germinal  margin,  or 
to  the  periphery  of  each  half  metamere;  and  b.  the  grade  of  development  of  each 
member  of  the  half  metamere  reaches  its  maximum  at  some  point  behind  the 
cephalic  end  of  its  series,  and  gradually  diminishes  toward  the  germinal  apex 
at  the  caudal  end  of  the  body.  (Fig.  157.) 

It  will  also  be  observed  that  in  passing  from  the  outside  of  the  sphere  inward, 
the  degree  of  morphological  and  physiological  complexity  in  the  four  superimposed 
layers  varies  inversely  as  the  distance  from  the  germinal  axis. 

Conclusion. — Continuous  aggregation  of  like  materials  is  an  impossibility, 
because  all  growth,  or  aggregation  of  materials,  automatically  creates  for  each 
of  the  constituent  parts  unlike  time  and  space  conditions,  which  in  turn  control 
further  growth  and  differentiation.  The  unlike  conditions  thus  created,  tend  to 
form  parallel,  or  concentric,  isogeminal  and  isomorphic  shells  and  zones,  the  re- 
sulting form,  mode  of  growth,  and  action  of  the  constituent  parts  being  the  visible 
expression  of  the  local  conditions  created  by  growth. 

Natural  selection,  external  environment,  and  heredity  play  no  part  in  the 
creation  of  the  physical  and  chemical  framework  of  living  things. 

II.  MORPHOLOGICAL  INTERPRETATION  OF  THE  EARLY  STAGES  OF  EMBRYONIC 

GROWTH. 

All  metazoa  may  be  reduced  to  one  of  two  types  of  structure,  the  radiate 
and  the  bilaterally  symmetrical.  These  two  types  have  not  arisen  independently 
of  each  other;  they  are  genetically  related,  the  latter  being  derived  from  the  former. 
The  bilateral  type  may  become  secondarily  asymmetrical  (certain  molluscs  and 
arthropods),  or,  by  the  complete  suppression  of  one  side,  it  may  develop  into  a 


220  EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 

new  type  of  radial  structure  (echinoderms) ,  without  thereby  destroying  or  com- 
pletely disguising  the  basic  elements  of  bilateral  structure  and  of  metamerism. 

In  all  these  varying  forms,  the  part  of  the  embryo  that  represents  the  coelen- 
terate  ancestor  is  the  head,  and  the  primitive  infolding,  or  ingrowth  in  the  center 
of  the  head  region,  represents  the  remnants  of  the  coelenterate  enter  on.  This  is  the 
only  part  of  the  embryo  that  may  be  properly  called  a  gastrula.  The  opening  of 
the  gastrula  becomes  the  opening  to  a  subsequent  ingrowth,  the  stomodaeum;  or 
the  stomodaeum  is  formed,  as  nearly  as  may  be  determined,  at  the  point  where 
the  gastrula  infolding,  or  the  so-called  blastopore,  closed. 

That  is,  in  bilaterally  symmetrical,  or  in  segmented  animals,  or  in  their  deriva- 
tives, the  true  gastrula  is  formed  at  the  head  end  only,  and  the  permanent  mouth  is 
formed  from  it,  or  in  its  place.  When  a  germinal  ingrowth  forms  in  the  post- 
cephalic  region,  it  is  not  to  be  regarded  as  a  blastopore,  but  as  a  telopore,  or  as  one 
of  the  various  stages  of  the  axial  infoldings  that  arise  as  a  secondary  result  of 
apical  growth. 

A  coelenterate  type  of  animal  represented  in  mercator  projection,  or  as  laid 
down  in  enbryo  on  a  large  yolk  sphere,  would  take  the  form  of  a  thin  circular  disc, 
with  the  nerves,  sense  organs,  and  appendages  arranged  in  concentric  and  radiat- 
ing lines  around  a  central,  enteric  infolding,  or  ingrowth.  (Fig.  119,  A.B.) 

In  the  arachnid  embryo,  the  coelenterate  stage  is  represented  by  the  circular 
germinal  disc,  with  its  central  infolding.  (Figs.  123-124.)  This  disc  becomes 
a  part  of  the  head,  or  procephalic  lobes  of  the  future  embryo,  while  the  trunk  is  an 
outgrowth  from  its  posterior  margin. 

In  the  later  stages,  we  may  recognize  the  remnants  of  the  primitive,  radiate 
nervous  system,  in  the  system  of  radiating  and  concentric  nerves  formed  from  the 
walls  of  the  stomodaeum.  If  we  evert  the  stomodaeum,  and  project  it  with  its 
nerves  on  the  central  area  of  the  germ  disc,  where  it  originally  belonged,  the 
radiate  arrangement  of  the  stomodaeal  nerves  is  apparent.  (Fig.  35.) 

In  vertebrates,  a  part  of  the  stomodaeal  ring  is  still  recognizable  in  the  cere- 
bellar  commissure  and  the  side  walls  of  the  diencephalon,  while  the  location  of  the 
blastopore,  and  of  the  invertebrate  stomodaeum  is  indicated  by  the  infundibular 
perforation  of  the  brain  floor,  and  by  the  pit-like  infolding  in  the  center  of  the 
procephalic  lobes  during  the  open  medullary  plate  stage.  (Figs.  25,  st,  46,  st.co.) 
During  the  gastrula  stage  of  the  metacoelente rates,  therefore,  the  primitive  mouth, 
or  blastopore,  is  surrounded  by  a  closed  nerve  ring  like  that  of  their  coelenterate 
prototype.  On  this  interpretation,  the  union  of  the  margins  of  the  germinal  disc 
represents  the  closing  of  a  yolk  navel  on  the  haemal  surface,  not  the  concrescence 
of  the  blastopore  on  the  neural  surface.  Such  a  concrescence  of  the  blastopore  is 
only  conceivable  on  the  untenable  assumption  that  the  yolk  sphere  completely 
fills  the  gastrula  mouth,  and  that  the  neural  surface  of  segmented  animals  repre- 
sents the  aboral  or  haemal  surface  of  a  coelenterate. 

Wherever  yolk  is  present  in  small  amounts  in  the  egg,  it  may  be  held  within 
the  endodeim  cells.  But  if  it  is  very  voluminous  it  forms  an  inert,  extra-cellular 


INTERPRETATION    OF    THE    EARLY    STAGES.  221 

mass,  over  which  the  cells  spread  in  all  directions  from  an  initial  center  that  always 
represents  the  beginning  of  the  oral  or  neural  surface.  Around  this  center,  the 
various  organs  are  arranged  in  a  definite  order,  from  the  center  outward. 

Embryonic  growth  on  a  yolk  sphere,  therefore,  always  begins  near,  or 
centers  in,  the  primitive  oral  region  and  spreads  from  it  toward  the  aboral  surface. 

If  there  is  but  little  yolk  present,  the  aboral  surface  may  be  formed  during 
cleavage,  and  at  practically  the  same  time  as  the  oral  surface.  But  in  proportion 
as  the  yolk  increases  in  volume  the  formation  of  the  aboral  surface  is  delayed, 
because  it  can  only  be  completed  by  the  growth  of  the  margins  of  the  germinal 
disc  around  the  yolk. 

Thus  growth  on  the  oral  and  growth  on  the  aboral  surface  of  the  embryo  are 
totally  distinct  processes,  and  always  take  place  under  different  conditions  and 
in  opposite  directions.  On  one  side  it  is  centrifugal,  on  the  other  centripetal. 
The  uncovered  yolk  mass,  or  yolk  navel,  always  lies  on  the  opposite  side  of  the 
egg  from  the  blastopore  and  the  germinal  axis. 

The  subject  of  apical  and  bilateral  growth,  or  of  radial  and  bilateral  symmetry 
has  more  than  a  purely  academic  interest  for  us,  because  the  interpretation  of  bilat- 
eral animals  in  terms  of  radiate  ones,  is  the  key  to  the  problems  of  germ  layers, 
gastrulation,  and  concrescence,  throughout  the  entire  series  of  segmented  animals. 
The  conditions  creating  bilateral  symmetry  and  metamerism  are  so  fundamental, 
that  it  is  hardly  conceivable  they  could  be  otherwise  than  they  are.  There  is  no 
reason  whatever  to  doubt  that  the  fundamental  relations  of  the  nervous  axis, 
blastopore,  primitive  mouth,  and  yolk  sphere,  and  the  main  axes  of  differential 
growth  are  the  same  in  coelenterates  and  in  all  bilateral  acrogenous  animals. 

The  axes  of  growth  and  of  differentiation  are  the  most  important  means  of 
orientation,  and  should  be  carefully  considered  in  all  attempts  to  compare  one 
great  group  of  animals  with  another. 

Gaskell,  Herrick,  and  others  fail  to  recognize  these  fundamental  relations. 
Gaskell  maintains  that  the  " ventral"  or  neural  side  of  an  arthropod  is  the 
same  as  the  haemal  or  "  ventral"  side  of  a  vertebrate,  and  that  the  vertebrate  nerve 
cords  represent  those  of  an  arthropod,  transferred  from  the  "ventral"  side  of  the 
one  to  the  " dorsal"  side  of  the  other.  There  are  only  two  possible  ways  in  which 
such  a  transfer  could  take  place.  One  way  would  be  for  them  to  migrate  over 
the  surface  of  the  yolk,  right  and  left,  uniting  on  the  opposite  side.  In  this  case, 
among  other  equally  obvious  difficulties,  the  original  lateral  margins  of  the  cords 
would  be  united  in  the  median  line,  all  the  ectodermic  territory,  originally  covered 
by,  and  giving  rise  to  the  peripheral  nerves  and  sense  organs,  would  be  extinguished; 
and  the  outgrowing  peripheral  nerves  would  be  directed  into  the  canalis  centralis, 
with  no  visible  means  of  escape.  The  second  possible  way  would  be  for  the  cords 
to  migrate  bodily  through  the  yolk,  in  which  case  they  would  reach  the  opposite 
side  inside  out,  or  upside  down;  that  is,  with  the  proliferating  nuclear  surface  on 
the  deeper  face  of  the  cords,  instead  of  the  outermost  surface  as  it  actually  is.  In 
order  to  meet  the  demands  of  his  theory,  Gaskell  turns  the  arthropod  embryo 


222  EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 

inside  out  and  upside  down,  and  reverses  its  axes  of  growth  and  of  specialization. 
Gaskell  entered  the  field  of  embryology  as  a  novice,  and  at  once  became 
hopelessly  confused  by  the  conflicting  usage  of  the  terms  "dorsal"  and  "ventral," 
and  he  still  remains  so,  because  he  did  not  establish  a  fixed  basis  for  orientation. 
The  result  is  familiar  enough.  In  spite  of  the  testimony  of  his  own  senses,  all 
his  rivers  persist  in  flowing  up  hill,  and  the  north  pole  of  his  compass  points  due 
south. 

Gaskell  at  least  makes  a  valiant  fight  to  save  the  pieces  of  the  invertebrate 
nervous  system,  even  if  he  does  annihilate  the  rest  of  the  animal  in  the  attempt. 
Prof.  C.  J.  Herrick's  effort  is  not  as  praiseworthy,  since  he  discards  altogether  a 
"perfectly  good"  nervous  system,  and  substitutes  for  it  a  new  one  created  out  of 
empty  space. 

III.  EARLY  STAGES  OF  LIMULUS. 

The  development  of  Limulus  will  serve  as  a  convenient  basis  for  a  comparative 
study  of  the  embryology  of  vertebrates  and  arachnids.  I  have  given  little  atten- 
tion to  the  maturation,  fertilization,  and  cleavage,  devoting  most  of  my  time  to 
the  general  form  of  the  body  at  successive  stages,  and  to  the  method  of  growth  of 
the  various  organs. 

Observations  were  made  on  the  living  eggs  during  the  cleavage  and  gastrula 
stages.  But  most  of  the  drawings,  up  to  the  appearance  of  the  appendages, 
were  made  from  eggs  hardened  in  picro-nitric,  or  Perenys  fluid,  and  viewed  as 
opaque  objects,  after  removing  the  membranes.  The  older  stages  were  drawn 
from  embryos  that  had  been  stained  in  various  ways,  and  cleared  in  cedar  oil  or 
balsam. 

The  earlier  stages  were  most  conveniently  obtained  by  artificial  fertilization; 
the  later  stages  were  obtained  from  the  nests  in  the  sandy  beaches  of  Woods  Hole, 
where,  in  1893-94,  most  of  the  embryological  work  on  Limulus  was  done. 

In  artificial  fertilization,  the  female  is  opened  and  the  eggs  poured  into  a 
shallow  glass  dish.  By  carefully  tilting  from  side  to  side,  the  ripe  eggs  may  be 
made  to  adhere  to  the  bottom  and  sides  in  a  compact  single  layer.  After  a  few 
minutes  they  may  be  rinsed  off  in  fresh  sea  water,  removing  the  clotted  blood  and 
immature  eggs,  and  then  fertilized. 

The  eggs  in  some  crabs  are  a  dull  slate  color,  in  others  pink  or  buff,  or  of 
various  intermediate  shades.  When  first  taken  from  the  body,  they  are  shriveled 
and  distorted,  but  after  a  short  time  in  sea  water,  they  become  plump  and  round. 

During  the  first  thirty-six  hours  after  fertilization,  their  form  and  appearance 
change  rapidly.  At  eighteen  hours,  most  of  them  appeared  to  segment  into  two 
unequal  blastomeres.  (Fig.  122,  a.)  About  two  hours  later,  they  again  assumed 
a  regular  outline,  some  of  them  meantime  showing  on  the  upper  surface  a  radiate 
appearance  which  soon  disappeared.  (Fig.  122,  b.) 


EARLY   STAGES    OF   LIMULUS.  223 

i.  Cleaveage. 

About  forty-eight  hours  after  fertilization,  broad  furrows,  filled  with  small 
spherical  bodies,  appear  on  the  upper  surface  of  the  egg.  There  is  no  regularity 
in  the  direction  of  these  cleavage  planes,  or  in  the  form  of  the  blastomeres, 
although  a  four  cell  stage  resembling  that  of  many  amphibia,  is  frequently  seen. 
(Fig.  126,  c.) 

Cleavage  is  most  marked  on  the  surface  that  happens  to  be  uppermost. 
Eggs  with  ten  or  twelve  prominent  blastomeres  on  the  upper  surface  may  be  per- 
fectly smooth  and  apparently  unsegmented  on  the  under  side.  However,  when 
stripped  off  the  glass  and  inverted,  segmentation  appears  on  the  smooth  under 
surface  in  about  fifteen  minutes,  and  in  half  an  hour  it  may  be  almost  as 
complete  as  it  was  on  the  original  upper  surface. 

There,  is  however,  a  distinct  polarity  to  the  egg  that  is  not  influenced  by  its 
position,  for  cleavage  ultimately  makes  greater  progress  on  that  side  of  the  egg 


FIG.  122. — Cleavage  stages,  and  yolk  navel,  h,  of  Limulus  eggs. 

that  is  to  become  the  neural  surface,  irrespective  of  whether  it  faces  up  or  down. 
On  the  fourth  or  fifth  day,  most  of  the  eggs  are  covered  with  a  single  layered 
blastoderm  on  one  side,  while  the  opposite  side  was  still  occupied  by  large  yolk 
spheres. 

In  many  cases,  the  blastoderm,  spreads  over  the  opposite  side  of  the  egg  in  an 
advancing  fold,  the  uncovered  yolk  either  protruding  from  the  opening,  or  lying 
a  little  below  the  general  surface.  The  uncovered  area  is  usually  circular,  but  it 
may  be  quite  irregular,  and  varies  in  size  from  an  opening  one-third  the  diameter 
of  the  egg  to  that  of  a  pin  hole.  (Fig.  126,  h.)  The  yolk  plug  thus  produced  has 
nothing  to  do  with  the  formation  of  germ  layers.  It  resembles  the  condition  seen 
in  the  early  stages  of  certain  ganoids,  cyclostomes,  and  other  fishes,  but  it  has 
no  parallel,  to  my  knowledge,  in  the  invertebrates. 

On  the  eighth  day,  a  single  layered  blastoderm  covers  the  entire  egg,  enclosing 
numerous  nucleated  yolk  spheres. 

Comparison  of  Arachnids  and  Vertebrates. — The  cleavage  in  the  arach- 


224  EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 

nids  presents  an  interesting  intermediate  series  of  stages  between  the  typical  cen- 
trolicithal  type  of  insects  and  that  of  the  lower  vertebrates. 

In  Limulus,  there  is  a  distinct  approach  toward  the  partial  cleavage  of  the 
amphibia  and  cyclostomes.  In  Telyphonus  (Schimkewitsch),  the  form  of  the 
blastomeres,  and  the  size  and  location  of  the  resulting  segmentation  cavity  are 
very  similar  to  the  corresponding  structures  in  the  frog's  eggs. 

In  the  scorpion,  which  possesses  one  of  the  largest  eggs  among  the  arthropods, 
alt  the  early  divisions  take  place  on  the  surface  of  the  yolk  (Brauer),  producing 
a  typical  meroblastic  cleavage,  and  a  small  sharply  defined  blastodisc  very  similar 
to  that  in  the  teleosts. 

In  many  insects  and  spiders,  the  early  divisions  take  place  in  the  interior  of 


FIG.  123. — A,  Surface  view  of  the  blastoderm  and  primitive  cumulus  of  Limulus,  showing  the  beginning  of 
gastrulation,  a.c.,  and  the  formation  of  the  primitive  germinal  area,  p. b. I;  B,  later  stage  showing  the  increasing 
number  of  inner  layer  cells  that  mark  the  boundaries  of  the  germinal  area,  a.nd  the  formation  of  the  posterior 
cumulus,  p.c.,  that  marks  the  beginning  of  the  trunk,  and  of  teloblastic,  or  apical,  growth. 

the  egg,  and  all  the  nuclei  thus  produced  may  move  to  the  surface  to  form  the 
blastoderm,  from  which  yolk  cells  and  mesentoderm  cells  arise  by  a  subsequent 
process  of  division  and  ingrowth.  But  in  some  insects  and  arachnids,  cleavage 
nuclei  remain  in  the  interior  of  the  egg  as  the  so-called  yolk  nuclei,  which,  as  a 
rule  do  not  give  rise  to  the  definitive  endoderm.  In  Limulus,  the  condition  appears 
to  be  exceptional,  in  that  most  of  the  yolk  cells  derived  from  the  early  cleavage 
nuclei,  persist  as  the  permanent  lining  of  the  midgut  and  its  diverticula. 
Although  accessions  are  subsequently  made  to  the  yolk  nuclei  by  the  ingrowth 
of  cells  from  the  germinal  disc  and  germ  wall,  no  definite  bands  of  columnar 
endoderm  cells,  such  as  those  seen  in  insects  and  Crustacea,  are  formed. 

On  the  whole,  the  cleavage  seen  in  the  arachnids  (Limulus  and  the  scorpion) 
is  very  similar  to  that  of  primitive  vertebrates,  and  affords  us  a  satisfactory  basis 
for  the  interpretation  of  the  later  stages  of  development  in  the  arthropod  and 
vertebrate  stock. 

2.  The   Germ  Disc  or  Primitive  Cumulus. 

The  embryo  first  appears  about  four  and  a  half  days  after  fertilization,  as 
a  very  faint,  white,  germinal  spot.  Later,  it  may  be  a  minute,  roughened  papilla, 


THE    GERM   DISC    OR   PRIMITIVE   CUMULUS. 


225 


partly  surrounded  by  a  shallow  groove,  and  situated  in  the  center  of  a  flat  germ 
disc. 

Early  in  the  fifth  day,  the  germ  disc  forms  a  prominent,  mound-like  elevation, 
or  cumulus,  with  the  germinal  spot  now  forming  a  crater-like  depression  at  its 
summit.  (Fig.  124,  A'.  A".}  In  sections,  the  cumulus  appears  as  a  thickening  of 
the  blastoderm,  with  scattering  cells  arising  from  the  whole  of  its  inner  surface, 
and  with  a  cloud  of  cells  migrating  from  the  central  depression,  or  gastrula, 
into  the  yolk. 


pTc:> 


pr.c. 


Q-.C. 


Bn 


-pc 


A 


r. 


>u 


, 


-1  B 


-pc. 


p.c. 


FIG.  124. — Limulus  embryos,  seen  as  opaque  objects,  in  surface  views  and  in  profile.     The  figures  show  the 
primitive   cumulus,  the  expanding  germinal  area,  and  the  beginning  of  the  separation  into  head  and  trunk. 

3.  Formation  of  Me tameres. 

On  the  sixth  and  seventh  days,  the  first  traces  of  metamerism  appear.  Some 
of  the  events  that  take  place  at  this  time  are  difficult  to  observe.  They  are  best 
seen  by  selecting  the  most  conspicuously  marked  eggs  and  examining  them  by 
reflected  light. 

In  stained,  surface  views  of  the  germinal  area,  targe  nuclei  may  be  seen  on 
its  margin,  sometimes  arranged  in  pairs,  two  nuclei  on  the  right,  two  on  the  left, 
and  one  or  more  on  the  anterior  margin.  (Fig.  123,  A.pb.l.)  These  nuclei 
appear  to  initiate  the  formation  of  the  germwall  and  the  periblast,  or  marginal 
yolk  cells.  In  later  stages,  the  inner  and  outer  cell  layers  become  a  little  thicker 
or  darker,  over  the  anterior  half  of  the  germinal  area,  and  two  germinal  spots  are 
now  visible,  an  anterior  and  a  posterior  one.  (Fig.  123,  B.,a.c.  and  p.c.) 

When  studied  as  opaque  objects,  an  earlier  stage  of  the  germinal  area  than 
15 


226 


EARLY  STAGES  OF  ARTHROPOD  AND  VERTEBRATE  EMBRYOS. 


the  one  just  described  has  the  appearance  shown  in  Fig.  124,  A1.  A  shallow 
groove  subsequently  divides  the  cumulus  into  a  darker  anterior,  and  a  lighter 
posterior  part  (Fig.  124,  A2.)-,  and  finally  a  faint  depression  appears  on  the  right 
and  left,  and  on  the  anterior  and  posterior  margins  of  the  cumulus.  A3. 

In  still  older  specimens,  the  anterior  half  of  the  previous  cumulus  now  appears 
as  a  faint,  wave-like  ridge,  in  front  of  the  reformed  cumulus;  the  ridge  repre- 
senting the  forehead,  or  procephalic  lobes.  B1.,  pr.c.  On  the  anterior  half  of  the 


pr.c   I 


c. 


op 


1 1 a 

s1' 


pbl. 


D 


,v  — t 
a,p. 
...t.p. 


&> 

ap. 


FIG.  125. — Limulus  embryos,  seen  as  opaque  objects,  showing  the  formation  of  the  first  six  thoracic  metameres, 
and  the  gradual  infolding  of  the  proliferating  cells  in  the  posterior  cumulus  to  form  a  primitive  streak,  or  telopore. 

cumulus,  is  the  beginning  of  the  second  thoracic  metamere,  s2;  the  posterior 
germinal  spot  marks  the  beginning  of  the  telopore,  p.c.  The  anterior  germinal 
spot  lies  between  the  procephalon  and  the  second  metamere,  a.c. 

In  a  short  time,  the  posterior  half  of  the  primitive  cumulus  rounds  out  into 
a  new  cumulus  with  a  germinal  depression  on  its  posterior  side  B2  and  B3.  The 
remaining  thoracic  metameres  are  formed  in  a  similar  manner,  by  successive, 
wave-like  elevations,  on  the  anterior  slope  of  the  terminal  cumulus,  or  anal  plate. 
(Fig.  125.) 

The  second,  third,  and  fourth  thoracic  metameres  form  in  regular  order,  and 
are  of  about  equal  proportions.  There  is  then  a  distinct  pause,  followed  by  the 
appearance  of  the  fifth  and  sixth  metameres.  (Fig.  125,  Ds.) 


THE    FORMATION    OF    METAMERES. 


227 


The  more  posterior  thoracic  metameres  are  cut  off  from  the  anterior  margin 
of  the  anal  plate,  as  rather  narrow  bands  or  ridges,  s5  and  SQ.  Later  their  pe- 
ripheral ends  join  the  germ  wall  and  spread  rapidly  in  a  lateral  direction.  The 
cheliceral  metamere,  s1,  appears  at  a  relatively  late  period  between  the  cephalic 
lobes  and  the  second  thoracic  metamere.  (Fig.  125,  D2.) 

The  sequence  in  the  development  of  the  abdominal  metameres  is  similar  to 
that  of  the  thoracic.  First,  the  operculum  and  first  gill  appear,  then  a  pause, 
followed  by  the  remaining  gills  in  order.  Finally  the  chilaria  appear  at  a  late 
period  in  front  of  the  operculum.  (Figs.  141,  142.) 


126 


127 


FIGS.  126  AND  127. — Diagrams  to  illustrate  the  methods  of  cleavage,  gastrulation,  and  the  growth  of  the  germ  layers 
and  the  cephalic  navel  in  a  yolkless  egg,  and  one  with  yolk.     The  yolk  is  shown  in  black. 

The  further  growth  of  the  metameres  and  appendages  is  shown  by  the  figures 
and  need  not  be  described  in  detail.  We  would,  however,  call  attention  to  the 
fact  that  there  is  a  period  when  the  second,  third,  and  fourth  thoracic  metameres 
are  especially  conspicuous,  and  the  appendages  first  to  appear  are  formed  on  these 
metameres.  (Fig.  140.)  During  this  period,  the  fifth  and  sixth  metameres  and 
the  whole  abdominal  region  may  be  deeply  depressed,  sometimes  forming  a  deep 
infolding,  on  the  floor  of  which  the  abdominal  appendages  are  developed.  All 
the  thoracic  appendages  have  appeared  by  the  tenth  or  eleventh  day. 

4.  The  Gastrula. 

Returning  to  stage  A.  We  have  seen  that  two  distinct  median  germinal 
spots  appear  near  the  summit  of  the  primitive  cumulus. 

The  anterior  one  (Figs.  123  and  124,  a.c.)  comes  to  lie  near  the  center  of  the 
future  procephalic  lobes.  It  soon  disappears  from  surface  views,  but  remains 
visible  in  sections  as  a  mass  of  "yolk  cells,"  128  Da.  a.c.  It  augments  by  division, 
and  by  the  migration  of  new  cells  from  the  surface,  up  to  stage  G  and  H ,  when 
under  favorable  conditions  it  may  be  seen  even  in  surface  views,  as  a  deep  lying 
cloud  of  cells,  now  showing  the  peculiar  histological  characters  of  degeneration. 
(Fig.  140,  G.a.c.) 


228 


EARLY    STAGES    OF  ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 


The  stomodaeum  appears  shortly  after  stage  E,  on  the  anterior  margin  of  the 
procephalic  lobes,  its  inner  end  coming  to  lie  in  the  midst  of  this  cloud  of  degen- 
erating nuclei.  (Fig.  140,  G.)  After  stage  7,  no  trace  of  these  yolk  nuclei  is  visible. 

I  have  pointed  out  that  a  similar  condition  exists  in  Acilius  and  other  insects, 
and  that  it  is  probably  characteristic  of  arthropods  in  general.  This  infolding  is 
the  only  one  in  arthropods  that,  in  respect  to  the  time  of  its  appearance,  its  loca- 
tion, and  its  products,  can  be  regarded  as  a  gastrula,  and  we  shall  designate  it  as 
such. 

5.  The  Germ  Wall. 

The  germ  wall  is  a  narrow,  unsegmented  zone  of  proliferating  cells,  first  seen 
along  the  margins  of  the  germinal  disc,  and  later  along  the  sides  of  the  embryo  up 
to  the  last  stage  in  the  closing  of  the  haemal  surface,  g.w. 

The  pairs  of  large  nuclei  in  stage  A  (Fig.  123,  p.bl.),  located  on  the  margins 
of  the  germinal  area,  mark  the  beginnings  of  the  germ  wall  and  of  the  periblast. 


^•l.  a.c. 


ms. 


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;/ 

iSfc-*,. 


g9ftii9i0w'V 

**S»~*"' 


V^^^p^£!^«^;  ^ 

••     ••,     •  %<^;^^:^*^M^%»^ 

-••'•-    w  «  <a  •  *-ii.. ._  .  .-»^E». ._.  ^Pi«^. 


FIG.  128. — Sections  of  the  germinal  area  of  Limulus  in  stage  D,  showing  the  formation  of  the  inner  layers, 
and  the  extension  of  the  germinal  area  by  the  growth  of  the  germ  wall,  g.w.,  over  the  surface  of  the  yolk.  The 
local  inward  proliferation  of  the  blastoderm  to  form,  in  part,  the  mesoblastic  somites,  is  shown  at  a.;  m.,  median 
line.  Sections  D&  and  DC  from  the  procephalic,  and  anterior  thoracic,  regions.  Section  D&.  from  the  region 
of  the  primitive  streak,  pr.s. 

At  thes^points  an  inward  proliferation  develops,  in  surface  views  appearing  as 
faint  spots  or  depressions;  later  they  form  a  conspicuous  marginal  band  or  thick- 
ening. (Figs.  125,  128,  140,  g.w.) 

The  inward  proliferation  along  this  margin  is  similar  to  that  at  the  apex  of 
the  body.  As  the  germinal  margin  spreads  over  the  surface  of  the  yolk,  it  leaves 
behind  a  trail  of  differentiated  ectoderm,  mesoderm,  and  yolk  cells,  or  periblast. 
It  ceases  to  produce  new  periblast  after  stage  J,  but  it  continues  to  proliferate  the 
definitive  ectoderm  and  new  ectoderm,  probably  up  to  the  time  the  cephalic  navel 
closes. 


THE    GERM    WALL. 


229 


On  either  side  of  the  germ  wall  there  is  a  sharp  distinction  between  the  defini- 
tive ectoderm  and  mesoderm  on  the  one  hand,  and  the  single  layer  of  columnar 
blastoderm  cells  on  the  other.  (Fig.  129.) 


It  thus  appears  that  in  Limulus  the  formation  of  germ  layers  is  not  a 
particular  event  in  the  development.  It  takes  place  at  different  times  and 
in  widely  different  parts  of  the  germinal  area.  In  fact,  the  production  of 
germ  layers  may  occur  wherever  early  growth  takes  place,  as  for  example, 
on  the  sides  of  the  anal  plate,  where  the  mesoblastic  somites  are  formed;  in  the 
germ  wall,  where  the  germ  layers  in  the  lateral  ends  of  each  half  metamere  are 


is    129 


130 


FIG.  129. — Section  through  the  primitive  streak  of  Limulus,  stage  G,  showing  mesoblastic  somites,  germ  wall, 

periblast,  etc. 
FIG.  130. — Sections  of  same  stage  through  posterior  parts  of  the  anal  plate. 


formed;  in  the  telopore,  where  the  axial  tissues  arise;  or  in  the  comparatively 
late  stages  of  development,  where  mesodermic  tissues  appear  to  arise  from  various 
local  modifications  of  the  ectoderm.  The  formation  of  germ  layers  is  not,  there- 
fore, synonymous  with  gastrulation,  nor  is  it  to  be  regarded  as  a  modification  of 
gastrulation,  which,  as  we  understand  it,  is  that  particular  process  by  which  an 
organ  representing  the  enteron  of  a  coelenterate  was  formed;  as  for  example,  the 
central  ingrowth  of  the  primitive  cumulus,  and  of  the  procephalic  lobes.  The 
formation  of  the  germ  layers  from  the  telopore  and  germ  walls,  is  ontogenetically 
and  phylogenetically  a  later  and  a  different  process.  It  is  the  embryonic  method 
of  growing  a  new  body  on  an  old  head,  the  head  representing  the  ancestral  coelen- 
terate body.  Neither  the  embryonic  method  of  postcephalic  growth  in  segmented 
animals,  nor  the  great  variety  of  tissues  and  organs  produced  by  it,  are  comparable 
with  anything  that  takes  place  in  the  ccelenterate. 


230  EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 

6.  The  Mesoderm. 

The  Sources  and  Kinds  of  Mesoderm. 

We  may  distinguish  four  sources  from  which  the  mesoderm  takes  its  origin, 
viz:  a.  the  procephalic  mesoderm,  arising  by  delamination  from  the  anterior 
portion  of  the  primitive  cumulus;  b.  the  axial  mesoderm  (primitive  streak)  from 
the  axial  teloblasts;  c.  the  mesoblastic  somites,  from  the  sides  of  the  anal  plate; 
and  d.  the  lateral  plates,  from  the  germ  wall. 

a.  Procephalic    mesoderm.     In    the    procephalon    the    mesoderm    forms    a 
single  unpaired,  thin- walled  chamber,  approximately  coextensive  with  the  cephalic 
lobes.     It  arises  by  delamination  from  the  anterior  half  of  the  primitive  cumulus; 
it  is  not  divided  into  an  axial  cord,  somites,  and  lateral  plates,  and  does  not 
appear  to  be  comparable  with  that  in  the  trunk.  (Fig.  123,  B.)     It  disappears 
without  giving  rise  to  any  permanent  organ  or  tissue,  except  the  investment  of  the 
stomodaeum  and  forebrain. 

b.  The  postoral  mesoderm  consists  of  the  axial  cord,  somites  and  lateral 
plates.     These  three  subdivisions  are  present  in  the  whole  postoral  region  or 
trunk,  but  they  may  be  very  unequally  developed.     A  striking  feature  in  the 
arachnids  and  vertebrates  is  the  large  size  of  the  somites  of  the  midbrain  region, 
and  the  atrophy  of  the  corresponding  lateral  plates. 

1.  The  axial  cord  arises  from  successive  pioliferations,  located  in  the  median 
line  at  the  posterior  end  of  the  body.     After  the  first  one  or  two  segments  are 
formed,  an  unsegmented,  axial  cord  appears,  extending  forward  from  the  center 
of  the  anal  plate  to  the  stomodaeum.     Its  presence  is  indicated,  either  by  a  faint, 
median  shadow  (Fig.  125,  C.p.c.),  or  by  a  groove,  or  "primitive  streak"  (Fig.  128, 
Db.  pr.s.),  or  by  a  sharp  depression,  or  telopore.     (Fig.  140,  t.p.) 

The  axial  cord  gradually  breaks  up,  from  before  backward,  into  a.  the 
ectodermic  middle  cord,  from  which  arises  the  median  nerve  and  the  epithelium 
of  the  canalis  centralis;  b.  into  the  mesodermic  lemmatochord,  or  notochord;  and 
c.  into  the  primary  germ  cells.  (See  notochord.) 

The  axial  mesoderm  shows  little  or  no  trace  of  segmentation  and  never  con- 
tains a  coelomic  cavity. 

2.  The  Somites. — The  thoracic  and  abdominal  metameres  are  formed  as 
paired,  wave-like  ridges  on  the  anterior  lateral  margin  of  the  posterior  cumulus, 
or  of  the  anal  plate.     At  the  same  time,  a  band  of  mesoderm  cells  separates  from 
the  inner  surface  of  each  ridge,  giving  rise  to  a  pair  of  mesodermic  segments,  which, 
as  fast  as  they  separate  from  the  outer  cell  layer,  form  hollow,  thick-walled  somites. 
In  the  later  stages,  the  posterior  abdominal  somites  arise  as  segments  of  the  wing- 
like  expansions  of  mesoderm  formed  beneath  the  ectoderm,  on  either  side  of  the 
telopore.     (Fig.  130.) 

From  the  somites  arise  the  longitudinal  muscles  and  cartilages  of  the  append- 
ages, the  endocranium,  the  genital  and  nephric  ducts,  and  the  nephric  tissue. 


THE    MESODERM. 


23I 


m 


3.  The  Lateral  Plates. — The  lateral  end  of  each  primitive  somite  retains  its 
connection  with  the  proliferating  surface  cells  that  constitute  the  germ  wall.  As 
the  germ  wall  spreads  over  the  yolk,  it  gives  rise  to  the  lateral  plates',  or  that  part 
of  the  mesodermic  segments  lying  lateral  to  the  zone  of  appendages.  The  lateral 
plates  are  merely  lateral  extensions  of  the  somites,  and  like  them  each  one  con- 
sists of  somatic  and  splanchnic  layers,  and  may  enclose  a  ccelomic  cavity.  The 
somite  is  the  product  of  the  apical  growth  of  the  anal  plate;  the  lateral  plates 

are  produced  by  the  lateral  extension 
of  the  germ  wall.  The  peripheral 
margins  of  the  lateral  plates  finally 
lose  their  connection  with  the  germ 
wall,  and  the  cell  layers  separating 
adjoining  coelomic  cavities  break  down. 
From  the  lateral  plates  arise  the 
extra  embryonic  blood  corpuscles, 
the  cardiomeres,  pericardial  chamber, 
and  the  longitudinal,  haemal  muscles. 
Lateral  Plates  in  the  Frog. — The 
mesoderm  forming  the  segmented 
lateral  plates  of  the  post-thoracic 


Fro.  131.  FIG.  132. 

FIG.  131. — Section  through  the  fourth  segmental  sense  organ,  s.o.4,  of  the  thorax  of  an  embryo  Limulus,  about 
stage  I,  together  with  the  adjacent  margin  of  the  germ  wall  with  its  mass  of  fiber  cells, /.c.;  a.  to  t.,  fiber  cells 
of  the  same  stage  more  highly  magnified;  d.e.f.,  similar  cells  from  the  adult,  in  a  free  amoeboid  condition,  and  in 
fission;  g.,  same  sense  organ  with  its  lens-like  chitenous  thickening  during  the  trilobite  stage. 

FIG.  132. — Clusters  of  fiber  cells  from  the  posterior,  haemal  region  of  the  thorax,  about  stage  N,  showing 
mode  of  division  and  their  transformation  into  muscles.  Limulus 

metameres  plays  such  an  important  part  in  the  development  of  the  arthropod 
embryos,  that  one  might  reasonably  expect  to  find  some  trace  of  them  in  verte- 
brates. With  this  object  in  view,  specially  prepared  frogs'  eggs  were  studied  in 
a  strong  oblique  light.  In  this  work  I  was  aided  by  Mr.  A.  O.  Kelley,  a  grad- 
uate student  at  Dartmouth,  and  the  figures  and  descriptions  on  this  point  were 
worked  out  by  us  together,  in  1906-07. 

A  considerable  number  show  two  or  three  pairs  of  lateral  plates;  a  very 


232  EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 

few  show  a  larger  number,  as  in  Fig.  159.  In  this  specimen,  the  anterior 
plates,  which  apparently  belong  to  the  post-branchial  metameres,  are  directed 
downward  and  forward  as  if  they  were  growing  around  the  egg  below  the  gill 
plates,  just  as  the  post-vagal  segments  of  the  arachnids  encircle  the  egg  on  the 
haemal  side  of  the  appendicular  arches.  (Figs.  17,  19.)  The  segments  decrease 
in  length  caudad,  and  the  most  posterior  ones  are  directed  downward  and  back- 
ward toward  the  closing  telopore,  forming  a  distinct  welt  on  either  side. 

The  lateral  plates  quickly  disappear,  so  that  it  was  not  possible  to  follow  them 
into  later  stages. 

Although  these  results  are  very  meager,  it  nevertheless  seems  probable  that 
the  figures  in  question  are  to  be  regarded  as  a  faint  recurrence  of  the  concrescing 
segmented  lateral  plates  of  mesoderm  so  conspicuous  in  the  arachnids. 

The  Fiber  Cells. — One  of  the  chief  products  of  the  germ  wall  is  a  thick  band 
of  rounded  or  oval  cells,  that  we  shall  call  fiber  cells.  They  lie  in  the  first  five 
thoracic  segments  in  an  intermediate  zone  median  to  the  germ  wall.  They  have 
a  remarkable  structure  and  history;  some  give  rise  to  definite  muscles;  some 
persist  in  the  adult  as  a  peculiar  type  of  spindle  shaped  semi  amoeboid  cells  re- 
sembling blood  corpuscless;  others,  after  forming  muscles,  degenerate  during  the 
later  embryonic  and  larval  stages. 

In  the  earlier  stages,  the  fiber  cells  cannot  be  distinguished  from  the  other 
cells  in  the  germ  wall.  (Figs.  128  and  129.)  In  stages  /  and  K,  they  form  a 
broad,  thick  band  of  large  oval  cells,  rather  loosely  arranged,  and  presenting  a 
very  striking  appearance.  (Fig.  131,  A.f.c.)  Each  cell  contains  a  small  eccentric 
nucleus  and  a  highly  refractive,  colorless  fiber.  The  latter  may  run  in  a  regular 
spiral  direction,  filling  the  entire  cell,  or  it  may  form  regular  loops  arranged  in 
compact  bundles  that  stand  at  various  angles  with  each  other;  a,  b,  h  and  c. 
Aside  from  the  fiber,  the  cells  appear  empty  and  colorless,  although  in  some 
cases  they  may  have  a  dense,  slightly  colored  envelop,  /  and  d. 

In  stage  G,  the  fiber  cells  are  clearly  visible  in  surface  views  as  a  dark  inner 
border  to  the  germ  wall,  and  extending  from  the  cheliceral  segment,  where  it 
is  especially  enlarged,  to  the  anterior  border  of  the  sixth  segment.  (Figs.  141- 
144,  a.v.) 

The  band  increases  in  width  and  continues  to  advance  toward  the  haemal 
surface.  In  stage  H,  it  forms  an  equatorial  girdle,  the  two  extremes  having 
meantime  united  behind,  and  almost  united  in  front.  (Figs.  141  and  144,  a.v.) 
After  it  passes  the  equator  the  posterior  limb  moves  rapidly  forward,  swinging 
into  a  hsemo-neural  direction,  thus  shifting  the  center  of  the  closing  ring  toward 
the  anterior  haemal  portion  of  the  thorax.  (Figs.  147  to  149.) 

Meantime  the  yolk  mass  of  the  thorax,  and  a  little  later  that  of  the  abdomen, 
divides  into  distinct  lobes,  the  future  enteric  pouches.  Important  agents  in  bring- 
ing this  about  are  the  haemo-neural  muscles  of  the  thorax.  There  are  eleven  pairs 
of  these  muscles  attached  to  the  middle  of  the  dorsal  shield  in  the  adult,  making 
five  pairs  of  complicated  markings  on  its  inner  surface.  (Fig.  155.)  Six  pairs 


THE    FIBER    CELLS. 


233 


of  tergo-plastrals  arise  from  the  endocranium,  and  five  pairs  of  tergo-coxals 
from  the  coxal  joints  of  each  thoracic  appendage,  two  in  front  and  three  behind. 
These  muscles  and  the  spaces  between  the  developing  gut  lobes  are  important 
conduction  paths  for  the  distribution  of  the  fiber  cells. 

The  muscles  arise  at  a  very  early  period  from  the  anterior  and  from  the  poster- 
ior wall  of  each  mesodermic  segment,  close  to  the  somites.  They  form  the  only 
suggestion  of  segmentation  that  is  to  be  seen  in  the  lateral  plate  area  of  the  thorax. 
(Figs.  142-151,  hm.m1'5.) 

At  first  the  tergo-coxal  muscles  lie  in  a  nearly  horizontal  plane,  their  median 
or  neural  ends  attached  to  the  ectoderm  between  the  bases  of  the  thoracic  ap- 
pendages. (Fig.  142.)  Their  lateral  ends  gradually  advance  with  the  germ 
wall  toward  the  haemal  surface,  and  as  they  swing  into  a  nearly  vertical  position, 
they  cut  the  thoracic  yolk  mass  into  five  great  lobes,  which  ultimately  become  the 
five  main  liver  lobes,  or  enteric  pouches,  of  the  thorax.  (Figs.  142-151, 
hn.m  I'5.) 

In  stages  K  and  L,  the  fiber  cells  begin  to  scatter  in  different  directions. 
Many  leave  the  surface  and  penetrate  the  interior  of  the  thorax,  following,  in  the 
main,  the  channels  between  the  yolk  lobes  formed  by  the  haemo-neural  muscles. 

During  stage  L,  embryos  seen  from  the  haemal  surface  show  the  fiber  cells 
as  two  large  mottled  patches  on  the  anterior  lateral  surface  of  the  thorax.  (Fig. 
149,  a.v.)  The  clear  oval  area  between  them  represents  the  cephalic  navel,  or  the 
depression  where  the  remnant  of  the  blastoderm  is  passing  into  the  interior  of  the 
yolk. 

The  fiber  cells  congregate  in  great  numbers  along  the  sides  of  the  cephalic 
navel,  and  around  the  haemal  ends  of  the  first  five  pairs  of  haemo-neural  muscles, 
where  they  form  dark  colored,  conical  or  wedge-shaped  masses  of  cells.  (Figs. 
146-149,  hn.m  J~s.) 

It  will  be  observed  that  up  to  the  present  time  the  fiber  cells  have  a  very 
definite  distribution,  and  except  for  a  few  scattering  clusters  they  are  absent 
from  the  sixth  thoracic,  the  vagus,  and  the  abdominal  segments.  (Figs.  148  and 
149.) 

The  metamorphosis  of  some  of  these  loose  oval  fiber  cells  into  muscles  is 
very  rapid  and  takes  place  a  little  after  stage  M ,  when  the  embryo  is  taking  on  the 
trilobite  form,  and  pigment  has  appeared  in  the  eyes.  At  this  period,  the  cells 
stain  more  readily;  the  fiber  is  less  distinct,  forming  finer  parallel  fibrils;  the  nucleus 
takes  up  a  central  position;  and  the  cells  elongate  somewhat  and  unite  end  to  end 
in  irregular  rows.  (Fig.  132.)  In  the  more  advanced  stages,  a  central  canal 
has  formed,  in  which  the  dividing  nuclei  are  arranged  in  a  single  row  and  the 
beginning  of  cross  striations  is  seen  on  the  free,  more  or  less  pointed  ends  of  the 
cells. 

Two  great  masses  of  these  muscle  cells  are  formed  on  either  side  of  the  ceph- 
alic navel  and  of  the  anterior  end  of  the  heart.  From  them  are  developed  two 
pairs  of  muscles.  One  pair,  the  inter- tergals,  lies  on  either  side  of  the  heart. 


234 


EARLY   STAGES    OF  ARTHROPOD  AND   VERTEBRATE   EMBRYOS. 


FIG.  133. — Diagrams,  in  mercator  projection,  illustrating  the  mode  of  growth  of  arachnid  embryos  during  the 
early  stages;  A,  primitive  cumulus,  in  the  radiate  or  gastrula  stages;  B,  appearance  of  the  posterior  cumulus  and  the 
teloblasts,  or  the  beginning  of  apical  growth  and  the  differentiation  of  the  primitive  head  and  trunk;  C,  appearance  of 
the  medullary  plate;  the  procephalic  lobes;  the  marginal  sense  organs  and  ganglia;  the  telopore;  and  the  segmenta- 
tion, and  lateral  expansion,  of  the  posterior  portion  of  the  germinal  area;  D,  appearance  of  the  thoracic  appen- 
dages; the  growth  of  the  palium  over  the  procephalic  lobes;  the  post-anal  concrescence  of  the  lateral  plates  of  the 
germinal  area;  and  the  forward  extension  of  the  vagal  and  anterior  abdominal  plates. 


gn.  nek.  ,c.c.  1J*- 9 


Kt. 


Cct.n 


FIG.   134. 


FIG.   135. 

FIG.  134.- — Schematic  cross-section  of  an  arachnid  embryo  in  the  anterior  thoracic  region,  showing  the 
relative  positions  and  the  mode  of  growth  of  the  principal  organs. 

FIG.  135. — Schematic  figure,  in  mercator  projection,  showing  further  concrescence  of  the  germ  wall,  in  a 
hypothetical,  large  yolked  embryo.  The  cephalic  navel  appears  just  in  front  of  the  procephalon,  as  the  anlage  of 
the  haemastoma,  and  the  anterior  appendicular  arches  are  beginning  to  close  in  the  hasmal  surface  of  the  procephalon. 


per 


FIG.  136. — Section  of  a  later  stage 
in  the  same  region. 


FIG.  137. — Section  of  a  still  later  stage 
in  the  branchial  region. 


THE    FIBER    CELLS.  235 

At  the  time  of  hatching,  it  is  very  voluminous,  and  is  apparently  confined  to  the 
thorax.  (Fig.  151,  in.t.)  In  the  later  stages,  it  undergoes  considerable  reduction, 
and  its  posterior  end  becomes  attached  to  the  anterior  margin  of  the  abdominal 
shield.  (Fig.  77,  in.t.) 

The  other  large  muscle  developed  from  the  fiber  cells  is  the  anterior  end 
of  the  hypobranchial.  (Fig.  77,  B.b.th.m.)  This  muscle  also  becomes  greatly 
reduced  in  volume  in  the  older  stages,  and  its  anterior  end  retreats  to  a  more 
posterior  position. 

Many  other  fiber  cells  become  scattered  irregularly  along  the  margins  of  the 
thoracic  shield,  in  the  spaces  left  free  by  the  contracting  liver  lobes.  Here  they 
elongate  and  form  the  small,  scattered  bundles  of  muscle  fibers  that  permanently 
unite  the  neural  and  haemal  walls  of  the  cephalic  buckler. 

The  metamorphosis  of  fiber  cells  into  these  particular  muscles  on  the  haemal 
surface  of  the  thorax,  takes  place  at  a  very  late  period,  long  after  the  endocranium, 
the  haemo-neural,  and  other  muscles  have  become  clearly  differentiated;  and 
no  other  muscles  than  those  mentioned  are  formed  in  this  manner.  At  the 
time  the  fiber  cells  undergo  their  metamorphosis,  the  yolk  lobes  are  invested  with 
a  thin  cellular  layer,  so  that  it  is  hardly  probable  that  any  of  them  are  absorbed  in 
the  yolk  with  the  haemal  blastoderm. 

A  large  number  of  fiber  cells,  however,  never  form  definite  muscles.  They 
persist  through  life,  loosely  distributed  throughout  the  lacuna  spaces  in  all  parts 
of  the  body,  although  they  appear  to  be  more  numerous  at  the  base  of  the  append- 
ages and  in  the  anterior  portions  of  the  cephalic  shield. 

In  the  half  grown  Limuli,  and  even  in  the  adult,  they  may  be  readily  recognized 
by  their  large  size,  peculiar  structure,  and  coloring.  They  generally  preserve 
the  spindle-shaped  form,  but  appear  to  be  somewhat  amoeboid,  or  rather  euglenoid, 
and  some  have  been  observed  in  division.  (Fig.  131,  e.)  The  fiber  is  now 
much  less  refractive  and  has  lost  its  distinctly  spiral  arrangement,  apprearing  as 
longitudinal  striae  that  converge  toward  either  end. 

The  remarkable  history  of  the  fiber  cells  indicates  that  they  are  the  degenerat- 
ing remnants  of  the  longitudinal  haemal  muscles  of  the  first  five  thoracic  segments. 
Only  a  part  of  the  large  number  of  cells  actually  form  muscles;  the  others  become 
free  cells  that  may  be  regarded  as  a  special  type  of  blood  corpuscle,  although  they 
do  not  appear  to  circulate  freely  in  the  main  blood  channels. 

In  the  spiders  and  in  the  scorpion,  the  same  kind  of  fiber  cells  are  present. 
Juts  what  their  history  is  in  these  forms  has  not  been  definitely  determined.  Vari- 
ous authors,  who  have  apparently  seen  them  in  the  spiders,  state  that  they  give 
rise  to  blood  corpuscles. 

Whether  these  cells  ultimately  give  rise  to  true  blood  corpuscles  or  not 
was  not  determined. 

Blood  corpuscles  of  the  usual  type  arise  from  the  peripheral  ends  of  the  lateral 
plates  of  the  thoracic  and  abdominal  segments,  and  at  these  early  stages  can  be 
readily  distinguished  from  the  fiber  cells.  (Fig.  131,  b.c.) 


236 


EARLY  STAGES  OF  ARTHROPOD  AND  VERTEBRATE  EMBRYOS. 


Assheton1  has  described  some  peculiar  wandering  cells,  of  unknown  fate  and 
significance,  that  appear  in  many  parts  of  the  body  on  the  sixth  and  seventh  days 
of  incubation  in  Gymnarchus  nitolicus,  p.  369-370.  In  their  distribution,  size, 
"peculiar  refrangent  qualities"  and  in  the  "ribs  which  run  from  pole  to  pole," 


FIG.  138. — Schematic  diagrams  in  mercator  projection,  at  two  different  stages  illustrating  the  arrangement  and 
mode  of  growth  of  the  mesoderm  and  its  derivatives  in  an  arachnid  embryo. 

they  greatly  resemble  the  fiber  cells  of  the  arachnids  and  I  have  no  doubt  they 
are  in  reality  the  same  kind  of  cells. 

Similar  cells  have  also  been  described  by  Plehn,  1906,  in  teleosts.  They  are 
oval,  thick-walled  cells  with  a  small  excentric  nucleus  and  with  numerous  fine, 
highly  refractive,  unstainable  rods  or  threads,  all  converging  toward  the  non- 


A. 

B.  C 

FIG.  139. — Schematic  figures  of  the  haemal  surface  of  arthropod  and  vertebrate  embryos,  to  illustrate  the 
method  ol  closing  the  haemal  surface  in  large  yolked  embryos.  A,  Insect  type,  with  elongated  cephalic  navel; 
B,  arachnid  type  with  larger  yolk  sphere  and  overhanging  or  projecting  cephalic  and  caudal  lobes;  C,  vertebrate 
type,  with  a  relatively  larger,  more  posteriorly  located,  yolk  sphere;  with  cephalic  appendages  concrescing  around 
the  cephalic  navel;  and  with  delayed  concrescence  of  the  centrally  located  cardiomeres,  thus  giving  rise  to  the 
bifurcated  heart  tube,  and  to  the  middle,  or  belly,  navel. 

nucleated  pole  of  the  cell.  They  are  widely  distributed  in  the  walls  of  blood  ves- 
sels and  in  lymphoid  tissue,  and  are  said  to  form  a  secretion  that  is  probably 
emptied  into  the  blood.  See  also  Phoronis  and  Lernaea,  p.  447. 

Vascular  Area. — From  the  preceding  descriptions  it  will  be  seen  that  an  ex- 
tra-embryonic area  is  established  in  the  arachnids  in  which  we  may  recognize  the 

1  The  Development  of  Gymnarchus  Niloticus.      The  work  of  John  Samuel  Budget,  Cambridge,  1907. 


THE    VASCULAR   AREA.  237 

beginnings  of  a  vascular  area,  a  pelucid  area,  and  a  germ  wall,  having  a  structure, 
arrangement,  and  mode  of  growth  similar  to  the  corresponding  ones  in  the 
vertebrates. 

A  comparison  of  the  numerous  sections  given  by  S.  Mollier  to  illustrate  the 
development  of  the  blood  will  show  that  the  structure  and  mode  of  growth  of  the 
vascular  cords  in  amphibians  and  cyclostomes  are  essentially  the  same  as  in 
Limulus. 

The  principal  difference  in  our  descriptions  relates  to  the  origin  of  the  cells 
in  or  near  the  germ  wall.  Many  students  of  vertebrate  embryology  state  that 
yolk  cells  migrate  into  the  germ  wall  and  give  rise  to  blood  cells.  In  Limulus, 
according  to  my  description,  while  it  is  true  that  many  cells  originating  in  the  germ 
wall  pass  out  of  it  into  the  yolk,  there  is  no  indication  that  the  yolk  cells  migrate 
in  the  opposite  direction  into  the  vascular  cell  cord.  As  the  pictures  presented 
by  the  vascular  cell  cords  in  vertebrates  and  arachnids  are  identical,  the  difference 
undoubtedly  lies  in  the  interpretations,  not  in  the  processes. 

It  will  be  seen  from  an  inspection  of  the  diagrams  illustrating  these  conditions 
in  mercator  projection,  that  along  the  periphery  of  the  germinal  area  in  typical 
arachnids  such  as  the  scorpion,  the  spiders,  and  Limulus,  there  is  a  zone  of  dividing 
cells  that  constitutes  one  of  the  earliest  and  most  important  sources  of  blood  cor- 
puscles. (Fig.  138,  av.)  It  may  therefore  be  called  the  vascular  area.  Owing 
to  the  forward  growth  of  the  abdominal  lateral  plates  and  the  absence  of  such 
plates  in  the  thoracic  region,  a  barren  extra  embryonic  area  is  formed  around  the 
head,  which  may  be  regarded  as  the  beginning  of  a  pellucid  area.  (Fig.  138, 
B.  a.pl.) 

It  will  be  seen  that  when  the  same  conditions  are  shown  from  the  haemal  sur- 
face of  the  egg  (Fig.  139,  A,B.),  the  concrescing  germ  walls  form  a  median  band 
or  cord  of  yolk  cells,  heart  cells,  and  blood  cells,  extending  from  the  posterior 
margin  of  the  dorsal  organ,  or  cephalic  navel,  to  the  caudal  navel. 

In  the  vertebrates,  we  may  recognize  a  modification  of  this  condition,  due 
largely  to  the  increase  in  the  size  of  the  yolk  sphere.  (Fig.  139,  C.)  When  the 
latter  is  of  considerable  size,  the  haemal  concrescence  of  the  germ  wall,  in  the 
middle  sections  of  the  body,  is  delayed,  or  does  not  take  place  at  all.  Thus  a 
potential,  or  real,  belly  navel  is  formed,  dividing  the  primitive  vascular  cord  into 
three  sections.  The  definitive  heart  arises  from  the  anterior  section  that  lies 
between  the  cephalic  navel,  or  mouth,  and  the  belly  navel.  In  the  middle  section, 
the  vessel  remains  in  a  paired  condition,  forming  around  the  navel  a  vascular 
ring,  the  anterior  part  of  which  represents  the  Cuvierian  ducts,  or  the  proximal 
ends  of  the  vitelline  veins.  The  posterior  ends  unite  behind  the  belly  navel  to 
form  the  posterior  section  of  the  primitive  vascular  cord,  which  is  continuous,  at 
its  posterior  end,  with  the  haemal  lip  of  the  blastopore,  or  caudal  navel.  From 
the  posterior  section  is  formed  the  unpaired  vitelline  vein,  which  represents  the 
non-contractile  caudal  end  of  the  arachnid  haemal  vessel.  (Compare  also  Figs. 
*7>  23>  31*  34,  43-) 


238  EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 

IV.  THE  CEPHALIC  NAVEL,  DORSAL  ORGAN,  OR  NEOSTOMA. 

Linmlus. — During  the  primitive  cumulus  stage,  the  blastoderm  outside  the 
germinal  area  consists  of  a  single  layer  of  sharply  defined  cells.     At  this  period, 


.••-.' •.'.•\::.. : --\;.;  t.',".;. i;^/!6.<"*  ;V"         -V/**? 

F    %jp 

FIG.  140. — Limulus  embryos  in  stage  F  and  stage  G,  the  latter  in  mercator  projection.     The  lateral  plates  of  the 
sixth  thoracic  and  anterior  abdominal  metameres,  and  the  segmental  sense  organs,  are  clearly  shown. 

^•:.-:^m^^^^m^^:-:. 

•. ^W&iMtm±mi&s-:;mms^.K^y:-.  - 


FIG.  141 . — Same,  stage  H,  mercator  projection.    The  first  five  thoracic  metameres  and  the  procephalic  lobes  have  no 

segmented  lateral  plates. 

the  "vicarious  chorion,"  a  thick  membrane  secreted  by  the  superficial  cells  of  the 
eggs,  is  smooth  and  structureless  over  the  germinal  area,  but  over  the  blastoderm 
it  is  divided  into  polygonal  plates,  one  for  each  blastoderm  cell.  (Fig.  128,  v.c.) 


THE    CEPHALIC    NAVEL. 


239 


The  membrane  soon  separates  from  the  embryo,  and  later  from  the  blasto- 
derm. The  blastoderm  cells  meantime  become  very  deep  and  columnar,  the 
nuclei  and  dense  cytoplasm  collecting  at  their  outer,  and  numerous  yolk  par- 
ticles, at  their  inner  ends.  (Fig.  129,  d.o.)  At  this  period,  the  blastoderm  cells 
beyond  the  embryonic  area  are  everywhere  sharply  cut  off  from  the  yolk,  and  there 
is  no  migration  of  cells  from  one  to  the  other. 

In  stage  G  (Fig.  143),  the  blastodermic  nuclei,  along  a  narrow  zone  just 
beyond  the  germ  wall,  take  on  a  sharper  and  darker  color,  do.  During  stages 


FIG.    142.— Same,  stage   K,   mercator  projection.     On  the  right,  the  ends  of  the  thoracic  appendages  are 

removed. 

H  and  7,  the  zone  widens,  gradually  spreading  over  the  entire  blastoderm.  This 
change  in  the  appearance  of  the  nuclei  marks  the  beginning  of  a  rapid  prolifera- 
tion, and  subsequent  degeneration  of  the  blastoderm  cells.  During  this  process, 
the  chromatin  collects  into  larger,  intensely  stained  particles;  the  columnar  cells 
divide,  take  on  a  spherical,  or  oval  form,  and  pass  in  great  numbers  into  the  yolk, 
where  they  form  a  very  conspicuous  mass  of  loosely  arranged  cells.  (Fig.  134,  d.o.) 
The  cytoplasm  of  these  cells  soon  becomes  fainter  and  more  transparent,  and 


240  EARLY    STAGES    OF  ARTHROPOD  AND    VERTEBRATE    EMBRYOS. 

finally  disappears.  The  coarse  irregular  nuclear  masses  break  up  into  very  fine 
granules,  which  become  scattered  through  the  yolk  and  absorbed. 

Meantime,  as  the  germ  wall  passes  the  equator  of  the  egg  and  advances  to- 
ward the  anterior  haemal  surface,  it  surrounds  a  gradually  narrowing  area,  where 
the  degenerating  blastoderm  cells  are  being  crowded  into  the  yolk  and  overgrown 
by  the  germ  wall  and  its  products.  The  ingrowing  blastoderm,  and  the  narrowing 
ring  formed  by  the  germ  wall  and  vascular  area,  constitute  the  cephalic  navel. 
(Figs.  139-149*  c.nv.) 

When  the  vascular  area  and  germ  walls  finally  close  in  the  haemal  surface, 
the  entire  extra-embryonic  blastoderm  and  its  products  have  disappeared  in  the 
interior  of  the  egg.  Owing  to  the  shape  of  the  yolk  sphere,  and  to  the  unequal 
expansion  of  the  thoracic  and  abdominal  lateral  plates,  the  greater  part  of  the 
blastoderm  is  crowded  into  the  anterior  portion  of  the  mesocephalon,  and  is  last 
seen  disappearing  into  the  yolk  just  behind  the  procephalon.  (Figs.  149,  150, 
c.nv.) 

In  stage  O,  the  last  remnants  of  these  cells  may  be  seen  scattered  about  in  the 
yolk  contained  in  the  first  four  pairs  of  enteric  pouches.  (Fig.  151.) 

Other  Arthropods. — The  cephalic  navel  of  Limulus  without  doubt  represents 
one  phase  of  the  structure  familiar  in  insects,  Crustacea,  and  myriapods,  and  which 
is  usually  spoken  of  as  the  "dorsal  organ."  It  has  only  recently  been  recognized 
in  the  arachnids.  I  have  found  a  similar  structure  to  that  of  Limulus  in  the 
scorpion,  and,  according  to  Schimkewitsch,  one  is  found  in  Pholcus  and 
probably  in  Telephonus.  One  has  also  been  described  in  the  copepods. 
(Fig.  272,  B.) 

In  the  Crustacea,  the  conditions  centered  in  or  around  the  cephalic  navel 
give  rise  to  a  variety  of  structures.  It  may  be  a  transitory,  embryonic  gland- 
like  organ,  as  in  isopods;  a  larval  organ  serving  for  temporary  attachment,  as  in 
cladocera  (Figs.  8,  9) ;  or  a  voluminous  outgrowth  that  serves  throughout  life  for 
the  attachment  of  the  animal  to  some  inanimate  object,  or  to  its  host,  as  in  cirri- 
peds  and  parasitic  copepods.  (Figs.  275,  282,  283.) 

The  cephalic  navel,  in  one  form  or  another,  is  therefore  found  in  all  classes 
of  arthropods.  What  its  original  significance  may  be  is  not  apparent.  But  its 
function,  when  it  has  one,  and  its  location  and  general  mode  of  growth  are 
constant. 

There  are  clearly  two  factors  involved  in  its  formation:  i.  the  low  pressure 
area  formed  by  the  degenerating  haemal  blastoderm;  and  2.  the  convergence  of  the 
margins  of  the  germinal  area  toward  the  center  of  degeneration.  The  invagination 
of  the  degenerating  blastoderm  into  the  yolk,  and  the  closure  of  the  germinal 
area  around  it,  gave  rise  to  a  fistula-like  communication  between  the  enteron  and 
the  exterior.  This  opening  may  close  up  completely  during  the  embryonic  stages, 
leaving  no  scar  behind  (insects  and  arachnids);  or  around  the  point  of  closure 
scar-like  glandular  structures  or  outgrowths  may  develop  that  serve  as  temporary 
or  permanent  means  of  attachment  (crustacea,  cirripeds),  or  in  parasitic  forms 


THE   CEPHALIC   NAVEL. 


cKl 


,^r/^^'f^iw 

u -#%'•";••-—- J 

uy---/--_.»^;.     '   ,    ,      « 

Pu. 


cKl 


Ve. 


24I 


vt.Kt 


147 


FIG.  143. — Limulus  embryo,  stage  G.     Note  the  segmental  sense  organs,  and  the  absense  of  segmented  thoracic 

plates. 

FIG.  144. — Same,  stage  H.     The  haemal  portion  of  the  blastoderm,  just  beyond  the  germ  wall,  is  beginning  to  pro- 
liferate, marking  the  beginning  of  the  dorsal  organ,  or  cephalic  navel,  d.o. 
FIG.  145. — Same,  stage  I.     The  entire  haemal  blastoderm  is  in  active  proliferation. 

FIG.  146. — Same,  stage  J.  The  germ  wall  and  vascular  area  are  very  conspicuous;  the  posterior  limb  of  the 
germ  wall  has  moved  forward  to  the  haemal  portion  of  the  thorax,  narrowing  the  area  of  the  cephalic  navel. 

FIG.  147. — Same,  stage  K.  The  abdominal  lobe  is  distinctly  marked  off;  the  lateral  eye,  I.e.,  has  moved  back- 
ward into  the  fourth  thoracic  segment,  and  the  cephalic  navel  is  confined  to  the  haemal  surface  of  the  anterior 
thoracic  region. 

FIG.  148. — Same,  stage  L.     The  full  complement  of  branchial  segments  have  appeared,  and  the  margins  of 
th  ir  lateral  plates  have  united  on  the  haemal  surface  to  form  the  heart;  the  lateral  eye  lies  hsemal  to  the  tho- 
racic sense  organ. 
16 


242 


EARLY    STAGES    OF  ARTHROPOD  AND   VERTEBRATE   EMBRYOS. 


p.e. 


Kit 
_     ivm. 


FIG.  149. — Same,  stage  L,  seen  from  the  hasmal  surface,  showing  the  completion  of  the  sixth  thoracic  and  all 
the  abdominal  segments.  Five  great  masses  of  fiber  cells,  surround  the  haemo-neural  muscles  h.n.  m.l~'°,  that 
divide  the  thoracic  yolk  mass  into  five  lateral  lobes. 


M 


FIG.  150. — Same,  stage  M.     Two  great  masses  of  fiber  cells  av.,  and  the  remnants  of  the  haemal  blastoderm,  are  seen 

on  either  side  of  the  cephalic  navel,  c.nv. 


CONCRESCENCE. 


243 


as  a  means  of  absorbing  nutrition.  In  the  vertebrates,  tunicates,  enteropneusta, 
amphioxus,  and  pterobranchia  the  cephalic  fistula  became  a  permanent  opening 
into  the  enteron,  thus  giving  rise  to  a  new  oral  opening,  or  hsemostoma,  at  the 
time  when  the  old  mouth  was  about  to  close  up. 

V.  CONCRESCENCE  AND  THE  CAUDAL  NAVEL  OR  "BLASTOPORE." 

We  have  seen  that  the  body  of  segmented  animals  consists  of  an  axial  cord 
and  a  right  and  left  series  of  segments,  or  half  metameres,  and  that  the  body 


Vmm. 


0 


FIG.  151. — Same.  Embryo  just  before  it  escapes  from  the  inner  egg  membrane,  stage  O.  The  cephalic 
navel  has  closed  and  the  degenerating  remnants  of  the  haemal  blastoderm  are  seen  enclosed  in  the  first  four  pairs  of 
liver  lobes,  giving  them  a  mottled  appearance.  The  lobes  are  still  partly  filled  with  yolk. 

grows  in  length  by  the  formation  of  new  segments  at  the  anal  plate,  and  in  breadth 
by  the  increase  in  length  of  the  half  metameres.  But  as  each  half  metamere 
grows  older  and  longer  its  peripheral  end  increases  in  width  faster  than  its  central 
end.  Hence,  as  new  segments  are  successively  formed,  the  lateral  margins  of  the 
germinal  area  increase  in  length  faster  than  the  axial,  and  the  germinal  area,  in- 
stead of  forming  an  elongated  band  of  nearly  uniform  width,  or  a  triangle,  forms 
first  a  heart-shaped  figure,  and  finally  one  in  which  the  peripheral  margins  con- 
cresce  in  front  of  and  behind  the  main  axis.  (Fig.  157,  A.C.)  When  this  has 


244  EARLY   STAGES    OF  ARTHROPOD  AND   VERTEBRATE   EMBRYOS. 


pr 


FIG.    152. — Limulus  larva  in  the  trilobite  stage.     Haemal  surface. 


FIG.  153. — Same  from  the  neural  surface,  the  free  portions  of  the  appendages  removed. 


CONCRESCENCE. 


245 


taken  place,  apical  growth  can  no  longer  proceed  over  the  surface  of  the  yolk,  it 
must  take  place  by  the  piling  up  of  one  segment  on  top  of  another,  in  the  shape 
of  small  closed  rings. 

Concrescence,  therefore,  is  the  inevitable  result  of  apical  and  bilateral  growth 
over  a  spherical  yolk  surface. 

The  amount  of  concrescence  in  a  given  animal  depends  on  the  ratio  of 
marginal  to  apical  growth,  and  upon  the  radius  of  the  yolk  sphere.  Another 
factor  that  materially  affects  the  form  of  the  embryo,  is  the  gradual  suppression 
of  the  lateral  margins  of  the  more  anterior  segments. 


FIG.   154. — Young  Limulus,  after  the  shedding  of  the  trilobite  shell.     The  segmental  sense  organs  of  the  fourth 
thoracic  segment,  and  the  piocephalic  sutures  of  the  trilobite  stage,  have  disappeared. 

Embryonic  apical  growth  represents  the  phylogenetic  method  of  adding  new 
body  segments  to  the  old,  but  it  is  accomplished  in  a  very  different  manner  in 
one  case  from  the  other.  The  embryo,  for  example,  produces  a  succession  of  flat 
bands  on  a  curved,  or  flat  surface,  each  one  arising  under  different  conditions  from 
the  preceding  one,  and  forming  closed  rings  as  best  they  may.  In  the  adult  an- 
cestral forms,  all  the  new  segments  were  produced  under  essentially  like  con- 
ditions, that  is,  as  small  closed  rings  at  the  apex  of  a  cylinder. 

In  Limulus,  an  oblong  area  is  enclosed  between  the  posterior  part  of  the  con- 
crescing  germ  wall  and  the  broad  anal  plate.  This  area  and  its  surrounding 
germ  wall,  I  shall  call  the  caudal  navel.  (Fig.  141,  c.nv.) 

The  inward  proliferation  in  this  region  is  very  conspicuous  and  might  be 


246        EARLY  STAGES  OF  ARTHROPOD  AND  VERTEBRATE  EMBRYOS. 

mistaken  for  an  extension  of,  or  as  a  product  of,  apical  growth.  This  is  not  the 
case.  The  yolk  cells,  or  endoderm  cells,  formed  around  the  posterior  end  of  the 
primitive  streak  and  from  the  walls  of  the  telopore,  belong  to  the  primitive  germinal 
area,  and  are  the  products  of  true  apical  growth.  The  post-anal  proliferation  in 
the  caudal  navel  is  of  a  different  nature.  It  represents  an  isolated  part  of  the 
degenerating  haemal  blastoderm,  enclosed  by  a  narrowing  area  formed  by  the  germ 
wall.  The  area  is  finally  closed  over  by  the  definitive  ectoderm,  leaving  behind 
a  special  cloud  of  mesoderm  and  yolk  cells  formed  by  the  united  germ  walls. 


The  conditions  just  described  for  Limulus  and  other  arachnids  give  us  the  clue 
to  the  correct  interpretation  of  the  phenomena  of  concrescence  in  the  vertebrates. 
Here  there  is  no  doubt  a  true  axial,  or  apical  growth,  and  a  false  axial  growth 
formed  by  the  post-apical  concrescence  of  two  bands  representing  the  proliferating 
margins  of  the  germinal  area.  But  in  vertebrates  it  is  difficult  to  distinguish  be- 
tween that  part  of  the  embryo  formed  by  apical  growth  and  that  formed  by  con- 
crescence, and  it  has  been  assumed  that  there  is  no  real  distinction  between  them. 
The  primitive  streak  of  vertebrates  for  example  is  often  regarded  as  an  ancient 
line  of  concrescence,  and  the  real  concrescence  that  takes  place  behind  it,  as  a 
continuation  of  the  same  process,  more  formally  expressed.  Both  the  primitive 
streak  and  the  actual  line  of  concrescence  are  supposed  to  represent  different  phases 
of  "a  modified  method  of  uniting  the  lips  of  a  greatly  elongated  gastrula  mouth." 
Minot.  Embryology,  p.  126.  But  according  to  our  interpretation,  there  is  no 
remnant  whatever  of  a  gastrula  mouth  at  the  caudal  end  of  any  segmented  animal. 
The  real  apex  of  the  body  is  an  actively  growing  point  composed  of  proliferating 
teloblasts  that  give  rise  to  the  axial  parts  of  the  body.  True  axial  growth  cannot 
take  place  by  concrescence,  because  the  parts  thus  united  represent  the  extreme 
lateral  or  haemal  ends  of  the  metameres  forced  into  the  neural,  or  axial,  position 
by  the  peculiar  exigencies  of  apical  growth  on  a  spherical  surface. 

We  may  recognize  in  vertebrates,  as  in  arachnids,  an  axial  telopore,  or  primi- 
tive streak,  and  post-apical  concrescence  of  the  margins  of  the  germinal  area. 
But  these  modes  of  growth  are  so  blended  with  one  another  in  vertebrates  that  it 
is  extremely  difficult  to  tell  where  one  begins  and  the  other  ends. 

The  actual  separation  of  the  products  of  teloblastic  growth  varies  widely  in 
different  segmented  animals.  But  the  differences  are  in  degree,  or  in  method,  not 
in  kind,  or  in  end  results.  For  example,  in  forms  like  Cymothoa,  there  is  a  trans- 
verse row  of  large  superficial  teloblasts,  which  like  the  cells  at  the  apex  of  a  growing 
plant  stem,  give  rise  in  the  most  precise  and  regular  manner,  to  the  various  parts 
of  the  trunk. 

In  Limulus,  in  birds,  reptiles,  and  mammals,  there  is  a  true  axial  infolding  or 
primitive  streak,  and  the  various  products  of  apical  growth  that  extend  forward 
from  it  may  be  recognized  almost  as  fast  as  they  are  laid  down.  In  amphioxus 
and  the  tunicates,  there  is  a  much  more  extensive  apical  infolding,  and  the  products 


CONCRESCENCE,    AND   APICAL   GROWTH. 


247 


of  apical  growth,  which  form  the  walls  of  the  infolding,  are  not  formally  separated 
into  endoderm,  mesoderm,  and  notochord  till  a  relatively  late  period.  To  call 
this  infolding  an  "archenteron,"  or  "  primitive  gut,"  and  to  then  conclude  that 
the  notochord  was  once  a  part  of  an  alimentary  canal,  because  it  is  for  a  brief 
period  united  with  the  definitive  endoderm,  is  no  more  justifiable  than  it  would  be 


Kn.m.'. 


FIG.  155. — Photograph  of  a  half  grown  Limulus 

to  assume  that  the  endoderm  is  historically  derived  from  an  internal  skeleton,  be- 
cause for  a  time  it  is  continuous  with  the  notbchord. 

As  I  pointed  out  in  my  first  contribution  on  this  subject,  in  1889,  the  only  avail- 
able criterion  as  to  what  constitutes  endoderm  is  evidence  that  it  forms  the  lining 
of  a  functional  alimentary  canal.  From  this  point  of  view,  it  is  clear  that  the  only 


248  EARLY    STAGES    OF   ARTHROPOD   AND    VERTEBRATE    EMBRYOS. 

parts  of  the  so-called  "archenteron"  that  can  be  justly  regarded  as  endodermic  are 
the  two  lateral  bands  that  in  both  vertebrates  and  arthropods  actually  form  the 
lining  of  a  functional  alimentary  canal.  (Fig.  270.) 

True  gastrulation,  comparable  with  the  formation  of  an  enteric  chamber  in 
ccelenterates,  is  a  much  less  important  process  in  arthropods  and  vertebrates  than 
has  been  supposed,  and  is  confined  to  the  central  region  of  the  procephalon. 


CHAPTER  XIV. 

THE  OLD  MOUTH  AND  THE  NEW;  LOCOMOTOR  AND  RESPIRA- 
TORY APPENDAGES. 

The  closing  of  the  invertebrate  mouth,  the  formation  of  a  new  one,  and  the 
evolution  of  segmental  appendages  into  leg-jaws,  gill-sacs,  and  locomotor  append- 
ages are  complex  independent  processes,  but  they  are  so  interwoven  with  one 
another  in  the  early  history  of  the  vertebrates  that  they  may  be  appropriately 
treated  together. 

The  salient  features  of  the  mouth  and  appendages  in  arthropods  and  verte- 
brates may  be  contrasted  as  follows: 

a.  In  arthropods,  the  mouth  lies  on  the  neural  surface,  and  the  foregut  passes 
through  the  brain  floor  between  the  two  nerve  cords,  just  behind  the  forebrain. 
(Fig.  43.)     In  vertebrates  the  mouth  lies  on  the  haemal  surface  of  the  head,  and 
the  foregut,  without  passing  through  the  brain  floor,  leads  directly  into  the  mid- 
gut.     (Fig.  44.) 

b.  In  arthropods  there  are  many  pairs  of  appendages,  the  most  conspicuous 
ones  being  arranged  in  rigid  metameric  order  either  on  the  sides  or  on  the  neural 
surface  of  the  first  thirteen  to  eighteen  metameres.     They  may  be  absent  in  some 
metameres,  while  in  others  they  assume  a  great  variety  of  forms  suitable  for  loco- 
motion, sense  organs,  jaws,  gills,  etc.     (Fig.  3,  A.C.)     In  primitive  vertebrates, 
metamerically  arranged    appendages   like   those   of  arthropods,   appear  to  be 
absent.     The  paired  locomotor  appendages,  when  present  (pectoral  and  pelvic 
fins)  are  not  segmentally  arranged;  they  are  merely  local  expansions  of  longi- 
tudinal folds,  and  they  always  lie  posterior  to  the  (i6±)  metameres  that  constitute 
the  head. 

c.  In  arthropods  the  jaws  are  formed  from  several  pairs  of  modified  legs  that 
belong  to  the  metameres  lying  just  behind  the  forebrain.     The  basal  joints  of  the 
leg-jaws  act  as  crushing  mandibles,  or  as  supplementary  jaws.      In  chewing, 
tasting,  or  preparing  food,  they  work  crosswise,  to  and  from  the  median  neural 
line.     In  true  vertebrates  the  jaws,  in  the  adult  stages,  consist  of  two  unpaired 
arches,  or  an  upper  and  a  lower  jaw.     They  lie  on  the  haemal  surface  instead  of 
the  neural,  and  in  chewing  move  forward  and  backward  instead  of  crosswise. 

d.  In   the  arthropods,   respiration  is  usually   accomplished  by  means  of 
specially  modified  appendages  that  either  project  outward  above  the  surface  of  the 
body,  or  inward,  forming  ectodermic  pouches,  with  vascular,  lamellate  walls. 
In  fishlike  vertebrates  the  gills  may,  for  a  brief  early  period,  consist  of  external 
appendages  of  ectodermic   origin,    but    in    their   later  stages   they  consist  of 

249 


250  THE  OLD  MOUTH  AND  THE  NEW. 

pouches  that  lead  from  the  gut  to  the  exterior;   and   the  whole  or  a  part  of  the 
respiratory  tissue  of  the  gill  pouch  is  said  to  arise  from  the  endoderm. 

These  striking  differences  are  apparently  irreconcilable,  and  have  led  many 
zoologists  to  the  conclusion  that  there  can  be  no  direct  genetic  relation  between 
these  two  groups  of  animals.  We  shall  shaw  in  this  chapter  that  there  is  no  real 
foundation  for  this  belief,  for  when  the  facts  are  known  and  their  meaning  is 
made  clear,  it  will  be  seen  that  the  vertebrate  organs  in  question  are  of  the  same 
nature  as  those  in  arthropods.  It  is  true  they  have  undergone  a  remarkable  meta- 
morphosis, but  it  is  one  brought  about  in  a  perfectly  natural  and  consecutive 
manner  by  the  action  of  definite  internal  forces  that  can  be  recognized  and  their 
probable  effects,  in  a  measure,  estimated. 


Argument. — Briefly  stated,  the  argument  and  the  evidence  to  be  presented  is 
as  follows:  We  shall  show  i,  that  during  the  evolution  of  the  arthropods  the  primi- 
tive entrance  to  the  midgut  was  being  gradually  closed,  and  in  some  cases  actually 
was  closed,  because  the  mouth  was  shifting  into  a  more  and  more  inaccessible 
position,  and  because  the  stomodaeum  was  becoming  more  and  more  constricted 
by  the  growth  of  the  surrounding  organs.  The  remnants  of  this  now  useless 
passageway  may  still  be  seen  in  its  proper  position  on  the  floor  of  the  closing 
neural  canal  of  vertebrates;  this  passageway  is  the  infundibulum,  and  the  remnants 
of  the  foregut  is  the  saccus  vasculosus  and  the  posterior  part  of  the  hypophysis. 

2.  That  the  foundations  of  a  new  mouth  are  already  established  in  arthropods 
in  the  cephalic  navel,  or  so  called  "  dorsal  organ,"  which  lies  on  the  haemal  side 
of  the  head  in  a  position  corresponding  to  that  of  the  mouth  in  vertebrates.     It 
affords  a  transitory  opening  from  the  exterior  into  the  midgut,  and  it,  or  the  adja- 
cent tissues,  may  serve  as  a  means  of  attaching  the  animal  to  foreign  objects,  or 
to  its  host.     Thus  in  the  arthropods,  an  organ  of  very  great  antiquity  and  habits 
long  established  are  prepared  to  perform  the  work  of  a  new  mouth  after  the  verte- 
brate fashion,  as  soon  as  the  old  mouth  becomes  permanently  closed. 

3.  That  in  the  arthropods  several  pairs  of  leg- jaws  surround  the  mouth  on  the 
neural  surface  of  the  body,  and  that  the  prevailing  conditions  in  the  arthropod 
head  tend  to  crowd  the  basal  portions  of  these  appendages  toward  the  haemal  sur- 
face so  that  they  converge  around  the  infolding  cephalic  navel  in  the  same  manner 
that  the  oral  arches  of  vertebrates  converge  around  the  mouth.     It  will  be  shown 
that  in  vertebrate  embryos  the  oral  arches  first  appear  on  the  neural  surface  as 
three  or  four  pairs  of  appendicular  arches,  and  that  they  then  gradually  shift 
toward  the  haemal  side,  converging  toward  the  "anlage"  of  the  new  mouth,  and 
forming  the  paired  rudiments  of  the  premaxillary,  maxillary  mandibular,  and 
hyoid  arches.     These  paired  arches  finally  unite,  the  first  two  pairs  forming  the 
upper,  and  the  third  pair,  the  lower  jaw. 

4.  That  in  the  ostracoderms,  the  oldest  fossil  vertebrate-like  arthropods,  the 
mouth  lies  between  paired  jaws,  which  in  chewing  move  to  and  from  the  middle 


THE    CLOSING    OF    THE    OLD    MOUTH.  251 

line.  Here,  therefore,  the  adult  condition  of  the  jaws  is  intermediate  between  the 
typical  arthropod  and  the  typical  vertebrate  condition,  and  is  similar  to  the  condi- 
tion of  the  jaws  in  the  higher  vertebrate  embryos. 

5.  That  the  gill  pouches  of  vertebrates  may  be  interpreted  as  invaginated 
respiratory   appendages,  which   have  become   secondarily   united   with   enteric 
pouches. 

6.  That  the  free  portions  of  the  cephalo-thoracic  appendages  of  arthropods 
are  represented  in  vertebrates  by  embryonic  oral  tentacles,  such  as  the  "balancers" 
and  the  external  gills  of  amphibian  larvae,  and  the  oar-like  appendages  of  the 
ostracoderms. 

7.  That   the   paired   appendages  of  typical  vertebrates,  i.e.,  pectoral  and 
pelvic  fins,  arise  from  a  new  generation  of  post-branchial  metameres  that  are  not 
represented  in  arthropods.     The  lateral  fold  from  which  they  arise  may  be  re- 
garded either  as  a  marginal  fringe  of  rudimentary  appendages  or  as  a  series  of 
keel-like  pleurites. 

I.  THE  CLOSING  OF  THE  OLD  MOUTH. 

It  will  be  recalled  that  the  alimentary  canal  of  arthropods  is  formed  in  three 
separate  sections,  the  midgut  arising  from  the  endoderm,  while  the  foregut  and 
hindgut  arise  from  separate  infoldings  of  the  ectoderm.  (Fig.  43.) 

The  infolding  for  the  foregut,  or  stomodaeum,  is  always  formed  in  the  median 
portion  of  the  procephalic  lobes  (Fig.  25);  the  lateral  cords  lying  on  either  side 
of  it,  and  cross  commissures  in  front  and  behind.  (Fig.  46,  A.)  As  these  impor- 
tant parts  of  the  nervous  system  are  formed  at  a  very  early  period  and  are  never 
known  to  be  absent,  the  stomodaeum  is  securely  trapped  in  a  nerve  ring  from  which 
there  is  no  escape. 

There  are  several  factors  in  the  evolution  of  arthropods  that  steadily  work 
toward  the  closing  up  of  this  old  passageway,  or  which  make  the  access  to  it  more 
and  more  difficult. 

In  the  more  primitive  arthropods  there  is  ample  room  for  the  stomodaeum  and 
for  the  passage  through  it  of  a  liberal  supply  of  food.  But  during  the  phylogeny 
of  the  phyllopod-arachnid  stock  there  is  a  great  increase  in  the  volume  and  com- 
pactness of  the  anterior  cranial  neuromeres,  which  narrows  in  a  very  marked 
degree  the  opening  between  the  nerve  cords  for  the  passage  of  the  stomodaeum. 
Moreover,  in  all  arthropods  there  is  a  tendency  for  the  rostrum,  which  represents  a 
fused  pair  of  appendages  lying  in  front  of  the  mouth,  to  gradually  work  its  way 
backward,  thus  covering  up  the  original  site  of  the  mouth,  or  carrying  the  entrance 
to  it  a  long  way  back  of  its  original  position.  (Figs.  3,  46.)  It  may  then  be 
surrounded  by  projecting  appendages,  or  it  may  lie  at  the  bottom  of  a  long  atrial 
chamber,  access  to  which  can  be  obtained  only  in  an  indirect  or  roundabout  man- 
ner, as  in  cirripeds  (Figs.  274,  275),  cladocera  and  phyllopods  (Figs.  7,  9  and  273.) 

In  many  cases,  the  animal  can  never  bring  its  mouth  in  direct  contact  with 
its  prey.  Liquid  foods  must  be  pumped  through  long  capillary  tubes,  formed  by 


252  THE  OLD  MOUTH  AND  THE  NEW. 

the  projecting  parts  of  leg-jaws,  as  in  many  insects  and  arachnids;  or  microscopic 
organisms  must  be  kicked  toward,  or  into  the  mouth,  by  the  movements  of  the 
legs,  or  of  the  posterior  part  of  the  body,  as  in  cirripeds  and  cladocera. 

In  some  cases,  a  combination  of  such  conditions  has  actually  led,  in  the  later 
stages  of  development,  to  the  complete  closure  of  the  foregut,  as  in  the  adults 
of  certain  lepidoptera  and  ephemeridae,  which  cease  to  feed  after  metamorphosis; 
or  in  certain  cirripeds,  where  the  stomodaeal  opening  into  the  midgut  is  closed 
soon  after  the  larval  stages.  (Figs.  280,  281.) 

Thus  we  may  recognize  in  the  arthropods  a  steady,  underlying  trend  to- 
ward a  more  compact,  voluminous  nervous  system,  and  toward  a  less  efficient 
stomodaeum.  These  internal  conditions  rigidly  prescribe  the  possible  modes  of 
life  that  are  open  to  the  animals  in  which  they  prevail.  The  pending  extinction  of 
the  foregut  is  the  dominant  factor  in  the  life  history  of  the  arachnids,  for  it 
has  made  a  liquid  diet,  sucked  through  capillary  tubes,  imperative.  For  that 
reason  a  blood  sucking,  or  a  parasitic  mode  of  life,  is  practically  universal  among 
them,  just  as  sucking  the  blood  of  animals,  or  the  juices  of  plants,  is  universal 
in  certain  groups  of  insects.  In  all  these  cases,  the  animals  appear  to  be  making  the 
best  of  a  desperate  situation,  adjusting  their  lives  with  great  precision  to  meet  the 
inevitable  march  of  events  within. 

Another  important  factor  in  the  closing  of  the  old  mouth  was  the  conversion 
of  the  medullary  plate  into  a  medullary  tube.  This  process  is  well  advanced  in 
the  arthropods,  reaching  its  highest  development  in  the  arachnids.  There  a 
true  cerebral  vesicle  is  formed  that  includes  the  whole  forebrain,  although  the 
advancing  palial  fold  of  the  embryo  just  fails  to  reach  and  enclose  the  oral  region. 
(Fig.  46,  B.) 

In  the  arachnids  we  may  recognize  all  the  important  preliminary  steps,  such 
as  the  formation  of  neural  crests,  axial  infolding,  and  a  forebrain  palium,  lead- 
ing up  to  the  conversion  of  the  medullary  plate  into  a  medullary  tube.  But  in  no 
arthropod  does  the  process  reach  a  condition  that  definitely  shuts  up  the  oral 
opening  inside  the  neural  tube,  thereby  cutting  off  access  to  the  foregut  from  the 
outside  world. 

But  this  event  does  take  place  in  the  vertebrates.  In  the  frog  embryos, 
during  the  open  medullary  plate  stage,  we  may  see  a  minute  pit  with  a  faint 
prominence  in  front  of  it  that  appears  to  represent  the  rostrum  and  stomodaeal 
infolding  of  the  arachnids.  (Fig.  25.)  As  the  medullary  plate  closes,  the  pit 
deepens,  giving  rise  to  the  infundibulum,  or  the  ancient  passageway  for  the 
stomodaeum,  while  the  epithelial  sac  that  lines  it  and  projects  out  of  it,  is  the 
saccus  vasculosus  and  the  posterior  part  of  the  hypophysis,  or  the  remnants 
of  the  stomodaeum  itself. 

We  have  shown  in  the  chapters  on  the  nervous  system  that  the  location  and 
relations  of  the  principal  nerve  centers  and  tracts,  and  especially  the  location  of 
the  primary  gustatory  and  swallowing  centers  is  entirely  in  harmony^with  this 
view. 


THE    NEW   MOUTH. 


253 


At  just  what  stage  in  the  closing  of  the  medullary  plate,  the  mouth  ceased  to 
communicate  with  the  exterior  cannot,  at  present,  be  determined.  The  final 
closure  was  no  doubt  hastened  by  the  crowding  of  the  optic  ganglia  upward  and 
backward  over  the  oral  region.  This  would  leave  the  broadest  part  of  the  primi- 
tive brain,  i.e.,  the  region  of  the  fourth  ventricle  and  the  rhomboidal  sinus,  wide 
open;  and  in  this  region,  which  is  now  covered  only  by  the  choroid  plexus,  there 
remained,  probably  for  some  time,  an  opening  through  which  the  old  stomodaeum 
could  communicate  with  the  exterior.  (Figs.  3,  46,  D.) 

Thus,  with  the  knowledge  acquired  after  the  event,  we  may  look  back  a  few 
million  years,  and  trace  with  our  mind's  eye,  the  slow,  inevitable  approach  and 
consummation  of  the  most  momentous  event  in  organic  evolution. 

II.  THE  NEW  MOUTH. 

A  group  of  animals  in  which  the  natural  growth  of  one  essential  part  inevita- 
bly eliminates  some  other  part  equally  essential,  is  doomed  to  extinction,  unless 
among  the  organs  already  at  hand  a  radical  redistribution  of  functions  is  possible 
at  the  moment  the  critical  period  arrives.  No  organ  was  ever  created,  we  may  be 
sure,  to  meet  an  organic  crisis  in  the  future,  or  was  ever  produced,  de  novo,  at  the 
demand  of  a  present  necessity.  We  are  bound  to  assume  that  unless  a  suitable 
organ  stands  ready  to  do  the  work  of  the  one  that  has  been  eliminated,  no  way 
out  of  the  difficulty  is  possible. 

Such  was  the  situation  when  the  great  crisis  in  the  evolution  of  vertebrates 
was  at  hand;  either  the  evolution  of  the  brain  must  cease,  or  a  new  entrance  to  the 
midgut  must  be  established  elsewhere.  The  alimentary  organs  proved  most 
pliable.  But  how  significant  it  is  that  the  momentum  of  nerve  growth  should  so 
dominate  the  growth  of  other  organs,  and  the  form  of  the  nervous  system  so 
modify  that  of  the  whole  body! 

No  doubt  one  important  factor  in  the  competitive  development  of  brain  and 
stomodaeum  is  the  increasing  volume  of  the  yolk  sphere;  for  the  presence  of  more 
yolk  postpones  to  a  later  and  later  embryonic  stage  the  time  when  the  stomodaeum 
becomes  functional,  and  thus  allows  the  precociously  developing  nervous  system  to 
undergo  its  early  stages  of  development,  unmodified  by  the  action  of  the  stomo- 
daeum in  feeding. 

In  the  arthropods,  a  very  old  organ,  the  "dorsal  organ,"  or  cephalic  navel, 
having  originally  a  very  different  function  from  that  of  alimentation,  stood  ready 
to  take  the  place  of  the  old  mouth  that  was  being  slowly  eliminated.  Its  presence 
alone  made  the  existence  of  the  vertebrates,  as  we  know  them,  a  possibility. 

We  have  shown  in  a  previous  chapter,  that  the  "dorsal  organ,"  in  part  at 
least,  is  the  product  of  the  mechanical  conditions  created  by  apical  growth  on  a 
spherical  surface.  It  is,  as  it  were,  a  vortex  center,  toward  which  all  the  adjacent 
organs  converge.  It  exists,  either  actually  or  potentially,  on  the  anterior  haemal 
surface  of  all  arthropod  embryos,  its  definitive  position  being  controlled,  in  any 


254 


THE    OLD    MOUTH   AND    THE    NEW. 


given  case,  by  the  shape  and  volume  of  the  yolk  sphere,  and  by  the  amount  of 
"  cephalization "  that  has  taken  place. 

"Cephalization"  takes  place  according  to  a  definite  law  of  growth  which 
applies  to  all  segmented  animals.  According  to  this  law,  the  lateral  members  of 
the  anterior  metameres  tend  to  atrophy  in  proportion  to  their  relative  distance  from 
the  median  line  and  their  nearness  to  the  anterior  end.  In  other  words,  there  is 
a  steadily  progressive  tendency  to  eliminate  by  degeneration,  the  lateral  members 
of  the  more  anterior  metameres,  and  to  enlarge  and  specialize  the  median  ones.1 

Thus  in  the  higher  arthropods  there  are  no  fully  developed  appendages  or 


FIG.  156. — Diagrams  to  illustrate  the  method  of  bilateral  apical  growth.  A,  Hypothetical  symmetrical 
checker  board  arrangement  of  unlike  parts  in  mercator  projection,  seen  from  the  neural  surface;  The  serial 
equality  (homology),  of  the  elements  is  supposed  to  be  perfect.  B  and  C,  Approximate  method  of  producing 
such  a  field  by  a  combination  of  apical  and  bilateral  division  of  units.  D,  The  same,  seen  as  a  cylindrical  object 
from  the  side.  E,  The  same,  in  cross-section,  composed  of  four,  unlike  concentric  superimposed  layers. 

other  somatic  organs  lateral  to  the  procephalic  lobes;  and  no  organs  in  the  lateral 
plate  region  of  the  first  nine  or  ten  metameres.  The  scanty,  or  degenerating, 
tissues  that  form  where  the  lateral  organs  should  be,  are  gradually  crowded  by  the 
growth  of  the  more  vigorous  posterior  ones  into  an  oval  area  on  the  haemal  surface 
of  the  embryo,  just  behind  the  forebrain,  where  the  degenerating  haemal  blasto- 
derm sinks  into  the  yolk  and  is  absorbed.  (Fig.  135.) 

In  insects,  the  ruptured  embryonic  membranes  play  an  important  part  in  this 
process;  they  complicate  it,  but  do  not  alter  its  essential  nature.  In  many  arth- 
ropod embryos,  the  dorsal  organ  becomes  a  formal  invagination  into  the  yolk, 
with  well-defined  epithelial  walls,  or  it  may  consist  merely  of  a  great  cloud  of  in- 
growing cells.  In  both  cases,  the  deeper  cells  are  the  first  to  dissolve  in  the  yolk 
mass  that  will  later  be  enclosed  within  the  midgut. 

A  "  dorsal  organ"  of  some  such  nature  as  this  occurs  in  all  classes  of  arthropods. 


Not  to  be  confused  with  median  fusion  and  degeneration. 


See  p.  277. 


THE    ORAL   ARCHES.  255 

I  have  called  it  a  cephalic  navel,  or  cephalic  fistula,  because  it  is  a  center  of  con- 
vergent growth;  a  region  of  ingrowth,  or  outgrowth,  around  the  point  where  the 
haemal  surface  of  the  head  and  the  gut  is  finally  closed. 

The  cephalic  navel  usually  closes  during  the  embryonic  stages,  leaving,  in  a 
morphological  sense,  a  barren  area  behind.  But  in  many  cases,  either  the  dorsal 
organ,  or  some  other  organ  in  the  same  place,  persists  as  an  adhesive  disc,  or  a 
voluminous  outgrowth,  by  means  of  which  the  animal  attaches  itself  to  some  for- 
eign object.  In  some  parasitic  cirripeds,  the  cephalic  outgrowth  is  buried  in  the 
tissues  of  the  host,  the  old  mouth  closes,  and  the  animals  are  then  said  to  absorb 
nutrition  through  the  walls  of  the  cephalic  outgrowth. 

We  have  merely  to  assume  for  certain  stages  a  somewhat  longer  duration  than 
now  occurs  in  any  arthropod,  to  make  the  dorsal  organ  a  new  gateway  to  the  gut; 
for  surely  a  cephalic  fistula  leading  into  the  midgut,  permanently  open  at  both 
ends,  and  used  to  hold  fast  to  animate,  or  to  inanimate  objects,  is  competent  to 
take  the  place  of  the  old  mouth. 

The  closing  of  the  old  mouth  and  the  evolution  of  the  new  one,  therefore,  was 
going  on  in  the  same  animals  at  the  same  time.  The  critical  period  of  substituting 
one  for  the  other  was  during  the  embryonic  stages,  when  both  of  the  organs  may 
have  opened  into  the  gut  at  the  same  time.  During  the  embryonic  stages,  a  con- 
siderable time  would  be  available  for  readjustment,  for  owing  to  the  large  amount 
of  food  yolk  in  the  eggs  of  primitive  vertebrates,  a  relatively  long  time  might 
elapse  before  the  absorption  of  food  from  without  became  imperative.  Whether 
the  actual  closing  of  the  old  mouth,  or  the  opening  of  the  new  one,  took  place  first 
or  last,  is  of  little  consequence,  for  the  consummation  of  one  event  would  probably 
accelerate  the  advent  of  the  other. 

III.  THE  JAWS  OR  ORAL  ARCHES. 

In  reconstructing  the  history  of  the  vertebrate  mouth,  it  is  not  enough  merely 
to  account  for  the  closing  of  the  old  mouth  and  the  origin  of  the  new  opening  into 
the  alimentary  canal.  To  make  the  account  complete,  it  is  necessary  to  explain 
the  origin  of  the  important  organs  which  surround  it,  such  as  the  jaws,  or  oral 
arches,  the  hypophysis,  tear  duct,  and  the  principal  outgrowths  from  the  adjacent 
pharyngeal  cavity. 

The  circumoral  organs  of  primitive  vertebrates  are  best  interpreted  as  the 
remnants  of  several  pairs  of  arthropod  leg-jaws,  or  other  appendages,  that  have 
been  crowded  onto  the  haemal  surface  by  the  peculiar  mechanical  conditions  which 
prevail  in  the  developing  head. 

An  arthropod  appendage  may  be  defined  as  an  ectodermic  outgrowth  consist- 
ing of  several  branches,  or  stems,  located  on  the  haemal  or  lateral  side  of  the  body, 
adjacent  to  the  main  nerve  axis.  In  the  higher  forms,  e.g.,  the  arachnids,  one  stem 
forms  the  typical  appendage.  On  its  median  basal  margin  is  a  prominent  sensory 
spur,  or  gustatory  organ,  from  which  in  the  early  embryonic  stages  arises  a  large 


256 


THE    OLD    MOUTH  AND    THE   NEW. 


ganglion,  that  lies  between  the  appendage  and  the  medullary  plate.  (Fig.  136.) 
On  the  outer  margin  there  may  be  a  second  row  of  sense  organs,  and  various 
infoldings  of  an  excretory  or  respiratory  nature.  On  its  posterior  basal  surface  a 
gill,  or  respiratory  plates  or  filaments,  may  be  developed. 

Each  appendage  is  associated  with  a  hollow  block  of  mesoderm,  or  somite, 
that  lies  beneath  the  basal  lobe,  and  gives  rise  to  the  associated  muscular  and  excre- 
tory tissues.  (Fig.  134,  138,  so.)  The  mesoderm  extends  beyond  the  appendages 
as  a  thin  double  layer  of  cells,  the  lateral  plates,  from  which  the  somatic  and 


FIG.  157. — Diagrams  illustrating  the  growth  of  organic  films  on  a  nutrient  surface.  The  figures  are  intended 
to  show  how  association,  and  the  time  element  involved  in  combined  apical  and  bilateral  growth  of  an  organic  film 
on  a  plain,  or  spherical  nutrient  surface,  automatically  creates  lines  of  unlike  conditions  that  are  coincident  with 
the  lines  of  morphological  and  physiological  specialization.  A  B  and  C,  Successive  stages  in  the  growth  of  such  a 
film,  showing  the  necessaiily  unlike  character  of  the  initial  and  terminal  element  (cephalic  and  caudal),  and  of 
the  median  and  lateral  ones,  and  that  this  unlikeness  increases  with  the  progress  of  growth.  D,  A  still  older 
stage,  in  side  view  perspective. 

splanchnic  tissues  of  that  metamere,  if  any  are  present,  arise,  l.pl.  The  cavities 
enclosed  in  the  mesoblastic  somites,  so  long  as  the  somites  retain  their  identity, 
do  not  communicate  with  one  another.  On  the  other  hand,  the  space  between 
the  somatic  and  splanchnic  layers  of  the  successive  lateral  plates,  is  not  divided 
into  separate  compartments  (thorax),  or  if  it  is  (abdomen),  they  speedily  break 
down,  forming  a  continuous  coelomic  chamber  on  each  side  of  the  body.  The 
third  elements  associated  with  the  appendages  are  the  outgrowths  from  the  mid- 
gut  that  form  the  so-called  "liver  lobes,"  or  the  enteric  diverticula,  or  the  gut 
pouches.  (Figs.  150,  154,  179,  180.) 


THE    ORAL  ARCHES. 


257 


In  dealing  with  the  morphology  of  a  true  segmental  appendage,  we  must 
recognize  and  account  for  these  various  parts. 

As  a  rule,  the  free  part  of  the  oral  appendages  in  arthropods  is  very  small, 
or  even  absent,  while  the  basal  portion,  the  somite,  sense  organ,  and  ganglion, 
may  be  of  considerable  size.  If  this  condition  is  associated,  as  it  usually  is  in  the 
higher  arthropods,  with  a  voluminous  and  precocious  forebrain,  a  large  yolk 
sphere,  and  with  the  absence  of  lateral  plate  structures,  the  tendency  during  the 
early  embryonic  periods  will  be  to  raise  the  forebrain  off  the  yolk  surface  and 
thrust  it  forward,  leaving  the  way  open  for  the  basal  lobes  of  the  more  anterior 
appendages  to  unite  on  the  haemal  surface,  around  the  center  formed  by  the  cephalic 
navel.  (Figs.  17,  31,  33,  135.) 


m-7do.      ht. 


FIG.    158. — M creator  projections   of   vertebrate   embryos. 

Under  the  influence  of  these  conditions,  the  more  anterior  appendages  in 
certain  adult  arthropods  have  been  transferred,  either  to  the  anterior  surface  of 
the  head,  to  a  position  halfway  between  the  neural  and  haemal  surfaces,  i.e.,  the 
chelicerae  of  arachnids  (Figs.  17,  43),  or  almost  to  the  haemal  surface,  i.e.,  the 
antennae  of  cladocera  (Fig.  9),  and  many  parasitic  copepods  (Figs.  282,  283);  or 
they  are  transferred  definitely  to  the  haemal  surface,  close  to  the  region  of  the 
cephalic  navel,  i.e.,  the  antennae  in  all  cirripeds.  (Figs.  274,  280.) 

It  is  seen,  therefore,  that  in  the  arthropods  the  increasing  size  and  precocity 
of  the  forebrain,  the  degeneration  of  the  lateral  members  of  the  anterior  meta- 
meres,  the  increasing  size  of  the  yolk  sphere,  and  the  time  factors  involved  in 
apical  growth  on  a  spherical  surface,  all  conspire  to  crowd  the  appendages  and 
their  associated  parts  toward  the  haemal  surface  of  the  head.  (Compare  the 
mercator  projections  in  Fig.  157  with  Figs.  17,  31,  32,  158,  160.) 

********* 

Development  of  the  Oral  Arches  in  the  Frog.— In  the  embryos  of  primi- 
tive vertebrates,  the  transfer  of  oral  arches  to  the  haemal  surface  of  the  head  is 


258  THE  OLD  MOUTH  AND  THE  NEW. 

accomplished  in  the  manner  indicated  for  arthropods,  and  the  successive  steps  in 
the  process  may  be  followed  with  comparative  ease  in  the  frog. l  In  the  study  of 
this  process  in  the  frog,  large  numbers  of  eggs  were  hardened,  usually  in  chromic 
or  picric  acid  solutions,  the  membranes  removed  by  eau  de  Javelle,  and  the  most 
sharply  sculptured  specimens  examined  as  opaque  objects,  under  a  strong  oblique 
illumination. 

Soon  after  the  closure  of  the  medullary  plate  one  may  see  the  outlines  of 
two  or  three  pairs  of  faintly  marked  ridges  that  represent  the  earliest  stages  of 
the  oral  arches.  (Fig.  159.)  Behind  them  are  two  pairs  of  more  prominent  ones, 
representing  either  the  first  two  gill  arches,  or  the  hyoid  arch  and  the  first  gill 
arch.  The  primitive  oral  arches  curve  downward  and  forward,  eventually  unit- 
ing between  the  anterior  end  of  the  brain  and  the  anlage  of  the  sucking  disc,  at 
the  point  where  the  mouth  appears  later.  (Fig.  160.) 

The  general  appearance  of  the  primitive  oral  arches,  and  the  rate  at  which 
they  concresce,  varies  considerably  in  different  embryos,  and  it  has  not  been  pos- 
sible to  identify  them  with  those  that  are  seen  in  this  region  at  a  later  period, 
but  the  two  shown  in  Figs.  159,  160,  appear  to  represent  the  anlage  of  the  pre- 
maxillary  and  maxillary  arches.  After  concrescence  takes  place,  the  arches,  for 
a  short  period,  become  indistinguishable. 


FIG.  159. — Frog  embryos,  seen  from  side,  showing  the  extension  of  the  primitive  oral  arches,  gill  arches,  and  lateral 
plates,  toward  the  haemal  surface.     Rana  septemtrionalis. 

In  the  following  stages  (Fig.  161,  A),  the  oral  region  is  bounded  in  front  by  a 
conspicuous  transverse  groove  that  terminates  at  either  end  in  the  nasal  pit,  ol.o.} 
on  the  posterior  side  it  is  bounded  by  the  sucking  disc.  A  longitudinal  groove  now 
extends  along  the  middle  of  the  intervening  space,  intersected  by  three  transverse 
ones,  a  small  pit  being  formed  at  each  intersection.  The  oral  field  is  thus  divided 
into  at  least  three  pairs  of  lobes  that  are  clearly  serially  homologous  with  one 
another.  The  first  pair  represents  the  premaxillae,  p.mx.,  the  second,  the 
maxillae,  mx.,  and  the  third,  the  mandibles,  md. 

In  the  following  stages,  the  lobes  become  more  prominent,  and  the  longitud- 
inal groove  becomes  deeper  and  wider,  B  and  C.  The  premaxillary  lobes  then 

1  Mr   E    E.  Just,  1906,  and  Mr.  A.  O.  Kelly,  1908,  students  in  biology  at  Dartmouth,  have  assisted  in  work- 
ng  out  the  history  of  the  embryonic  and  larval  jaws  of  the  frog. 


THE    ORAL   ARCHES. 


259 


unite,  obliterating  a  part  of  the  median  groove.  Between  the  anterior  margins 
of  the  premaxillary  lobes  a  small  pit  is  left  that  gradually  deepens,  forming  the 
anlage  of  the  hypophysis,  D,  E,  F,  hyp. 

The  prominent  maxillary  lobes  move  laterally  and  forward  and  unite  with  the 
premaxillae,  although  they  are  still  distinctly  marked  off  from  them  by  the  second 


op.iv 


a'd.0 


FIG.  160. — Frog  embryos  seen  from  the  anterior  end,  showing  the  concrescence  of  the  oral  metamers  in  front,  and  on 
the  haemal  side  of  the  fore  brain.     Rana  septemtrionalis. 


FIG.  161. — Frog  embryos,  seen  from  the  anterior  haemal  sufrace  of  the  head,  showing  successive  stages  in  the 
concrescence  of  the  three  pairs  of  oral  arches,  the  pre-maxillae,  maxillae,  and  mandibles,  and  their  relation  to  the 
apex  of  the  forebrain,  to  the  mouth,  hypophysis  and  sucking  discs.  R.  septemtrionalis. 

transverse  groove,  E  and  F,  mx.  This  groove  finally  extends  past  the  olfactory 
.pits  toward  the  eyes,  probably  initiating  the  formation  of  the  tear  duct.  The 
mandibular  lobes,  meantime,  have  become  very  prominent.  Later  they  unite  in 
the  median  line  to  form  the  lower  jaw,  E  and  F}  md. 


260  THE    OLD    MOUTH  AND    THE    NEW. 

In  the  early  larval  stages,  all  traces  of  the  paired  oral  arches  have  disappeared 
and  we  have  instead  the  characteristic  larval  mouth  of  the  amphibia,  with  the 
V-shaped  mandibles  and  the  maxillae  sheathed  in  horn,  surrounded  by  prominent 
lips.  The  latter  form  a  shallow  antechamber  fringed  with  sensory  papillae,  that 
resembles  the  pre-oral  in  chamber  Amphioxus,  Bothriolepis  and  the  cyclostomes. 

(Figs.  164, 165, 166,  i;;-1?^) 

The  mouth  itself,  owing  to  the  gradual  union  of  the  paired  arches  on  the 
haemal  surface,  undergoes  a  remarkable  transformation^  It  first  appears  as  a 
very  long  median  furrow.  As  the  anterior  end  is  obliterated  by  the  union  of  the 
premaxillae  and  the  posterior  end  by  the  union  of  the  mandibles,  the  remaining 
median  portion  widens,  taking  on  first  an  hexagonal  contour,  and  then  the  form 
of  a  transverse'  slit,  with  a  continuous  maxillary  arch  in  front  and  a  mandibular 
arch  behind.  (Fig.  161.) 

The  median  groove  that  initiates  the  opening  into  the  enteron,  may  be 
regarded  as  the  remnant  of  the  primitive  cephalic  navel  of  arthropods,  and  its 
subsequent  changes  in  form,  as  the  result  of  the  way  in  which  the  oral  arches 
unite  around  it. 

The  anterior  end  of  this  groove,  or  the  part  lying,  during  the  earliest  stages, 
between  the  premaxillary  lobes,  becomes  deeper  than  the  rest,  and  marks  the 
beginning  of  the  mouth,  C.  A  little  later,  this  depression  is  most  pronounced 
between  the  maxillary  lobes,  E  and  F. 

The  hypophysis  may  be  regarded  as  the  remnant  of  a  pair  of  excretory 
glands  similar  to  those  on  the  outer  margin  of  the  anterior  cephalic  appendages  in 
arachnids.  In  ancestral  vertebrates  they  were  probably  situated  on  the  margins 
of  the  premaxillary  lobes,  their  unpaired  condition  being  due  to  the  median  fusion 
of  the  appendages  to  which  they  belonged. 

The  "tear  duct"  may  be  regarded  as  a  specialization  of  the  groove  that 
originally  separated  the  maxillary  and  premaxillary  arches. 

The  development  of  the  mouth  and  oral  arches  of  the  frog  may  be  regarded 
as  the  typical  mode  of  development  in  vertebrates.  A  condition  like  that  just 
described  is  clearly  present  in  other  amphibia,  as  in  Amblystoma  (Fig.  168),  and 
in  the  sturgeon  (Fig.  174).  Even  in  the  mammals  we  may  see  indications  of  the 
same  structure,  the  fronto-nasal  process  probably  representing  in  part  the  fused 
premaxillary  lobes  of  the  frog. 

In  Bdellostoma,  three  pairs  of  oral  lobes,  comparable  with  those  in  the  frog 
are  preserved  even  in  adult  stages.  (Fig.  175.)  In  petromyzon  there  has  been  a 
greater  median  fusion,  for  the  remnants  of  the  premaxillary  lobes,  the  olfactory 
pits,  and  the  hypophysis  have  apparently  been  absorbed  in  a  single  median  infold- 
ing lying  in  front  of  the  maxillary  arch. 

In  the  most  primitive  vertebrate-like  animals  of  all,  the  ostracoderms,  the 
mouth  parts  of  the  adult  were  in  a  condition  similar  to  those  of  Bdellostoma,  or  to 
those  in  the  embryonic  stages  of  the  frog  and  sturgeon.  (See  Chapter  XX,  p.  373.) 

Conclusion. — We  may  therefore  conclude  that  the  mouth  of  vertebrates  is 


THE    GILL   ARCHES   AND    THE    EXTERNAL    GILLS.  261. 

surrounded  by  at  least  three  distinct  pairs  of  segmentally  arranged  arches,  com- 
parable on  the  one  hand  with  the  gill  arches  of  the  postoral  region,  and  on  the 
other  with  the  cephalic  appendages  of  arthropods. 

The  prevalent  view,  first  advanced  I  believe,  by  Gegenbaur,  that  there  is  in 
vertebrates  but  a  single  pair  of  oral  arches  consisting  of  the  mandibles — the  maxil- 
lae being  regarded  merely  as  a  forward  extension  of  their  proximal  ends — was 
based  largely  on  the  relations  of  the  skeletal  structures  of  adult  fishes,  and  is 
clearly  untenable. 

IV.  THE  GILL  ARCHES  AND  THE  EXTERNAL  GILLS. 

The  oral  arches  are  clearly  comparable  with  the  gill  arches  that  lie  behind 
them.  In  the  embryos  of  primitive  vertebrates,  however,  the  gill  arches,  owing 
to  their  more  posterior  position  and  to  the  presence  of  cardiac  elements  on  their 
lateral  margins,  do  not  unite  on  the  haemal  surface  of  the  head,  although  the  more 
anterior  ones  move  almost  as  far  in  that  direction  as  the  oral  arches  do.  (Figs. 
162,  163,  168.) 

A  conspicuous  feature  of  the  gill  arch  in  primitive  vertebrates  is  the  external 
gill.  Budgett  has  well  described  them  as  follows.  He  states  (p.  274),  that  in 
Lepidosiren,  Protopterus,  and  in  the  more  primitive  amphibia,  each  gill  "arises  as 
an  outgrowth  from  the  outer  side  of  the  visceral  arch,  and  is  composed  of  a  mesen- 
chymatous  core  with  ectodermal  covering.  .  .  .  They  develop  well  before  the 
perforation  of  the  gill  clefts.  .  .  .  and  the  aortic  arch  itself  is  in  early  stages 
simply  the  vessel  of  the  external  gill."  He  concludes  that  the  external  gills  are 
organs  of  great  antiquity,  which  were  probably  characteristic  of  primitive  verte- 
brates, not  merely  larval  adaptations  of  no  special  significance. 

For  my  own  part,  I  see  no  reason  to  doubt  that  the  external  gills  of  vertebrates 
represent  the  remnants  of  the  thoracic  appendages  of  their  arthropod  ancestors, 
for  they  strongly  resemble  them  in  form,  position,  and  direction  of  growth.  Com- 
pare, for  example,  the  external  gills  of  an  embryo  Protoperus  (Fig.  173),  with  those 
of  an  arachnid  (Figs.  26-32),  and  observe  also  the  identity  in  their  arrangement 
on  the  mercator  projections  (Figs.  157,  158),  and  the  details  of  their  structure  and 
mode  of  branching. 

In  many  primitive  vertebrates,  similar  organs  are  found  on  the  oral  arches. 
In  the  larvae  of  Amblystoma  (Figs.  168,  169),  there  are  long  rod-like  appendages 
attached  either  to  the  mandibular  or  to  the  hyoid  arch.  These  so-called  "  balan- 
cers" are  clearly  serially  homologous  with  the  external  gills  of  the  more  posterior 
arches.  Similar  organs  are  found  in  other  amphibia,  as  in  Zenopus  (Fig.  170); 
the  great  oar-like  appendages  of  the  ostracoderms  are  in  all  probability  of  a 
similar  nature. 

In  the  frog,  the  adhesive  discs  appear  to  represent  the  remnants  of  vestigial 
appendages  or  external  gills,  belonging  to  either  the  mandibular,  or  the  hyoid 
arch.  Note  for  instance  their  positions  in  Figs.  161,  E  and  F  and  in  Figs.  164, 


262 


THE    OLD   MOUTH  AND    THE   NEW. 


165,  167,  a.d.o.  In  adult  cyclostomes  in  place  of  these  organs,  there  are  three 
pairs  of  tentacle-like  appendages,  belonging  apparently  to  the  premaxillary, 
maxillary,  and  mandibular  arches  (Fig.  175);  and  in  embryo  sturgeons,  there 


g-      pr.n.  n-p- 


162 


163 


b.rxv. 

FIGS.  162  AND  163.  —  Frog  embryos,  seen  from  the  side,  showing  the  beginning  of  the  external  gills,  or  cephalic  appen- 
dages, operculum,  cranial  ganglia,  and  oral  arches.     R.  septemtrionalis. 


op.          ex.g 


165 


C 


FIGS.  164,  165  AND  166. — Young  tadpole  of  R.  septemtrionalis.  Fig.  164. — Tadpole  from  the  side,  showing 
early  stage  in  the  formation  of  the  peribranchial  chamber.  Fig.  165. — Same;  older  stage;  gills  completely  en- 
closed in  peribranchial  chamber.  Fig.  166. — Oral  region,  from  in  front. 

appears  to  be  a  pair  of  similar  structures  on  the  premaxillary  and  one  on  the 
maxillary  arch.  (Fig.  174.)  In  Polypterus,  Budgett  has  described  a  "cement 
organ,"  similar  to  the  mandibular  glands  of  the  frog,  located  at  first  in  front  of 


THE   GILL   SACS.  263 

the  mouth,  but  finally  coming  to  lie  inside  of  it,  apparently  on  the  premaxillary 
lobes  (Figs.  171,  172.)     Premaxillary  discs  also  occur  in  Lepidopterus  and  Amia. 


There  can  be  but  one  inference  from  the  facts  that  have  been  enumerated, 
namely,  that  the  so-called  " visceral  arches"  of  vertebrates  represent  the  basal 
lobes  of  the  cephalo-thoracic  appendages  of  their  arthropod  ancestors  and;  that  the 
external  gills,  the  balancing  organs  of  amphibia,  the  cephalic  oars  of  ostracoderms, 
the  tentacles,  and  adhesive  discs  of  the  oral  arches  in  embryonic  fishes  and 
amphibia  are  serially  homologous  structures,  representing  the  remnants  of  the 
cephalothoracic  appendages  themselves. 

Similar  modifications  of  the  appendages  frequently  occur  in  various  classes 
of  arthropods.  For  example,  in  insect  embryos  (Blatta  and  Acilius)  the  first  pair 
of  abdominal  appendages  are  reduced  to  gland-like  discs  or  cups,  very  similar 
in  appearance  to  the  cement  glands  on  the  oral  arches  of  embryo  fishes  and 
amphibia.  Beyond  this,  their  function  and  significance  is  unknown.  In  many 
Crustacea,  entomostraca,  and  cirripeds,  one  or  two  pairs  of  degenerate  cephalic 
appendages  terminate  in  adhesive  discs  by  means  of  which  they  fasten  themselves 
to  foreign  objects,  or  to  their  hosts,  just  as  larval  vertebrates  use  their  cement 
glands  for  a  similar  purpose. l 

It  remains  to  be  seen  how  far  a  more  detailed  study  of  the  structure  and 
growth  of  the  oral  arches  in  primitive  vertebrates  will  confirm  the  above  interpre- 
tation. So  far  as  we  know,  there  is  nothing  in  the  embryonic  history  of  the  meso- 
dermic  head  cavities,  or  of  the  oral  arch  nerves  and  ganglia,  that  conflicts  with  it, 
while  all  the  data  available  concerning  the  superficial  form  and  the  mode  of 
growth  of  these  organs  lend  it  their  unqualified  support. 

V.  THE  GILL  SACS.     THE  THYROID  AND  THE  THYMUS. 

In  many  fishes  and  amphibia  the  external  gills  disappear  and  their  place  is 
taken  by  internal  gills  developed  at  a  later  period  from  the  walls  of  gill  sacs  or 
pouches.  The  latter  are  formed  between  the  gill  arches  by  an  infolding  of  the 
ectoderm  that  unites  with  a  tubular  outgrowth  from  the  mesenteron.  It  is  not 
clear  whether  the  new  gill  lamellae  arise  solely  from  the  entodermic,  or  from  the 
ectodermic  part  of  the  gill  chamber,  or  from  both,  although  the  prevalent  opinion 
strongly  favors  the  first  alternative. 

However  that  may  be,  it  is  a  fact,  the  significance  of  which  is  readily  apparent, 
that  in  the  arachnids  there  is  the  same  kind  of  gill  sacs,  having  the  same  relation 
to  external  appendages,  located  in  the  same  region  of  the  head,  and  having  the 
same  relation  to  enteric  outgrowths,  as  in  the  embryos  of  primitive  vertebrates. 

1  The  origin  of  the  secreting  surface  of  the  cement  glands  from  the  endoderm  is  not  in  conflict  with  this 
view.  It  merely  confirms  the  interpretation  of  the  several  oral  arches  as  serially  homologous  with  the  gill 
arches. 


264 


THE    OLD    MOUTH   AND    THE    NEW. 


In  the  Crustacea  the  gill  usually  consists  of  a  special  plate,  or  plume-like  pro- 
cess arising  from  the  basal  lobe  of  a  biramus  appendage.  In  Limulus  and  in  the 
eurypterids,  it  consists  of  many  lamellae  arising  from  the  covered  posterior  basal 
surface  of  the  abdominal  appendages.  (Fig.  5.)  In  abnormal  limulus  embryos, 
the  thoracic  appendages  are  often  completely  infolded,  forming  a  leg  pocket  in- 
stead of  a  leg  process,  suggesting  the  infolded  respiratory  appendages  that  have 
become  the  normal  condition  in  the  air  breathing  arachnids.  In  the  case  of  the 


p.  TUX 


FIG.  167. — Tadpole  of  frog.  A,  Haemal  surface;  B,  older  stage,  seen  from  the  haemal  surface  as  a  semi- 
transparent  object,  and  showing  the  relations  of  the  oral,  respiratory,  and  digestive  orgas;  C,  same  specimen 
from  the  neural  surface.  Figs.  164  to  167  show  frog  tadpoles  in  the  ostracoderm  stages. 

scorpion,  the  respiratory  lamellae  arise  from  an  infolding  just  behind  a  small, 
rudimentary  appendage;  the  latter  then  disappears  without  forming  a  part  of  the 
gill  chamber. 

Without  going  any  further  into  the  discussion  of  the  infinite  variety  of  arthro- 
pod respiratory  appendages,  these  facts  stand  out  clearly,  namely:  i.  there  is  a 
tendency  to  restrict  the  respiratory  function  to  a  small  group  of  metameres,  four 
or  five  in  number,  more  or  less,  following  the  oral  or  locomotor  ones;  2.  that  in 


THE   GILL   SACS. 


265 


the  true  respiratory  appendages,  the  non-respiratory  part  is  often  rudimentary  or 
absent;  and  3,  that  in  the  higher  forms  of  arachnids,  the  respiratory  part  of  the 
appendage  is  deeply  infolded  to  form  the  walls  of  a  gill  sac. 

We  conclude  from  the  above  facts  that  approximately  the  same  group  of 


FIG.  168. — Amblystoma  larva?,  showing  the  vestigeal  cephalic  appendages  in  the  form  of  external  gills,  or  the 

"balancers"  of  the  oral  arches. 


FIG.  169.— Amblystoma  tadpole,  showing  the  balancers,"  c.a.p.,  at  the  height  of  their  development. 

appendages,  which  in  the  arachnids  have  become  partly  or  wholly  infolded  to 
form  the  respiratory  sacs,  have  retained  that  function  in  primitive  vertebrates,  and 
have  given  rise  to  the  ectodermic  portion  of  the  visceral  clefts,  or  branchial  cham- 
bers. We  also  conclude  that  no  vertebrate,  however  primitive,  ever  possessed 


266  THE  OLD  MOUTH  AND  THE  NEW. 

functional  gill-clefts  in  front  of  the  hyoid  arch  metamere  or  any  considerable 
distance  back  of  the  metameres  which  are  now  provided  with  gills  in  typical  fishes. 

VI.  THE  GUT  POUCHES. 

The  midgut  of  the  arthropods,  in  its  typical  condition,  may  be  regarded  as  a 
straight  tube  with  lateral  diverticula,  or  pouches  segmentally  arranged.  In  primi- 
tive Crustacea  there  may  be  either  a  single  unbranched  diverticulum  directed 
forward  and  haemally  from  the  anterior  end  of  the  gut,  cladocera  (Fig.  9) ;  or  one 
or  more  pairs  of  branching  lobes,  as  in  phyllopods  (Fig.  273),  and  many  cirripeds. 
(Fig.  275.) 

In  the  arachnids  the  gut  pouches  of  the  thoracic  and  abdominal  regions 
become  very  highly  developed,  forming  one  of  the  most  conspicuous  features  of 
their  internal  structure.  Their  structure  and  development  is  clearly  seen  in 
Limulus.  Here  the  cephalothoracic  yolk  mass  gradually  breaks  up  into  six  pairs 
of  lateral  lobes.  (Figs.  149,  150,  lv.ll~6.)  The  five  anterior  pairs  form  a  group 
by  themselves  and  open  into  the  midgut  by  a  single  channel.  The  sixth  pair  may 


FIG.   170. — Tadpole  larva  of  Zenopus  laevis,  Daud.     After  Bles.  6omm.  long. 

be  distinguished  from  the  others  by  its  larger  size,  the  absence  of  enclosed  cells 
derived  from  the  haemal  blastoderm  and  by  the  fact  that  it  opens  into  the  gut  by  a 
separate  channel.  (Fig.  151  Iv.l6.) 

In  the  later  stages,  a  pair  of  lobes  develop  from  its  haemal  surface  and  extend 
forward  and  backward,  forming  the  anlage  of  a  special  system  of  haemal  gut  tubes. 
(Figs.  151,  154  a. b.)  In  the  adult,  the  extensive  ramifications  of  the  lateral  tubes 
fill  the  greater  part  of  the  cephalothorax,  the  branches  of  the  posterior  haemal 
tubes,  6,  being  apparently  the  only'  ones  that  extend  into  the  abdomen.  In 
the  young  scorpion,  a  similar  arrangement  is  seen.  (Fig.  179.)  The  sixth  pair, 
the  so-called  salivary  glands,  /././.,  are  large  and  open,  as  in  Limulus,  by  separate 
ducts.  The  five  anterior  pairs  a.t.L,  are  reduced  to  small  blind  pockets.  The 
haemal  lobes  h.t.L,  are  well  developed,  the  large  anterior  horns  extending  forward, 
over  the  haemal  surface  of  the  forebrain.  (Fig.  43.)  In  the  branchial  region,  there 
are  six  pairs  of  pouches,  the  first  corresponding  to  the  comb  segment,  the  next  four 
to  the  lung  books,  and  the  sixth  extending  backward  into  the  last  mesothoracic 
segment.  (Figs.  43,  179.) 

In  the  pedipalpi,  the  large  haemal  lobes  of  the  thorax  are  united  by  a  transverse 
anastomosis.  In  the  pycnogonida  and  in  the  spiders,  the  long  thoracic  diverticulas 
are  unbranched,  and  may  extend  a  considerable  distance  into  the  base  of  the  legs. 
(Fig.  1 80.)  In  the  spiders,  there  are  also  four  pairs  of  abdominal  pouches. 


THE    GUT   POUCHES. 


267 


In  primitive  vertebrates,  a  variable  number  of  lateral  gut  pouches  are  formed 
belonging  to  the  postoral  group  of  metameres.  They  unite  with  the  adjacent  gill 
pockets,  and  thus  establish  a  communication  between  the  gut  and  the  exterior,  via 
the  enteric  diverticula  and  the  ancestral  gill  pockets. 


171 


172 


FIG.  171,  172. — Larva  of  Polypterus  senegalus,  showing  rudimentary  maxillary  appendages,  or  adhesive  papillae. 

After  Budgett. 

What  caused  the  opening  of  one  organ  into  the  other  in  ancestral  vertebrates 
we  do  not  know,  any  more  than  we  know  why  it  actually  takes  place  now  in  the 
embryos  of  modern  vertebrates.  For  our  present  purpose,  it  is  sufficient  to  show 
that  in  the  arachnid  ancestors  of  the  vertebrates  the  gill  pockets  and  enteric  diver- 
ticula stand  in  the  same  relation  to  each  other  and  to  the  rest  of  the  head  that 


FIG.  173. — Larvae  of  Protopterus  annectens,  showing  the  highly  developed  external  gills.     A,  Seventh  day  em- 

byro;  B,  tenth  day.     After  Budgett. 

they  now  do  in  vertebrate  embryos  before  the  perforation  takes  place.  We  are 
here  dealing  with  one  of  those  cases  where  there  are  no  intermediate  stages  between 
two  very  different  conditions,  for  either  the  gut  tubes  open  into  the  gill  chamber, 
or  they  do  not;  if  they  do,  we  are  dealing  with  animals  of  the  vertebrate  type;  if 
not,  with  those  of  the  invertebrate  type. 


268  THE    OLD    MOUTH  AND    THE    NEW. 

The  most  anterior  of  the  lateral  gut  pouches  in  the  vertebrates  corre- 
spond with  those  of  the  locomotor  appendages  of  the  arachnids.  They  probably 
atrophied  in  the  immediate  ancestors  of  the  vertebrates,  neither  communicating 
with  the  exterior,  nor  leaving  any  definite  organs  behind.1  The  paired  haemal 
outgrowth  of  the  thoracic  gut  probably  persists  as  the  thyroid  gland,  with  which 
it  agrees  in  position  and  direction  of  growth.  (Figs.  43,  44,  308.) 

In  the  arachnids  the  free  ends  of  the  lateral  and  of  the  haemal  enteric  pouches 
of  the  thorax  are  usually  branched,  or  lobular,  or  racemous,  the  subdivisions  con- 
sisting of  a  single  layer  of  cylindrical  secreting  cells  that  suggest,  in  their  general 
appearance,  the  condition  presented  by  the  thyroid  in  vertebrates,  and  by  those 
organs  resembling  the  thyroids  that  arise  from  the  walls  of  the  visceral  clefts. 

The  thymus  probably  represents  a  modification  of  the  several  pairs  of  thoracic 
coxal  glands  that  occur  in  the  arachnids. 

The  postbranchial  outgrowths  of  the  vertebrate  mesenteron,  i.e.,  the  lungs, 
liver,  and  pancreas  may  be  regarded  as  local  specializations  of  enteric  diver  - 
ticula  comparable  with  the  endodermic  portions  of  the  visceral  clefts.  (Figs. 
181,  182.) 

VII.  THE  LOCOMOTOR  APPENDAGES. 

A .  The  Cephalic  Appendages. — In  the  higher  arthropods,  locomotion  is  effected 
by  several  pairs  of  jointed  appendages  arranged  in  strictly  segmental  order,  and 
usually  located  anterior  to  the  respiratory  region.  (Figs.  3,  4,  5.)  In  true  verte- 
brates the  locomotor  appendages  are  always  situated  behind  the  gills;  there  are 
never  more  than  two  pairs ;  and  they  have  no  fixed  relation  to  the  metameres.  (Fig. 
4,  B  and  C.) 

The  meaning  of  this  sharp  contrast  will  appear  after  a  more  careful  examina- 
tion of  their  structure  and  serial  location  in  the  two  groups,  and  in  the  intermediate 
one  formed  by  the  ostracoderms. 

In  free  swimming  phyllopods  and  arachnids,  there  may  be  one  pair  of  large 
oar-like  appendages  that  arise  from  the  anterior  end  of  the  thorax,  as  for  example 
the  antennae  of  many  cladocera  and  entomostraca,  or  the  elongated  chelicerae  of 
Pterygotus.  Or  such  a  pair  may  arise  from  the  posterior  part  of  the  thorax,  as  in 
Eu-rypterus.  (Fig.  5.)  In  all  these  cases  the  locomotor  appendages  lie  in  front  of 
the  respiratory  region. 

In  typical  vertebrates  the  paired  locomotor  appendages,  if  present,  consist 
primarily  of  two  lateral  folds  that  extend  from  the  postbranchial  to  the  precaudal 
region.  The  pectoral  and  pelvic  fins  are  local  expansions  of  these  folds.  The 
paired  fins  are  not  definite  segmental  structures,  for  they  consist  of  muscle  buds  and 
cartilages  derived  from  a  large  and  varying  number  of  metameres. 

As  there  are  approximately  sixteen  or  more  metameres  in  the  vertebrate  head, 

1  Possibly  they  may  be  represented  by  the  "lateral  thyroids"  and  by  the  diverticula  leading  into  the 
cement  glands  of  amphibia. 


THE  CEPHALIC  APPENDAGES  AND  THE  LATERAL  FOLD. 


269 


it  is  clear  that  the  paired  fins  of  vertebrates  cannot  represent  any  arthropod  ap- 
pendages in  front  of  the  sixteenth  pair. 

The  clue  to  the  whole  problem  of  vertebrate  and  arthropod  locomotor  ap- 
pendages will  be  found  in  the  ostracoderms,  where  both  cephalic  appendages  of 
the  arthropod  type,  and  lateral  folds  of  the  trunk,  like  those  in  true  vertebrates, 
are  present. 

In  Bothriolepis  oar-like  cephalic  appendages  are  present  that  clearly  belong 
to  the  anterior  division  of  the  head,  for  they  lie  immediately  behind  the  oral 
arches  and  in  front  of  the  gills.  (Fig.  4,  A,  247.)  In  Cephalaspis  the  large 
paddle-shaped  appendages  are  of  the  same  general  nature  and  lie  in  a  similar  posi- 
tion. (Fig.  232,  234.)  In  Tremataspis  portions  of  armored  appendages  have  been 
found  similar  to  those  in  Bothriolepis,  that  fit  into  the  most  anterior  of  a  series  of 
nine  pairs  of  notches,  or  openings.  The  eight  posterior  pairs  served  either  for 
the  attachment  of  smaller  appendages,  ^comparable  with  external  gills,  or  as 


FIG.    174.  FIG.    175 

FIG.   174. — Embryos  of  the  sturgeon;   showing  the  rudimentary  cephalic  appendages  of  the  oral  arches. 

After  Salensky. 

FIG.  175. — Oral  region  of  an  adult  Cyclostome  ( Bdellostoma)  showing  three  pairs  of  rudimentary  oral-arch  appen- 
dages.    Compare  with  the  oral  arches  of  an  amphibian  embryo,  Fig.  161. 

openings  to  respiratory  chambers  containing  the  internal  gills.  (Fig.  236.)  In 
Cyathaspis,  Drepanaspis,  Palaeaspis,  and  Pteraspis,  there  are  indications  of  ex- 
ternal cephalic  appendages  in  the  form  of  armored  "oars"  like  those  in  Bothrio- 
lepis, or  in  the  form  of  naked  tentacles.  (Figs.  244-246.) 

The  cephalic  appendages  of  the  ostracoderms  no  doubt  represent  the  same 
kind  of  organs  as  the  external  gills,  the  balancers,  and  the  oral  arch  tentacles  of 
primitive  vertebrate  embryos,  and  all  these  structures  probably  represent  various 
modifications  of  the  cephalothoracic  appendages  of  the  arachnids. 

B.  The  Fringing  Processes  and  the  Lateral  Fold. — In  those  ostracoderms  that 
are  well  enough  preserved  to  show  the  shape  of  the  body,  the  postcephalic  portion 
has  a  triangular  outline  in  cross-section,  with  either  a  series  of  distinct  separately 
movable  plates  of  peculiar  structure  on  either  margin,  or  a  continuous  membran- 
ous fold,  with  or  without,  supporting  specules  or  minute  ossicles. 

In  Bothriolepis  the  trunk  was  practically  naked  and  was  provided  with  two 
narrow  membranous  folds  projecting  freely  from  the  hsemo-lateral  margins.  The 
folds  extended  from  the  root  of  the  trunk  backward,  uniting  with  each  other  on 


270  THE  OLD  MOUTH  AND  THE  NEW. 

the  haemal  surface  at  the  base  of  the  caudal  fin.  (Fig.  248.)  This  fold  is  usually 
sharply  denned  and  shows  no  trace  of  subdivisions  or  of  supporting  rays. 

In  a  small  undescribed  species  of  Cephalaspis  from  Dalhousie,  N.B.,  a  similar 
fold  is  present,  but  it  appears  to  be  strengthened  by  minute,  ill-defined  spicules. 
(Fig.  234.) 

In  the  larger  species,  like  C.  lyellii  and  C.  murchsonii,  in  place  of  a  membran- 
ous fold,  there  is  a  series  of  separately  movable  processes,  segmentally  arranged 
and  covered  with  a  dermal  skeleton  having  the  same  surface  ornamentation  as 
that  on  the  rest  of  the  body.  There  is  nothing  in  true  fishes  exactly  comparable 
with  these  remarkable  structures. 

In  C.  lyellii  (Fig.  232),  the  fringing  processes  hang  freely  away  from  the  trunk, 
in  a  nearly  vertical  position,  with  their  distal  ends  bending  backward  in  graceful 
curves.  Each  process  has  a  slender  neck  and  rounded  head  that  fits  into  a  cup-like 
depression  on  the  posterior  ventral  margin  of  the  large  dorso-lateral  trunk  scales. 
(Fig.  233,  C.)  There  are  from  twenty  to  thirty  pairs,  beginning  just  back  of  the 
cephalic  shield  and  gradually  decreasing  in  size  from  that  point  toward  the  tail 
end.  The  most  posterior  ones  are  reduced  to  mere  spines  or  rhomboidal  plates, 
loosely  articulated  to  the  lateral  trunk  scales. 

In  C.  murchisonii  the  fringe  plates  are  distinctly  lobed,  and  overlap  one  an- 
other so  that  their  flattened  surfaces  are  directed  diagonally  forward  and  outward. 
(Fig.  233,  D.)  In  Cephalaspis  pagei  they  appear  to  have  a  similar  shape  and 
arrangement,  but  are  armed  with  coarse  projecting  spines  that  give  them  a 
decidedly  arthropod  appearance.  (Fig.  233,  E.) 

There  can  be  no  doubt  that  the  fringing  processes  represent  an  initial  stage 
in  the  formation  of  a  lateral  fold  like  that  in  the  embryos  of  true  fishes.  They 
may  be  regarded  as  small  segmentally  arranged  locomotor  appendages  compar- 
able with  arthropod  abdominal  appendages,  but  belonging  to  metameres  lying 
farther  back  than  any  that  occur  in  that  class  of  animals.  An  alternative  inter- 
pretation would  be  to  regard  them  as  a  series  of  overhanging  pleurites  like  those  in 
the  trilobites  and  in  many  other  arthropods. 

We  may  therefore  conclude  that  the  oral  arches,  gill  arches,  external  gills, 
"balancers,"  the  lateral  folds  of  vertebrate  embryos,  the  cephalic  oars  and  fring- 
ing processes  of  the  ostracoderms,  are  various  local  modifications  of  one  set  of 
serially  homologous  structures,  that  are  in  turn  comparable  with  the  segmental 
appendages  of  arthropods.  The  pectoral  and  pelvic  fins  of  true  vertebrates  are 
to  be  regarded  as  comparatively  recent  modifications  of  the  lateral  folds,  and 
as  containing  the  remnants  of  a  large  and  varying  number  of  such  appendages. 

This  interpretation  of  the  origin  of  the  paired  appendages  gives  us  precisely 
what  Gegenbaur  claims  has  heretofore  been  lacking  in  the  lateral  fold  theory, 
namely:  i.  a  reason  for  the  existence  of  the  primary  fold  of  ectoderm  that  initiates 
the  formation  of  the  lateral  fold;  2.  a  reason  for  the  migration  into  it  of  segmental 
detachments  of  muscle,  nerve,  and  cartilage;  and  3.  a  primary  function  for  the 
lateral  fold,  out  of  which  a  set  of  locomotor  organs  could  be  logically  developed. 


CONCLUSION. 


271 


Conclusion. — A  general  survey  of  the  appendages  throughout  the  arthropod- 
vertebrate  stock  reveals  a  steady  and  logical  progress  in  their  specialization.  Sen- 
sation, feeding,  locomotion,  respiration,  and  reproduction  are  alike  essential 
functions,  but  they  do  not  make  their  local  appearance  at  the  same  time  phylo- 
genetically  or  ontogenetically;  or  make  the  same  demands  for  space  or  for  special 
locations.  Sensory  and  feeding  appendages,  for  example,  must  be  located  near 
the  mouth,  which  is  the  oldest  organ  of  the  body,  and  the  first  one  to  be  formed. 
Locomotor  appendages  must  be  located  where  they  can  raise  and  move  the  primi- 
tive head,  or  in  the  later  phases  of  evolution,  lift  the  whole  body  and  support 


pb.C 


p.o.c 


an. 


FIG.   178. 


FIGS.  176  TO  178. — A,  Hypothetical  form  in  median  section,  indicating  the  probable  arrangement  of  organs  in 
an  intermediate  condition  between  that  in  an  arachnid  and  an  ostracoderm;  B,  median  section  of  an  ostracoderm 
(Bothriolepis)  showing  the  arrangement  of  the  internal  organs;  in  part,  hypothetical;  C,  Amphioxus,  in  median 
section.  Diagrammatic. 

it  in  a  properly  balanced  position.  Respiration,  circulation,  excretion,  and  repro- 
duction make  no  such  imperative  demands  for  special  locations  or  for  early 
development. 

Thus  there  is  established  in  the  appendages,  at  a  very  early  period,  a  definite 
linear  sequence  of  functions  that  coincides  with  the  sequence  in  the  historical 
evolution  of  the  functional  demands  made  upon  them. 

In  other  words,  the  oldest  organs  phylogenetically,  and  those  first  in  demand 
ontogenetically,  are  laid  down  first  in  time,  and  consequently  at  the  head  end  of  the 
series,  because  growth  in  segmental  animals  always  takes  place  by  a  process  of 
addition  at  the  tail  end.  Those  organs  that  make  no  special  demand  for  location 
or  for  prior  use,  even  though  they  may  have  an  equally  long  pedigree  with  the  rest, 
are  gradually  relegated  to  the  more  recently  added  territory  in  the  posterior  part 
of  the  body. 


272 


THE    OLD    MOUTH  AND    THE    NEW. 


oil. 


Fro.    179. 


FIG.    180. 


FIGS  179  AND  180 Diagrams  showing  the  location  of  the  principal  viscera,  such  as  the  enteric  pouches,  coxa! 

glands,  malpighian  tubules,  lung  books  and  gills.     A,  Scorpion;   B,  spider. 


lil-i 


FIG.   181.  FIG.   182. 

FIGS.  181  AND  182. — Hypothetical  forms  leading  to  the  condition  in  vertebrates. 


CONCLUSION.  273 

In  accordance  with  these  principles,  the  linear  distribution  of  functions  in  the 
following  order:  sensory,  feeding,  respiratory,  and  reproductive,  becomes  very 
early  established,  and  thereafter  suffers  but  minor  modifications  throughout  the 
entire  range  of  the  arthropod-vertebrate  series.  For  example,  in  the  arachnids  the 
procephalic  appendages  are  mainly  sensory,  the  dicephalic,  masticatory,  the  meso- 
cephalic,  locomotor,  and  the  metacephalic,  respiratory  and  reproductive.  The 
boundaries  to  these  divisions  are  not  always  sharply  defined,  and  there  may  be 
some  overlapping  of  functions,  but  not  enough  to  invalidate  the  general  law. 

One  of  the  more  recent  changes  in  this  primitive  distribution  of  functions  to 
the  appendages  was  the  transfer  of  the  locomotor  organs  to  a  postcephalic  position, 
owing  largely  to  the  increased  number  of  metameres  and  the  consequent  shifting 
of  the  center  of  balance  backward. 


18 


CHAPTER  XV. 
VARIATION  AND  MONSTROSITIES. 

The  study  of  variation  is  an  important  aid  to  phylogeny,  for  with  the  ever 
shifting  conditions  within  and  around  a  center  of  life,  that  which  is  now  an  occa- 
sional phenomenon,  or  "abnormality,"  may  under  other  conditions  become  a 
" normal"  or  constant  result.  Thus  the  abnormal  of  to-day  may  have  been  the 
normal  of  yesterday,  or  the  normal  of  to-day  the  abnormal  of  tomorrow. 

The  minute  variations  expressed  toward  the  close  of  development,  and  which 
at  most  are  only  productive  of  new  species,  or  even  genera,  are  not  likely  to  be 
the  sources  of  those  fundamental  changes  which  give  rise  to  new  classes.  In  our 
problem,  therefore,  we  should  consider  those  early  embryonic  variations  in  verte- 
brates that  are  likely  to  reveal  the  structure  of  their  remote  ancestors;  or  failing 
that,  we  should  seek  for  embryonic  variations  in  arthropods  that  might  have 
produced  a  vertebrate;  for  if  we  know  the  broad  limitations  to  the  range  of  varia- 
tion in  a  given  animal,  we  may  feel  a  reasonable  confidence  that  we  can  roughly 
define  all  the  principal  forms  in  which  the  ancestors  or  the  descendants  of  such 
animals  could  be  expressed. 

To  begin  such  a  study,  it  is  necessary  to  have  an  inexhaustible  supply  of  em- 
bryonic material  that  is  easy  to  prepare,  easy  to  observe,  and  easy  to  separate  the 
abnormal  from  the  normal.  I  know  no  other  animal  in  which  these  conditions  are 
so  happily  fulfilled  as  in  Limulus.  The  eggs  may  be  obtained  in  hundreds  of  thou- 
sands and  when  properly  prepared,  the  checkerboard  arrangement  of  organs  can  be 
easily  observed  in  their  normal  and  abnormal  conditions.  The  abnormal  embryos 
may  be  obtained  in  great  numbers  by  placing  the  eggs  from  many  different  nests  in 
running  water.  In  due  time,  eight  to  ten  weeks,  the  normal  eggs  hatch  and  the 
free  swimming  larvae  are  carried  off  in  the  waste.  The  five  or  ten  per  cent,  that 
remain  are  apparently  sound  and  healthy,  but  among  them  will  be  found  all  the 
different  phases  of  abnormality  likely  to  occur.  There  are  pygmies  and  giants, 
in  early  and  late  stages;  some  are  legless,  others  headless,  or  tailess,  or  consist 
of  fractional  parts,  such  as  halves,  quarters  and  smaller  divisions,  in  endless 
combination  and  variety.  Then  there  are  doublets  and  triplets  in  various 
stages  of  progressive  or  regressive  growth. 

As  all  the  eggs  develop  under  similar  conditions,  the  cause  of  these  various 
abnormal  forms  must  be  referred  not  to  the  unusual  environment  of  a  modern 
hatching  jar,  but  to  variable  conditions  in  the  eggs  themselves,  that  were  probably 
as  frequent  millions  of  years  ago  as  they  are  to-day. 

These  variations  are  much  greater  and  more  numerous  than  one  might  have 

274 


INVAGINATED  APPENDAGES.  275 

expected  in  such  an  ancient  type  as  Limulus,  although  there  is  no  reason  to  believe 
that  the  range  of  variation  in  Limulus  is  exceptional. 

Our  observations  show  that  there  is  a  very  small  range  in  the  kind  of  variation, 
for  it  is  of  a  plus  or  minus  nature  in  practically  all  cases.  They  also  show  that 
the  normal  forms  follow  a  set  course;  extreme  divergence  from  it  leads  to  complete 
extinction  by  a  process  of  degeneration  that  is  the  reverse  of  the  process  of  genera- 
tion, the  various  organs  disappearing  in  the  same  order  in  which  they  made  their 
appearance. 

I.  INVAGINATED  APPENDAGES. 

This  common  abnormality  consists  in  the  transformation  of  the  usual 
outgrowing  appendages,  in  whole  or  in  part,  into  ingrowing  pockets. 

The  infolding  may  take  place  in  the  comparatively  late  stages  of  its  develop- 
ment, only  the  tip  of  a  well  developed  leg  being  infolded  (Fig.  183,  C.) ;  or  the  entire 


FIG.  183. — Three  Limulus  embryos  in  about  the  same  stage  of  development  (stage  G-H)  and  drawn  to  the  same 
scale.  They  show  the  variations  in  the  size  of  the  appendages,  and  of  the  embryos  as  a  whole;  also  the  varying 
extent  to  which  the  appendages  are  infolded.  A,  All  the  thoracic  appendages  are  invaginated,  except  the  first  and 
last  pairs;  B,  the  fourth  pair  completely  invaginated;  C,  the  third  pair  completely  invaginated,  and  the  tip  of 
the  fourth  appendage  of  the  right  side.  Camera. 

leg  may,  from  the  beginning  of  its  development,  grow  inward  instead  of  outward) 
forming  a  deep  pocket  opening  outwardly  by  a  transverse  slit.  (Fig.  183,  B., 

Any  thoracic  appendage,  except  possibly  the  first,  may  be  invaginated.  In  a 
given  embyro  the  infolding  may  affect  one  appendage,  or  a  pair,  or  several  append- 
ages on  one,  or  on  both  sides.  Infolded  legs  are  found  in  otherwise  normal  em- 
bryos, or  in  those  presenting  other  abnormalities;  never  in  the  adult. 

The  conditions  under  which  they  occur  clearly  show  that  they  are  in  all 
probability  produced  by  some  special  condition  within  the  appendage  itself, 
not  by  local  pressure,  or  by  any  other  external  cause. 

The  frequent  occurrence  of  invaginated  appendages  in  Limulus  embryos  is 
an  important  fact.  We  may  infer  that  similar  "  abnormalities  "  occurred  in  other 
arachnids  and  for  some  unknown  reason  became  "fixed"  or  "  normal,"  giving 
rise  to  the  infolded  abdominal  appendages  which  form  the  basis  of  the  lung 
books.  In  the  tunicates,  in  Balanoglossus,  and  in  vertebrates,  similar  respiratory 


276 


VARIATION  AND    MONSTROSITIES. 


pouches  or  ingrowths  occur,  but  in  these  cases  they  have  become  perforate  at  their 
inner  ends,  and  open  into  the  cephalic  diverticula  of  the  alimentary  canal. 

II.  ASYMMETRY. 


It  has  been  shown  that  in  typically  segmented  animals  the  organs  are  arranged 
symmetrically,  in  checkerboard  fashion,  on  either  side  of  a  median  line.  (Fig. 
157.)  Asymmetry  occurs  when  any  of  these  paired  organs  differs  in  form  from 


FIG.  184. — Limulus  embryos  of  different  ages,  showing  various  forms  of  asymmetry,  due  either  to  the  absence 
of  organs  usually  present,  or  to  the  presence  of  extra  organs  on  one  side  of  the  median  line.  A,  The  right  side  of 
the  abdomen  and  the  right  half  of  the  last  three  thoracic  metameres,  absent.  X,io  B,  Left  half  of  abdomen 
and  left  chelicera  absent;  posterior  thoracic  appendages  abnormally  small;  cam.  Xso.  C,  Right  half,  except 
the  sixth  thoracic  appendage,  absent;  cam.  X  16  1/2.  D,  Stage  L.  M.  (about  ready  to  hatch).  Right  half  of 
nerve  cord  is  present  but  all  the  other  structures  of  the  right  side  are  absent,  except  traces  of  third(?)  and  sixth 
thoracic  appendages,  and  the  margin  of  the  thoracic  and  abdominal  shield;  cam.  Xso.  E,  Stage  G.  H. 
There  are  two  lateral  eyes  and  four  cheliceras  on  the  left.  The  apex  of  the  third  thoracic  appendage  on  the  right, 
is  invaginated;  cam.  Xso. 

its  mate  on  the  opposite  side.  If  this  local  variation  occurs,  all  the  other 
parts  of  the  body  respond  to  it  by  a  change  in  position  or  form.  A  common  form 
of  asymmetry  is  the  reduction  in  size,  or  the  absence  of  certain  organs  on  one 
side  of  the  median  line.  In  such  cases  the  opposite  half  of  the  body  tends  to  form 
a  spiral  or  circle,  with  the  defective  area  as  its  center. 

It  is  generally  assumed  that  the  morphological  unit  of  segmented  animals 
is  the  metamere  or  a  complete  transverse  row  of  organs.  But  it  appears  that  in 
Limulus,  and  no  doubt  it  is  true  of  other  animals  also,  the  right  and  left  halves  of  a 
metamere  attain  a  higher  stage  of  organic  unity,  are  more  variable  than  the  meta- 
meres themselves,  and  should  be  regarded  as  the  true  morphological  units. 


DEGENERATION.  277 

Asymmetery  is  a  common  abnormality  in  Limulus  and  is  expressed  in  a  variety 
of  ways.  It  is  most  easily  observed  in  the  appendages,  but  in  most  cases,  appar- 
ently, changes  in  them  are  accompanied  by  similar  ones  in  the  corresponding  half 
neuromeres,  the  somites,  and  sense  organs. 

A.  Multiple  Organs. — Asymmetry  due  to  the  presence  of  extra  organs  on 
one  side  is  very  rare.     A  good  illustration  is  seen  in  Fig.  184,  £.,  where  the  first 
thoracic  half  metamere  has  divided  twice,  giving  rise  to  two  half  neuromeres, 
two  left  lateral  eyes,  and  four  imperfectly  divided  left  chelicerae. 

B.  Defective  Organs. — Asymmetry  due  to  the  absence,  or  reduction  of  ap- 
pendages is  very  common,  and  apparently  occurs  as  frequently  on  the  right  side 
as  on  the  left,  and  as  frequently  in  the  thorax  as  the  abdomen.     (Fig.  184.)     But 
half  embryos  like  that  in  Fig.  184,  C.,  are  very  rare.     Here  all  the  organs  on  the 
right  side,  except  what  appears  to  be  the  sixth  leg,  are  absent;  the  left  side  appears 
perfectly  normal  except  for  its  curvature  toward  the  right. 

Asymmetry  similar  to  that  seen  in  Limulus  has  become  a  fixed  character  in 
certain  groups  of  arthropods,  e.g.  hermit  crabs  and  bopyridae.  The  radial  symmetry 
of  the  echinoderms  was  brought  about  by  the  loss  of  one  side  of  the  body  probably 
in  some  arthropod-like  ancestor.  The  remaining  side  taking  the  form  of  a  closed 
ring  established  a  successful  organic  union  and  laid  the  foundations  for  a 
new  type  of  radiate  structure  and  a  new  class  of  animals.  See  Chapter 
XXIII,  p.  42 1. 


III.  DEGENERATION. 

A.  Median  Fusion  and  Antero-posterior  Degeneration. — This  remark- 
able phenomenon  is  so  common,  and  has  been  observed  in  so  many  different  stages, 
that  there  can  be  no  doubt  of  the  manner  in  which  it  takes  place.  The  process 
starts  in  the  anterior  metameres  and  is  taken  up  by  the  following  ones  in  numerical 
order.  In  the  typical  cases,  each  organ  unites  with  its  fellow  of  the  opposite  side 
to  form  an  unpaired  organ,  which  then  disappears.  Those  nearest  the  median 
line  unite  first,  and  after  they  degenerate  those  lateral  to  them  unite  and  degener- 
ate in  the  order  of  their  position,  till  the  whole  of  the  metamere  has  disappeared. 
As  the  process  in  one  metamere  is  always  a  step  in  advance  of  that  in  the  next 
posterior  metamere,  A-shaped  embryos  are  produced  showing  various  stages  in 
the  progress  of  degeneration.  The  successive  steps  are  most  clearly  shown  by 
the  appendages,  the  dorsal  organs,  and  the  nerve  cords.  The  other  paired  organs 
probably  fuse  and  degenerate  in  the  same  manner,  but  their  history  is  not  so  easily 
followed.  Toward  the  close  of  degeneration  we  may  find,  at  what  was  the  pos- 
terior end  of  the  embryo,  either  a  median  row  of  unpaired  organs,  or  an  exhausted 
mass  of  cells,  and  finally  they  may  in  their  turn  disappear,  leaving  no  trace  of  the 
embryo  behind. 


278  VARIATION  AND    MONSTROSITIES. 

An  examination  of  the  different  stages  of  the  process  (Figs.  185,  189) 
shows  that  as  the  neuromere  disappears,  the  corresponding  appendages  approach 
each  other  and  unite,  fusing  at  the  base  first  and  at  the  tip  last.  The  large  un- 
paired leg  thus  formed  first  becomes  long,  slender,  and  often  coiled  and  twisted; 
later,  it  shortens,  becomes  smaller  and  smaller,  and  finally  disappears. 

The  process  of  degeneration  may  be  best  understood  by  a  consideration  of 
the  normal  structure  of  bilaterally  symmetrical  animals  and  the  way  in  which  it 
is  produced.  This  may  be  illustrated  by  a  diagrammatic  mercator  projection  of 
its  superficial  organs.  (Fig.  192,  A.)  Here  the  line,  A. P.,  represents  the 
cephalo-caudal  neural  line;  each  lettered  square  represents  a  paired  segmental 
organ,  and  each  transverse  row  a  metamere.  The  middle  section  shows  five 
body  metameres  with  the  typical  arrangement  of  segmental  organs,  from  the 
neural  series,  a. a.,  to  the  haemal  series,  e.e. 

The  apex  of  the  figure,  m.A.n.,  illustrates  the  progressive  elimination  of  lateral 
segmental  organs  in  the  head  region,  and  the  predominance  of  the  neural  ones. 
The  lower  part,Z,.P.7?.,  shows  the  order  in  which  the  organs  arise  by  apical  growth. 
In  this  process  we  may  recognize  two  distinct  factors,  or  two  different  kinds  of 
growth.  One  gives  rise  to  a  longitudinal  series  of  similar  metameres,  the  new  ones 
always  appearing  just  in  front  of  the  apex  and  behind  the  one  previously  formed. 
The  other  produces  a  transverse  row  of  unlike  organs,  i.e.,  neuromere,  ganglion, 
leg,  sense  organ,  nephridia,  heart,  etc.,  extending  from  the  median  line  outward, 
the  most  highly  specialized  ones  being  at  the  median  end  of  each  row. 

The  relative  age  of  each  organ,  and  its  degree  of  specialization  is  therefore  a 
function  of  its  position  in  relation  to  these  two  axes  of  growth. 

Degeneration  takes  place  in  the  following  manner:  the  most  median  organs 
of  the  first  metamere,  a  and  a,  unite  to  form  an  unpaired  organ,  A,  which  then  dis- 
appears, followed  in  the  same  way  by  c.  d.  and  e.  The  same  thing  takes  place  in 
all  the  following  metameres,  the  process  in  the  second  metamere  being  one  step 
ahead  of  that  in  the  third,  and  so  on. 

If  the  process  proceeds  till  the  first  two  e's  of  Fig.  192,  A.,  are  united,  all  the 
organs  within  the  area,  a.m.A.n.a.j  will  have  disappeared,  and  those  that  were  on 
the  margins  of  this  area  will  form  a  median  row  of  unpaired,  unlike  organs,  E.A., 
arranged  from  before  backward  in  the  reverse  order  of  that  on  the  half  metamere. 
(Fig.  192,  B.)  Embryos  in  which  this  condition  is  approximately  realized  are 
shown  in  Figs.  185  and  189.  The  condition  is  shown  in  diagrammatic  form  in 
Fig.  191. 

This  mode  of  degeneration,  therefore,  takes  place  according  to  a  definite  law, 
and  creates  an  entirely  new,  heretofore  unrecognized  architectural  type.  This 
type  is  fleetingly  represented  in  degenerating  Limulus  embryos,  and  probably  in 
many  other  segmented  animals  also.  The  animals  in  which  this  condition  be- 
came established  in  the  adults  would  form  a  new  class. 


HOUR-GLASS    EMBRYOS. 


279 


B.  Hour-Glass  Embryos. — Precisely  the  same  kind  of  median  fusion  and 
degeneration  may  appear  at  other  points  in  the  body,  forming  marked  transverse 
constrictions,  or  even  complete  fission.  A  common  condition  is  where  the  con- 
striction appears  in  the  middle  of  the  thorax  forming  the  hour-glass  embryos,  as 
in  Fig.  189. 

Several  other  minor  zones  of  transverse  constrictions  may  be  recognized. 
They  are  formed  at  the  points  where  a  reduction  in  the  size  of  organs  and  a  tend- 


FIG.  185. — Limulus  embryos  in  various  stages  of  degeneration;  all  drawn  to  the  same  scale.  The  process  con- 
sists in  the  union  in  the  median  line,  and  the  subsequent  degeneration,  of  the  right  and  left  organs  of  each  metamere. 
The  organs  nearest  the  median  line  are  the  first  to  unite,  forming  a  single  extra  large  organ  having  the  characteristic 
features  of  each  member  of  the  pair.  The  unpaired  organ  then  decreases  in  size  till  it  completely  disappears.  In 
its  place  the  organs  lateral  to  it,  on  the  same  metamere,  unite,  and  in  turn  degenerate;  and  so  on  till  the  whole 
metamere  has  disappeared.  The  process  begins  in  the  most  anterior  metarneres,  and  progresses  in  a  cephalo- 
caudal  direction.  In  typical  cases,  as  soon  as  the  first  unpaired  organ  formed  in,  say,  the  first  thoracic  metamere, 
has  disappeared,  the  same  organ  is  found  unpaired  in  the  second  metamere;  and  by  the  time  that  has  disappeared, 
the  unpaired  condition  of  that  organ  obtains  in  the  next  following  metamere;  and  so  on,  till  every  paired  organ  has 
become  median  and  unpaired,  and  then  disappeared.  In  the  later  phases  of  the  process,  nothing  is  left  of  the 
embryo  but  the  mesodermic  area  and  a  posterior  unpaired  process,  representing  either  the  last  thoracic  appendage, 
or  the  tail  lobe;  and  these  in  turn  finally  disappear. 

In  very  rare  cases,  one  of  the  posterior  pair  of  appendages  may  fuse  in  the  median  line,  without  any  indication 
of  fusion  in  front  of  that  point.  But  there  is  no  evidence  of  a  progressive  median  fusion  and  degeneration  extend- 
ing toward  the  anterior  end.  Camera  X30. 

ency  to  undergo  median  fusion  takes  place  in  the  adults  of  other  groups  of 
arthropods.  They  divide  the  body  more  sharply  than  usual  into  cephalic  lobes, 
oral,  thoracic,  vagal,  and  abdominal  regions.  Each  of  these  regions,  or  tag- 
mata,  may  show  traces  of  degeneration  from  before  backward,  independently  of 
the  others,  and  in  the  same  manner  that  we  have  seen  in  the  whole  embryo. 

It  would  appear,  therefore,  that  the  broad  subdivisions  of  the  arthropod  body, 


280  VARIATION   AND    MONSTROSITIES. 

which  constitute  the  very  foundation  of  arthropod  morphology,  are  the  varying 
results  of  apical  growth,  locally  checked  by  median  fusion  and  degeneration. 

C.  Acephalic  and  Acaudal  Embryos. — A  common  abnormality  consists  in 
the  extensive  reduction  of  the  anterior  or  posterior  metameres,  or  of  both,  leaving 
only  the  middle  portion  of  the  trunk  intact.  It  is  not  clear  whether  this  result  is 
brought  about  by  median  fusion  or  not. 

It  is  a  common  thing  to  find  embryos  without  the  cephalic  lobes;  or  without 
cephalic  lobes  and  the  first  three  pairs  of  thoracic  appendages;  or  the  abdomen 


.^ 


,- 

> 


e 

. 


""-     MX-       ^ 

^  '•      ''  *< 

I  ""',/  ""  K 

FIG.  186. — Limulus  embryos  in  advanced  stages  of  degeneration.  A ,  Stage  G.  The  anterior  half  of  the  thorax 
is  absent  and  the  isolated  procephalic  lobes  are  reduced  to  a  thin  depressed  plate,  with  an  underlying  mass  of  yolk 
cells.  Cam.  Xso.  B,  Stage  G.  Anterior  half  of  thorax  absent,  and  the  greatly  reduced  cephalic  lobes  covered  by 
a  thick  fold  of  ectoderm.  Cam.  Xso.  C,  Stage  G.  The  abdomen  and  the  anterior  half  of  the  thorax  absent. 
Last  three  thoracic  appendages  on  the  left,  reduced  to  shallow  pits;  cam.  Xso.  D,  Germinal  disc,  with  three  un- 
paired thoracic  appendages;  cam.  Xso.  E,  Remnants  of  an  embryo,  probably  in  stage  G;  only  the  tip  of  the  ab- 
domen, or  the  unpaired  remnants  of  one  of  the  more  posterior  pairs  of  appendages,  projects  above  the  surface  of 
the  faint  embryonic  area;  cam.  X6o.  F-K,  Similar  embryos  in  more  advanced  stages  of  degeneration. 

alone  may  be  present,  without  the  head  and  thorax.  The  most  common  form  con- 
sists of  little  more  than  the  three  posterior  thoracic  metameres.  Such  embryos 
may  have  a  deceptive  resemblance  to  a  crustacean  nauplius.  (Fig.  186,  B.) 

Such  cases  as  these,  and  the  hour-glass  type,  show  that  certain  groups  of 
metameres  have  an  organic  unity  that  enables  them  to  survive,  at  least  for  a  certain 
period,  independently  of  other  parts  of  the  body.  It  suggests  the  phenomena  of 
transverse  division  in  the  annelids;  the  remarkable  process  in  cirripeds  (Sacculina) 
by  which  the  abdomen  is  cast  off,  leaving  only  the  thorax  to  complete  the  life  cycle; 


ACEPHALIC  AND   ACAUDAL   EMBRYOS.  281 

and  the  amputation  of  the  head,  in  larval  star-fishes,  leaving  only  the  posteiior 
part  to  complete  its  development. 

D.  Final  Stages  of  Degeneration. — The  vast  majority  of  all  abnormal 
embryos,  whether  single,  doublets,  or  triplets,  continue  to  degenerate  by  gradually 
cutting  off  the  more  anterior  segments,  or  by  some  modification  of  the  process  of 
median  fusion  and  antero-posterior  degeneration.  The  details  of  the  final  stages 
cannot  be  followed,  but  the  general  nature  of  the  process  and  the  final  results  are 
readily  observed.  In  the  class  of  cases  we  shall  now  consider,  after  the  disappear- 
ance of  all  the  appendages,  the  embryo  may  be  reduced  to  a  mere  pit  or  sac,  yet 
preserving  certain  features,  such  as  concrescing  mesodermic  areas  and  protruding 
tail  lobe,  which  show  clearly  the  advanced  stage  of  development  in  which  the 
whole  embryo  would  have  been,  had  no  degeneration  taken  place.  (Fig. 
186,  J.K.) 

Some  embryos  may  consist  of  two  pits,  or  two  groups  of  cells,  like  two 
primitive  cumuli,  one  corresponding  to  the  head  and  the  other  to  the  tail  end  of  the 
body.  (Fig.  186,  G.H.)  Finally  these  sac-like  remnants  are  reduced  to  faint 
clouds  of  scattered  cells,  or  nuclei,  which  in  turn  disappear,  leaving  no  trace  of 
living  substance  in  the  yolk. 

The  conditions  we  have  just  described  are  important  in  that  they  give  us  a 
glimpse  of  the  negative  processes  of  life,  side  by  side  with  the  positive  ones.  They 
afford  us  a  new  picture  of  death,  unlike  the  one  with  which  we  are  most  familiar. 
In  these  embryos  cell  production,  cell  specialization,  and  cell  decay  proceed  side 
by  side,  for  in  every  part  of  the  body  karyokinetic  figures  and  the  fragments  of 
decaying  nuclei  are  found.  The  result  depends  on  the  relative  intensity  of  these 
three  factors.  The  embryos  apparently  dwindle  in  size  because  the  death  rate 
of  the  cells  is  greater  than  the  birth  rate.  Nerve  centers,  sense  organs  and  ap- 
pendages disappear  because  the  specialization  of  individual  cells  ceases  and  only 
the  simplest  kinds  remain. 

The  process  of  degeneration  is  never  exactly  the  same,  but  if  completed  it 
invariably  carries  the  organism  back,  in  the  main,  over  the  old  lines  of  progressive 
development  till  it  is  reduced  to  its  primitive  condition,  namely,  a  small  community 
of  similiar,  unspecialized  cells  which  disappears  with  the  death  of  the  last  survivor. 

This  may  be  called  the  true  natural  death  of  an  organism,  all  others  are  more 
or  less  catastrophic,  and  are  due  to  the  increasing  lack  of  coordination  and  of  ad- 
justment to  the  new  conditions  that  have  been  created  by  growth. 

IV.  DOUBLE  EMBRYOS. 

We  may  distinguish  two  kinds  of  fission,  transverse  and  longitudinal. 

i.  Transverse  fission  divides  the  embryo  into  anterior  and  posterior  por- 
tions, the  point  where  the  division  most  frequently  occurs  being  between  the  third 
and  fourth  thoracic  appendages,  or  between  the  abdomen  and  thorax.  The 
steps  leading  up  to  this  form  have  been  described  under  the  preceding  sections. 


282 


VARIATION   AND    MONSTROSITIES. 


2.  Longitudinal  fission  is  radically  different  from  transverse  fission,  for 
the  latter  is  the  result  of  a  local  concrescence  and  degeneration  of  segmental 
organs,  while  longitudinal  fission  consists  in  the  formation  of  two  new  halves 
of  an  embryo  along  the  median  line  of  one  already  existing.  The  formation  of 
the  new  halves  begins  at  the  anterior  end  and  extends  gradually  backward.  The 
old  halves  are  thus  thrust  apart  and  each  old  half,  together  with  the  adjacent  new 
half,  makes  a  new  embryo. 


FIG.  187.  —  Limulus  embryos,  stage  H,  showing  various  steps  in  the  formation  of  double  embryos.  Double 
embryos  are  formed,  in  all  the  observed  cases,  by  the  generation,  beginning  at  the  anterior  end,  of  two  new  halves 
between  the  old  ones.  If  there  are  five  paired  organs  on  each  segment,  and  a  is  median  and  e  lateral,  then  a  will 
be  the  first  new  organ  to  appear,  and  it  will  appear  in  the  median  line  of  the  first  segment  as  an  unpaired  organ. 
It  divides,  and  in  its  place  in  the  same  segment  will  be  found  an  unpaired  organ,  like  organ  b.  But  at  the  same 
time  a  new,  unpaired  organ,  like  a,  will  be  formed  in  the  median  line  of  segment  number  two.  At  the  next  division, 
organ  a  will  be  produced  in  the  median  line  of  the  third  segment,  b  in  the  second,  and  c  in  the  first;  organs  a  and  b 
being  now  completely  formed  in  pairs  in  the  first  segment,  and  organs  b  in  the  second.  This  process  goes  on  till 
two  complete  new  halves  are  wedged  in  between  the  old,  and  two  new  individuals  are  produced,  each  individual 
consisting  of  an  old  and  a  new  half. 

A,  and  B,  Early  stages  in  the  formation  of  the  new  halves;  cam.  X  16.  C,  Later  stage;  cam.  X  15  .  D,  The 
left-hand  embryo  has  begun  to  disappear  by  median  fusion  and  progressive  cephalo-caudad  degeneration;  cam. 
X  1  6  1/2.  E,  The  two  embryos  have  completely  separated  and  the  one  on  the  left  is  disappearing  by  the  char- 
acteristic method  of  degeneration;  cam.  X  16  1/2.  F,  One  embryo  normal;  the  other  reduced  to  a  single 
appendage,  and  a  narrow  embryonic  area;  cam.  X  16.  Stage  H. 


This  process  may  be  repeated  a  second  time  in  one  of  the  new  embryos,  thus 
producing  three  embryos  tail  to  tail,  consisting  of  the  two  original  halves  plus 
four  new  ones. 

All  multiple  embryos  in  Limulus  are  formed  in  the  above  manner,  never  by  the 
partial  union  of  embryos  originally  separate.  This  is  shown  by  the  fact  that 
in  all  these  cases  the  embryos  match  each  other  exactly,  and  always  in  the 
same  way,  which  could  hardly  be  the  case  if  two  separate  embryos  had 
united  through  accidental  contact. 


DOUBLE    EMBRYOS. 


283 


Where  do  the  new  halves  come  from,  and  by  what  processes  of  growth  are  they 
formed  ?  There  is  no  evidence  of  the  existence  of  special  formative  material  along 
the  median  line  where  the  new  parts  are  forming.  This  is  especially  clear  when 
the  process  begins  at  a  late  period.  The  old  halves  are  then  quite  distinct  from 
the  new,  and  there  seems  to  be  no  way  open  to  explain  the  origin  of  the  new  half 
of  a  segment  by  lateral  budding,  or  by  regeneration,  or  by  growth  from  the  cor- 
responding old  one.  We  might  perhaps  infer  that  the  new  neuromeres  in  Fig. 
187,  A.B.  come  from  a  kind  of  regeneration  of  the  old  one,  but  that  could  not 
possibly  be  the  case  with  any  of  the  new  organs  lateral  to  the  neuromere,  such  as 
the  appendages,  sense  organs,  and  the  margin  of  the  mesodermic  area. 


FlG.    188. — Double  embryos  of    Limulus,    accompanied   by   median   fusion   and    cephalo-cauded    degeneration; 

cam.  X  16  1/2. 

A  comparison  of  double  embryos  in  various  stages  shows  that  the  sequence  in 
the  production  of  new  organs  is  as  follows :  The  most  anterior  metameres  are 
formed  first.  Each  new  organ  of  a  metamere  first  appears  as  a  single  organ  com- 
mon to  both  embryos  and  having  a  normal  position  for  each.  Additional  organs 
are  formed  in  the  same  way,  in  the  order  of  their  arrangement  on  the  metamere. 
For  example,  the  organ  nearest  the  median  line  is  formed  first;  this  then  divides 
into  two,  and  the  one  lateral  to  it  appears  between  them  as  a  single  organ  common 
to  both  embryos;  this  divides,  and  the  next  one  appears  in  the  same  place,  till 
all  the  organs  of  a  given  metamere  are  formed.  (Fig.  187,  A.B.)  The  same 
process  takes  place  in  the  next  posterior  metamere,  but  it  is  always  one  step  be- 
hind that  in  the  metamere  in  front  of  it. 

An  embryo  that  has  nearly  completed  its  division,  as  shown  in  the  diagram 
(Fig.  190),  presents  a  row  of  median  unpaired  organs  which  follow  the  same 
order  in  a  cephalic  direction  that  they  do  in  a  lateral  direction  on  the  metamere, 
namely  a.b.c.d.e. 

The  successive  eruption  of  new  organs  along  this  median  line,  and  the  man- 
ner in  which  they  divide  and  move  away  from  it  to  right  and  left,  is  so  entirely 
different  from  what  we  have  been  accustomed  to  see  that  it  is  very  impressive. 


284  VARIATION   AND    MONSTROSITIES. 

New  organs  make  their  first  appearance  in  the  same  stage  of  development 
as  the  corresponding  old  ones.  Each  newly  formed  median  appendage  first 
attains  a  considerable  size,  then  divides  at  the  apex,  the  separation  gradually 
extending  toward  the  base.  This,  it  will  be  observed,  is  the  exact  reverse  of  what 
occurs  when  degenerating  appendages  of  the  right  and  left  sides  unite  to  form  a 
single  median  one. 

During  the  later  stages  of  double  embryos  the  growth  of  the  new  halves  forces 
the  old  ones  apart,  and  the  two  embryos  then  swing  into  a  straight  line,  tail  to 
tail.  (Fig.  187,  C.)  They  may  then  separate,  moving  tail  first  in  opposite 
directions  till  they  lie  on  opposite  sides  of  the  egg.  (Fig.  187,  £.) 

At  any  stage  in  this  process  of  division,  one  or  both  embryos  may  undergo 
the  typical  median  fusion  and  antero-posterior  degeneration.  The  process  may 
go  on  in  one  embryo  quite  independently  of  that  in  its  mate,  but  always  in  the 
typical  manner  described  for  the  single  embryos.  (Figs.  187,  188.) 

V.  TRIPLE  EMBRYOS. 

Triple  embryos  are  very  rare.  Their  mode  of  origin  is  shown  by  the  accom- 
panying diagram.  (Fig.  190,  B.)  It  is  assumed  that  in  the  beginning  a  single 
normal  embryo,  in  the  manner  already  described,  gives  rise  to  two  embryos,  of 


• 


FIG.   189.  —  Triple  embryos  of   Limulus,  showing  extensive  median  fusion  and  cephalo-caudad 

degeneration. 

A.  Of  the  three  embryos  in  this  egg,  A  is  normal  and  perfect  in  everything  except  the  abdomen.     B  has  under- 
gone median  fusion  and  degeneration,  and  transverse  fission.     The  cephalic  lobes  and  first  four  segments  have  dis- 
appeared, except  two  incompletely  fused  appendages.     The  abdomen  and  the  posterior  part  of  the  thorax  persists. 
The  latter  is  bounded  in  front  of  the  fifth  pair  of  appendages  by  a  great  fold  that  extends  completely  across  the 
median  line.     The  nerve-cord  in  this  posterior  remnant  of  an  embryo  forms  a  conspicuous,  unpaired  r^dge.     Em- 
bryo C  has  undergone  extensive  fusion  and  antero-posterior  degeneration,  nothing  remaining  but  the  fused  appen- 
dages of  the  sixth  segment,  and  a  rudimentary  abdomen.     It  is  probable  that  the  original  embryo  divided  length- 
wise, giving  rise  to  A  and  BC,  and  the  latter  then  divided,  giving  rise  to  B  and  C.     Cam.  X  15- 

B.  Embryo  A  has  undergone  median  fusion  and  transverse  fission.     The  fused  appendages  of  the  first  four  seg- 
ments are  arranged  in  a  single  row.     The  cephalic  lobes  are  narrowed,  and  covered  by  a  hood-like  fold  of  ectoderm, 
through  which  one  sees  the  oesophagus.     The  marginal  fold  has  grown  across  the  median  line  in  front  of  the  fourth 
pair  of  appendages.     In  front  of  this  fold,  and  near  the  median  line,  are  the  dorsal  organs.     Embryo  B  has  degen- 
erated completely  in  front  of  the  fused  fifth  pair  of  appendages,  with  the  exception  of  the  dorsal  organs,  which  have 
almost  reached  the  median  line.     In  embryo  C,  the  median  fusion  and  degeneration  has  progressed  still  farther, 

or  the  dorsal  organs  have  fused  and  also  the  six  pairs  of  appendages.     At  the  central  ends  of  all  three  embryos,  are 
paired  and  unpaired  ridges,  representing  abdominal  appendages.     Mercator  projection;  cam.  X  15. 

C.  A  triple  embryo  in  which  each  individual  is  reduced  to  an  unpaired  dorsal  organ,  to  the  last  thoracic  and 
first  branchial  appendages.     There  are  faint  indications  of  cardiomeres  and  of  caudal  segments;  cam.  X  16  1/2. 


TRIPLE    EMBRYOS.  285 

which  one  is  A ,  the  other  occupying  the  position  of  B.  The  right  hand  embryo 
then  divides  again,  forming  embryos  B.  and  C.  Thus  two  new  generations  of 
halves  are  produced,  //  and  ///,  each  consisting  of  an  embryo  with  inverted 


FIG.  190. — Diagrams  illustrating  the  mode  of  growth  of  double  and  triple  embryos. 

A.  Diagram  of  a  dividing  limulus  embryo  showing  the  sequence  in  which  the  organs,  a-e,  of  the  two  new  halves 
are  generated.     The  newly  formed,  unpaired  organs  are  in  capitals;  the  organs  formed  by  their  division  are  in  corre- 
sponding small  letters.     The  old  halves  shaded,  new  halves  unshaded;  see  also  explanation  of  Fig.  187. 

B.  Diagram  of  a  triple  embryo.     First  generation  of  metameres,  black  and  shaded;  second,  blank;  third, 
dotted. 

right  and  left  sides.  The  three  generations  now  form  a  tri-radiate  figure,  with 
the  new  and  old  halves  making  three  apparently  normal  embryos,  A.  B.  and  C. 
But  the  halves  of  the  original  embryo  are  now  separated  by  an  angle  of  about 


286 


VARIATION  AND   MONSTROSITIES. 


FIG.  191. — Diagram  of  a  degenerating  limulus  embryo,  showing  the  order  in  which  the  paired  organs  unite  in 
the  middle  line  and  then  disappear.  Organs  formed  by  normal  apical  growth  are  represented  by  small  letters; 
the  unpaired  organs  formed  by  their  union,  and  which  are  about  to  disappear,  are  in  capitals. 


y 

/ 

\ 

\1 

\2 

X 

/ 

a 

a 

/ 

b 

a 

a 

b 

c 

b 

a 

a 

b 

C 

d 

c 

b 

a 

a 

b 

C 

d 

e 
\ 

d 

c 

b 

a 

CL 

b 

c 

d 

e 

e 

d 

\ 

c 

b 

a 

a 

b 

c 

/ 

e 

e 

d 

c 
\ 

b 

a 

a 

b 

c 

d 

e 

e 

d 

c 

\ 

a 

a 

b 

c 

d 

e 

e 

d 

c 

b 

a 

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b 

c 

d 

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L\ 

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c 

b 

a 

a 

b 

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A 

c 

b 

a 

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c 

( 

\ 

b 

a, 

a 

b 

/It. 
3 

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a 

a 

/I, 

FIG.  192. — Diagram    illustrating  the  law  of  growth  and  degeneration  of  segmented,  bilaterally  symmetrical 

animals. 

A.  The  segmental  organs,  seen  in  mercator  projection  from  the  neural  surface,  are  represented  by  letters,  and 
the  metameres  by  numbers.     A-P,  The  principal,  or  neural  axis;  A,  cephalic;  P,  caudal  end.     A,  m.n.,  area  of 
initial  growth;  m.L.R.n,  area  of  multiple  growth;  L.R.P.,  area  of  diminishing  growth. 

B.  The  arrangement  of  organs  after  the  degeneration  of  the  area  in  figure  A,  enclosed  by  the  lines  A.m.  a.n.* 
With  the  disappearance  of  this  area,  the  lines  w.a.9  and  a.n.4  unite  to  form  the  line  of  unpaired  organs  E-A. 


SUMMARY.  287 

240°;  and  embryo  A.  consists  of  a  mother  and  a  daughter  half;  B.  of  a  daughter 
and  a  grand-daughter  half,  and  C.  of  a  granddaughter  and  a  mother  half. 

Some  of  the  conditions  that  are  actually  realized  are  shown  in  Fig.  189. 
In  all  these  cases,  median  fusion  and  degeneration  accompanies,  or  follows  the 
formation  of  triplets. 

In  Fig.  189,  A,  embryo  A.  is  practically  normal;  embryo  B.  has  undergone 
median  fusion  and  degeneration,  almost  resulting  in  transverse  fission  at  the 
fourth  segment.  The  abdomen  and  last  two  thoracic  appendages  are  practically 
normal,  while  the  anterior  part  of  the  thorax  and  cephalic  lobes  has  disappeared, 
except  one  pair  of  fused  appendages.  In  embryo  C.  median  fusion  and  degen- 
eration have  obliterated  all  but  the  abdomen  and  the  last  pair  of  fused  thoracic 
appendages. 

In  another  triplet  (Fig.  189,  B),  embryo  A  has  undergone  median  fusion  and 
degeneration,  forming  a  good  example  of  an  hour-glass  embryo.  The  same  proc- 
ess has  affected  embryo  B,  entirely  obliterating  the  cephalic  lobes  and  the  anterior 
portion  of  the  thorax;  the  dorsal  organs,  however,  are  not  quite  fused  in  the  median 
line.  But  this  has  taken  place  in  embryo  C,  and  in  other  respects  the  degeneration 
is  carried  farther  than  in  B. 

In  still  another  triplet  (Fig.  189,  C),  all  three  embryos  are  reduced  by  antero- 
posterior  fusion  and  degeneration  nearly  to  the  same  level;  each  one  retains  an 
unpaired  sixth  thoracic  appendage,  a  remnant  of  the  abdomen,  and  an  unpaired 
dorsal  organ. 

Multiple  embryos,  therefore,  are  formed  by  the  appearance  of  new  halves 
between  the  old,  the  various  organs  being  formed  in  a  definite  and  orderly  manner. 
After  a  time  degeneration  begins,  the  organs  disappearing  by  a  method  and  in  an 
order  that  are  the  reverse  of  those  in  which  they  were  generated. 

VI.  SUMMARY  AND  CONCLUSION. 

1.  The  variations  here  described  are  primarily  due  to  structural  variations 
resident  in  the  ovum,  and  not  to  differences  in  the  environment. 

2.  There  is  a  great  difference  in  the  growth  rate,  under  apparently  the  same 
conditions. 

3.  There  is  a  great  difference  in  size,  some  embryos  of  the  same  stage  being 
much  larger  than  others. 

4.  Certain  organs  or  regions  of  the  body  may  be  entirely  absent,  and  are  not 
subsequently  restored. 

5.  When  organs  disappear  it  is  usually  by  median  fusion  and  degeneration, 
in  the  reverse  order  of  their  age  and  specialization. 

'  6.  Multiple  embryos  are  produced  by  the  formation  of  new  halves  between 
the  old,  the  process  beginning  at  the  head  end.  The  new  organs  are  produced  in 
the  reverse  order  to  that  in  which  they  are  formed  by  normal  apical  growth,  that  is, 
the  lateral  ones  before  the  median  ones.  The  arrangement  is  also  reversed,  the 


288  VARIATION   AND    MONSTROSITIES. 

haemal  organs  lying  in  the  axis  of  growth  and  the  neural  ones  on  the  margins. 
The  way  in  which  the  new  organs  make  their  appearance  is  the  reverse  of  that 
by  which  organs  disappear  by  median  fusion  and  degeneration. 

7.  Multiple  embryos  thus  formed  disappear  again  by  median  fusion  and 
anteroposterior  degeneration. 

8.  Variation  in  Limulus  is  primarily  of  a  plus  or  minus  nature.     The  endless 
variety  of  results  attained  is  due  to  the  presence  of  a  larger  or  smaller  number  of 
organs.     We  are  apparently  always  dealing  with  the  same  things  and  with  the 
same  kind  of  stuff.     When  an  unusual  form  of  the  aggregate  appears,  as  in  the 
semicircular  form  of  a  half  embryo,  it  is  the  indirect  result  of  the  absence  of  some 
other  part.     In  no  case  does  a  new  organ  or  a  new  part  appear  that  is  different  in 
kind  from  those  already  existing ;  in  no  case  is  an  organ  out  of  place  in  reference  to 
others;  in  all  cases  the  organs  come  and  go  in  a  definite  orderly  sequence. 

The  organs  of  the  embryo,  crystal-like,  are  always  expressed  in  approximately 
the  same  forms  and  in  similar  geometrical  aggregates.  The  successive  steps  in 
apical  growth,  in  degeneration,  in  twin  formation,  and  again  in  degeneration,  are 
minutely  graded  transitional  phases  in  which  the  living  substance  in  the  egg  of 
Limulus  finds  formal  expression. 

9.  The  parallel  rise  and  fall,  or  the  opening  and  closing  of  the  checker-board 
pattern,  is  a  fundamental  attribute  of  organic  growth,  and  is  a  basic  factor  in  the 
morphogenesis  of  all  segmented  animals.     The  laws  of  differential  apical  growth, 
and  of  the  reverse  process,  or  degeneration,  are  the  keys  to  their  morphology, 
accounting  for  their  bodily  subdivisions,  and  for  the  unequal  growth,  specializa- 
tion, and  union  of  their  various  organs. 


CHAPTER  XVI. 
THE  DERMAL  SKELETON. 

We  recognize  four  distinct  structures  in  the  skeleton  of  primitive  vertebrates: 

1.  The  dermal  skeleton,  consisting  of  bony  plates  more  or  less  intimately  united 
to  form  a  continuous  external  armor  for  the  head  and  trunk;  2.  the  primordial 
endocranium,  consisting  of  a  broad  unsegmented  plate  of  fibrocartilage  lying  on 
the  haemal  side  of  the  brain;  3.  the  gill-bars,  segmentally  arranged  cartilage  bars 
lying  in  the  visceral  arches;  4.  the  notochord;  5.  neural  arches,  segmentally  ar- 
ranged blocks,  or  half-rings  of  cartilage  distributed  along  the  sides  of  the  nerve 
cord  and  notochord.     These  structures  differ  from  one  another  in  their  chemical 
and  histological  composition,  and  in  their  origin.     The  general  trend  in  the  evolu- 
tion of  the  vertebrate  skeleton  is  toward  the  elimination  of  the  two  oldest  constitu- 
ents, the  dermal  skeleton  and  the  notochord,  and  the  union  of  the  remaining 
elements,  without  distinction  of  origin,  structure,  or  previous  function,  into  a 
common  axial  skeleton. 

These  five  sets  of  skeletal  structures  are  already  established  in  the  arachnids, 
viz:  i.  The  external  chitenous  armor  with  its  primordial  canaliculi  and  lacunae. 

2.  the    fibrocartilaginous    endocranium  or  plastron;    3.  the  cartilaginous  bars 
supporting  the  branchial  appendages;    4.  the  middle  cord,  or  lemmatochord, 
and  5.  the  segmentally  arranged  cartilages  over  the  spinal  cord.     Nothing  re- 
sembling this  assemblage  of  skeletogenous  tissues  is  found  in  any  other  animals 
outside  the  vertebrates  and  arachnids. 

I.  THE  DERMAL  SKELETON  OF  VERTEBRATES. 

The  dermal  skeleton  of  primitive  vertebrates  consists  of  a  series  of  bony 
plates,  not  preformed  in  cartilage,  arising  in  or  beneath  the  epidermis.  It  has 
been  generally  assumed  that  the  most  primitive  dermal  skeleton  is  one  consisting 
of  small  isolated  placoid  scales,  similar  to  those  in  the  elasmobranchs;  and  that 
the  larger  plates  seen  in  the  ganoids,  dipnoi,  and  ostracoderms  were  formed  by 
the  secondary  union  of  such  scales.  This  assumption  is  based  on  the  prevalent 
belief  that  the  elasmobranchs  and  cyclostomes  are  the  most  primitive  vertebrates, 
and  that  the  above  mentioned  heavily  armored  forms  were  derived  from  them. 
This  view  is  untenable,  since  it  takes  no  account  of  the  fact  that  the  ostracoderms, 
which  are  the  oldest  known,  and  the  most  primitive  fish-like  vertebrates,  were  pre- 
eminently heavily  armored  forms. 

We  shall  reverse  the  usual  way  of  approaching  this  question  and  start  with  the 
assumption  that  the  most  primitive  vertebrate  dermal  armor  is  like  that  of  the 

19  289 


THE    DERMAL    SKELETON. 

ostracoderms,  and  consists  of  a  practically  continuous  bony  envelop  for  the  head 
and  gill  region,  with  segmentally  arranged  plates,  loosely  articulated,  covering  the 
joints  of  the  trunk  and  tail.  Such  an  exoskeleton  resembles  that  of  the  marine 
arachnids,  the  hypothetical  ancestors  of  the  vertebrates. 

It  is  assumed  that  the  cephalic  buckler  of  the  ostracoderms  corresponds  to  the 
cephalothoracic  shield  of  the  merostomes  and  arachnids  generally,  i.e.,  the  primi- 
tive head,  the  first  six  thoracic,  and  the  vagus  segments.  The  buckler  of  such 
forms  as  Pteraspis,  Tremataspis  and  Bothriolepis  also  includes  the  abdominal  or 
respiratory  segments. 

The  line  of  union  between  the  thoracic  and  branchial  regions  is  clearly  indi- 
cated in  Bothriolepis  by  the  hinge-like  joint  separating  the  cephalic  buckler  from 
the  respiratory  chamber.  (Fig.  247.)  Just  in  front  of  this  joint  are  two  pores 
leading  into  the  interior  of  the  shell.  Two  similar  pores  are  found  in  Tremataspis 
(Fig.  236),  and  just  back  of  these  pores  is  probably  the  dividing  line  between  the 
cephalic  and  branchial  regions  in  these  animals. 

In  Cephalaspis  and  Tremataspis,  the  trunk  region  is  covered  with  segmentally 
arranged  dermal  plates  corresponding  with  the  plates  covering  the  neural  surface 
of  the  post-branchial  region  in  arthropods.  Only  a  few  of  the  more  anterior 
segmental  plates  of  the  ostracoderms  are  represented  in  the  arthropods,  the  more 
posterior  ones  being  new  formations  added  after  the  separation  of  the  vertebrate 
from  the  arthropod  stock  had  taken  place. 

In  Bothriolepsis,  and  in  a  small  undescribed  species  of  Cephalaspis  from 
Dalhousie  (Fig.  234),  the  trunk  and  tail  are  naked,  save  for  a  few  minute,  irregu- 
larly distributed  ossicles  near  the  anterior  end  of  the  trunk. 

In  Pterichthys  and  Pteraspis,  the  trunk  appears  to  have  been  covered  with 
rounded  or  polygonal  scales,  probably  formed  by  the  breaking  up  of  larger  seg- 
mental scales  like  those  of  Trematapsis. 

The  Minute  Structure  of  the  Dermal  Bones  of  the  Ostracoderms. 

The  structure  of  the  dermal  bones  of  Tremataspis,  Pteraspis,  and  Bothriolepis 
has  been  studied  with  special  care,  and  some  observations  were  made  on  the 
dermal  skeleton  of  Cephalaspis  and  Tolypaspis,  but  they  are  incomplete,  owing  to 
the  lack  of  adequate  material. 

It  is  surprising  how  beautifully  the  details  of  the  minute  structure  of  these 
ancient  fish  bones  is  preserved.  When  properly  prepared,  the  color  and  the 
minutest  details  may  occasionally  be  seen  as  clearly  as  though  the  animals  had 
been  dead  a  few  weeks  only,  instead  of  a  few  million  years. 

Tremataspis. 

In  Tremataspis  the  outer  surface  of  the  shell  generally  has  a  light  yellowish- 
brown  color.  It  is  beautifully  polished  and  under  the  lens  appears  to  be  orna- 
mented with  low  winding  ridges  and  mounds,  similar  to  those  of  Bothriolepis, 


TREMATASPIS. 


291 


but  much  fainter.  The  surface  is  dotted  with  minute  circular  openings  arranged 
in  irregular  rows  that  usually  correspond  with  the  meshes  of  the  underlying  canals. 
The  shell  is  about  0.3  mm.  thick.  The  inner  portion  consists  of  horizontal 
lamellae  of  uniform  width,  interrupted  by  large  irregular  chambers.  (Fig.  193.) 
The  lamellae  appear  to  have  consisted  originally  of  fibrous  strands  arranged  with 


FIG.  193. — Cross  section  of  the  exoskeleton  of  Tremataspis. 

great  regularity  into  parallel  bundles,  those  in  adjacent  layers  running  at  right 
angles  to  one  another.  Viewed  from  the  inner  surface  of  the  shell  the  strands  of 
the  lamellae  look  like  the  warp  and  woof  of  a  coarse  cloth  (Fig.  194).  Between 
the  layers  are  flattened  lacunae,  sometimes  filled  with  a  reddish-brown  substance 
that  gives  them  a  very  striking  resemblance  to  living  pigment  cells.  Canaliculi 


,.,-exp. 


tr. 


FIG.  194. — Exoskeleton  of  Tremataspis,  seen  from  the  inner  surface.   Enough  of  the  outer  surface  has  been  removed 

to  make  the  remainder  semi-transparent. 

radiate  from  the  lacunae  and  anastomose  with  those  in  the  layers  of  bone  above 
and  below. 

The  substance  of  the  shell  is  penetrated  by  two  sets  of  canals.  The  deeper 
set  forms  a  horizontal  meshwork  of  uniform  caliber,  opening  to  the  outer  surface 
at  frequent  intervals  by  short  conical  chimneys.  (Fig.  193,  sn.c.)  This  system 
of  canals  is  generally  filled  with  a  peculiar  matrix  that  is  either  colorless  and  similar 


292  THE    DERMAL    SKELETON. 

to  that  outside  the  shell,  or  impregnated  with  a  dark  red  or  black  substance.  The 
walls  of  these  canals,  which  appear  to  have  contained  sense  organs  and  mucous 
glands,  are  very  sharply  defined.  Bone  lacunae  do  not  open  into  them,  and  they 
lead  only  at  irregular  intervals,  if  at  all,  into  the  other  system  of  canals,  or  into  the 
cancellae.  They  appear  to  be  uniformly  distributed  in  the  shell,  throughout  all 
parts  of  the  buckler.  When  the  outermost  layers  are  removed  and  the  shell  is 
viewed  as  a  semi-transparent  object  from  its  inner  surface,  the  sensory  canals  are 
seen  to  overlie  the  partition  separating  the  cancellae.  (Fig.  194.) 

The  canals  belonging  to  the  outer  set  are  smaller  and  much  fainter  than  the 
ones  just  described,  and  never  contain  pigment,  or  foreign  materials  derived  from 
the  surrounding  matrix.  They  form  a  horizontal  polygonal  mesh  network  of 
slender  irregular  vascular  canals,  v.c.,  that  open  by  larger  ones  into  the  summit  of 
the  cancellae,  c,  and  hence  through  the  floor  of  the  cancellae  into  the  interior,  i.p. 


exp 

' v 


FIG.  195. — A,  Tangential  section  of  the  outer  layers  of  the  exoskeleton  of  Tremataspis;  B,  outermost  layer,  in  sur- 
face view,  more  highly  magnified;  C,  same  in  cross-section. 

Several  strands  of  the  arching  horizontal  canals  lead  into  irregular  spaces  lying  in 
the  center  of  the  areas  enclosed  by  the  chimney  pores.  From  the  floor  of  these 
spaces,  numerous  irregularly  looped  canals  arise  that  project  inward,  forming  ill- 
defined  tufts  of  vascular  canals  (Fig.  195,  v.c.),  on  about  the  same  level  with  the 
large  sensory  canals.  Some  of  these  canals  appear  to  open  occasionally  into  the 
floor  of  the  sensory  canals. 

From  the  roof  and  sides  of  all  the  outermost  vascular  canals,  and  from  their 
points  of  union  with  one  another,  arise  numerous  tapering  vertical  canals,  the 
osteo-dentinal  canals.  They  are  larger  than  the  ordinary  canaliculi,  especially 
at  their  proximal  ends,  and  after  expanding  into  a  row  of  three  or  four  overlying 
lacunae,  open  into  innumerable  anastomosing  canaliculae.  The  main  axial  canal 
terminates  in  a  single  minute  canaliculus,  that  runs  through  a  faintly  defined 
cylinder  to  the  outer  surface.  (Figs.  193,  195,  e.) 

The  layer  of  prisms  with  their  axial  pore  canals  make  up  the  glassy  outer 
surface  of  the  shell;  the  layer  of  osteo-dentinal  canals  forms  the  dentinal  layer. 

The  bone  cells,  or  lacunae,  lying  in  the  spaces  between  the  vascular  and  sen- 
sory canals  are  smaller  than  the  ones  just  described  and  are  more  like  the  typical 
bone  lacunae.  They  appear  to  lie  between  the  concentric  bone  lamellae  surround- 
ing the  canals. 


DERMAL  SKELETON  OF  PTERASPIS. 


2Q3 


Pteraspis. 


Of  the  genus  Pteraspis  only  a  part  of  the  cephalic  armor  and  a  few  scale-like 
structures  belonging  to  the  anterior  part  of  the  trunk  are  known. 

The  boat-shaped  dorsal  shield  (Fig.  245),  is  composed  of  seven  portions, 
marked  off  on  the  outer  surface  of  the  shield  by  furrows,  and  on  the  inner  surface 
by  ridges.  In  young  specimens  the  rostrum  and  the  central  disc  may  be  found 
separately.  In  each  piece  the  ornamental  surface  ridges  and  furrows,  which  look 
much  like  the  wavy  lines  on  the  finger  tips,  are  arranged  in  concentric  lines  parallel 
with  its  margins.  This  fact,  together  with  other  considerations,  led  Lankester  to 


^ 

FIG.  196. — A,  Cross-section  of  the'shield  of  Pteraspis.'at'right  angles  to  the  surface  ridges;     B,  Tangential  section 
through  the  outer  layers,  and  nearly  parallel  with  the  outer  surface. 

believe  that  each  piece  ossified  from  a  separate  center,  and  that  their  complete 
anchylosis  occurred  only  in  the  adult.  •  p 

Along  the  lateral  margins  of  the  shield,  in  the  rostrum,  and  near  the  posterior 
dorsal  spine,  the  shell  is  greatly  thickened,  and  consists  of  a  network  of  bony  tra- 
beculae  with  irregular  spaces  between.  In  the  median  portions  it  is  of  a  more 
uniform  thickness,  about  0.6  to  0.8  mm. 

In  sections  across  the  ridges  (Fig.  196,  -4),  the  shell  is  seen  to  consist  of  four 
principal  layers:  i.  A  thick  inner  wall,  &./.,  composed  of  many  parallel  layers  of 
uniform  thickness,  perforated  here  and  there  by  openings  that  lead  from  the  interior 
of  the  head  into  the  overlying  cancellous  spaces.  2 .  A  cancellated  layer,  c.l. ,  consist- 
ing of  polygonal  chambers,  c.,  separated  by  thin  vertical  walls  that  are  perforated 
here  and  there  by  narrow  lateral  passages.  3.  Three  or  four  layers  of  canals, 
v.L,  each  layer  forming  a  close  four-sided  meshwork  from  which  vertical  canals 


294 


THE    DERMAL    SKELETON. 


connect  with  the  layer  above  and  below.  The  diameter  of  the  canals  in  the  inner 
layer  is  the  largest  and  the  main  canals  of  this  layer  run  at  right  angles  to  the  sur- 
face ridges.  (Fig.  196,  B.)  The  diameter  of  the  canals  in  the  several  layers  dimin- 
ishes toward  the  outer  surface;  at  the  same  time  the  apparent  trend  of  the  main 
canals  of  a  layer  gradually  shifts,  so  that  in  the  outermost  one  the  main  horizontal 
canals  run  parallel  with  the  surface  ridges,  one  canal  running  lengthwise  along  the 
basal  portion  of  each  ridge.  In  sections  parallel  to  the  outer  surface  as  well  as  in 
cross-section,  it  is  readily  seen  that  each  ridge  canal  opens  right  and  left  into  the 
bottom  of  the  grooves,  s.c.,  between  the  ridges,  and  at  pretty  regular  intervals  sends 
a  short  loop  upward  into  the  ridge  itself,  c.ca.  From  the  summit  of  these  ridge- 


FIG.    197.  FIG.    198. 

FIG.    197. — Cross-section  of  a  small  portion  of  a  shield  especially  well  preserved,  and  probably  belonging  to 
Pteraspis;  highly  magnified,  showing  the  axial  core  and  the  sharply  laminated  structure  of  the  trabeculae. 
FlG.  198. — Trabeculae  of  Ateleaspis. 

loops  arise  the  radiating  dentinal  canals  of  the  ridges.  These  terminal  canals 
resemble  those  of  Tremataspis,  but  they  do  not  contain  any  lacuna-like  dilatations. 
4.  The  layer  of  surface  ridges  and  the  intervening  grooves  form  the  fourth  or 
outer  layer  of  the  shield. 

The  substance  of  the  shell  consists  of  a  series  of  plates  and  trabeculae;  the 
cut  surfaces  of  the  latter  present  a  very  distinct  concentric  lamination  that  is 
precisely  like  the  laminated  trabeculae  in  Limulus.  In  each  plate  or  bar  there  is 
an  axial  core  of  a  darker,  yellowish-brown  color;  it  is  also  distinguished  by  a 
change  in  the  distinctness  of  the  lamination.  The  outermost  laminae  are  crossed 
by  innumerable  fine  lines  that  produce  the  appearance  shown  in  Fig.  197.  In 
other  specimens  that  have  been  preserved  in  a  little  different  manner,  some 
of  the  canals  are  filled  with  air  so  that  they  become  very  distinct,  like  the  pore 
canals  of  Limulus,  or  the  air-filled  canaliculi  of  typical  bone  cells. 
These  canaliculi  begin  at  the  surface  of  the  trabeculi  and  extend  at  right  angles  to 
the  lamellae  into  the  axial  core.  There  they  bend  nearly  at  right  angles  and  some 
of  them  terminate  in  slender,  spindle-shaped,  or  elongated  dilatations,  the  long 
axis  of  which  generally  lies  parallel  with  the  long  axis  of  the  core.  These  terminal 
dilations  or  primitive  lacunae  are  readily  seen  with  a  magnification  of  about  600 


ATELEASPIS. 


295 


diameters.     The  same  kind  of  unbranched  canaliculi  and  terminal  lacunae  are 
seen,  often  with  great  distinctness,  in  the  basal  layers.     (Fig.  196,  Al.) 

It  has  been  generally  assumed  that  in  the  pteraspids  true  lacunae  are  absent. 
It  will  be  seen  from  the  above  account  that  they  possess  a  primitive  form  of  lacunae 
that  are  similar  to  those  in  Limulus.  They  differ  from  typical  bone  lacunae  in 
their  small  size  and  in  the  absence  of  radiating  canaliculi. 

Ateleaspis. 

In  a  specimen  of  Ateleaspis  from  Scaumenac  Bay  (Fig.  198),  the  shell  in 
tangential  sections  forms  a  continuous  network  of  trabeculae.  They  are  not  as 


FIG.  199.- 


-A .   Surface  ornamentation  of  the  thoracic  shield  of  Limulus,  near  the  lateral  eyes, 
haemal  surface  of  one  of  the  cornua.     C.   Cross-section  of  one  of  the  cornua. 


B.  Same  on  the 


FIG.  200. — A,  Surface  view  of  semi-transparent  exoskeleton  of  Limulus,  from  the  flexible .  portion  in  the 
olfactory  region.  It  shows  the  peculiar  groups  of  pore  canals,  lyng  just  below  the  surface,  and  radiating  from  the 
base  of  the  denticle-like  spines;  also  the  polygonal  network  of  low  ridges,  or  trabeculae,  projecting  from  tl 
inner  surface  of  the  exoskeleton.  B,  Surface  view  of  thoracic  shield  of  an  ostracoderm  (Ateleaspis,  sp  ?)  showing 
the'polygonal  areas,  surface  tubercles,  and  the  peculiar  grouping  of  canals  lying  just  below  the  surface.  C,  Surface 
ornamentation  from  the  haemal  surface  of  thoracic  shield  of  Limulus.  D,  Same,  more  highly  magnified. 

distinctly  laminated  as  in  Pteraspis,  although  there  is  a  dark  brown  axial  core  in 
which  most  of  the  lacunae  are  located.  The  lacunae  are  spindle-shaped,  with 
more  than  one  canaliculus,  some  of  which  can  be  traced  to  the  outer  surface  of  the 
trabeculae. 


296 


THE    DERMAL    SKELETON. 

II.  DERMAL  SKELETON  OF  LIMULUS. 


We  shall  describe  in  some  detail  the  dermal  skeleton  of  Limulus  because  it 
has  the  usual  arthropod  characters,  and  at  the  same  time  several  other  very  im- 
portant ones  that  are  not  found  in  any  other  animal,  so  far  as  I  know,  outside  the 
vertebrates. 

The  outer  surface  of  the  shell  of  half  grown  Limuli  (Fig.  199,  B.)  is  marked 
by  broad  zig-zag  ridges  separated  by  shallow  grooves.  In  some  places,  notably 


FIG.  201. — Dermal  skeleton  of  Limulus.  A,  Cross- section  through  the  posterior  median  portion  of  the  thoracic 
shield;  B,  through  the  pineal  eye  chamber;  C,  surface  view  of  the  trioculate  parietal  eye;  D,  bony  trabeculae 
on  the  inner  surface  of  the  shield,  in  the  lateral  eye  region;  E,  Cross-section  of  the  lateral  eye  chamber. 

in  the  region  about  the  lateral  eyes,  the  ridges  break  up  into  polygonal  areas, 
each  of  which  contains  a  crater-like  depression  with  radiating  grooves  extending 
toward  the  base  of  the  cone,  Fig.  200,  D.  The  distinctness  of  the  ornamentation 
is  accentuated  by  the  deeper  color  and  other  optical  properties  of  the  matrix  in 
the  grooves  and  craters;  but  the  same  figures  can  be  obtained  in  wax  impressions 
of  the  outer  surface,  except  that  they  are  fainter. 


FIG.  202. — A,  Inside  margin  of  the  thoracic  shield,  showing  the  mode  of  growth  of  the  trabeculse;    B,  mass  of  bony 
trabeculae  from  the  inner  surface  of  the  thoracic  shield. 

The  inner  surface  of  the  shell  at  certain  places  gives  rise  to  great  masses  of 
interwoven  chitenous  bars,  or  trabeculae,  separated  by  irregular  spaces  filled 
with  loose  connective  tissue,  blood-vessels  and  nerves.  (Figs.  203,  204.) 

The  bars  are  concentrically  laminated  and  contain  numerous  fine  canals  and 
spindle-shaped  cavities,  so  that  the  whole  mass  of  tissue  resembles  in  a  very 
striking  way  the  cancellous  bony  tissue  of  vetebrates. 


LIMULUS. 


297 


The  trabeculae  are  most  highly  developed  along  the  lateral  margins  of  the 
thoracic  and  branchial  shields,  and  in  the  cornua.  In  these  places  the  spaces 
between  the  dorsal  and  ventral  walls  are  filled  with  dense  masses  of  this  tissue. 
(Fig.  199  C.)  In  the  cornua  the  trabecular  network  of  each  wall  is  united  to  the 
other  by  long  columns  with  branching  ends.  (Fig.  199,  C.) 

In  the  margins  of  the  thoracic  shield  (Fig.  202,  A)  one  sees  how  the  new 
trabeculae  arise  at  separate  points,  unite,  and  later  form  new  supports  which 
gradually  lift  the  older  bars  off  the  surface. 


I 


FIG.  203. — Section  of  the  dermal  skeleton  of  Limulus,  tangential  to  the  surface,  showing  the  cancellous  spaces, 
and  the  axial  location  of  the  lacunae  in  the  trabeculae.     Margin  of  thoracic  shield. 

There  are  similar  deposits  of  this  tissue  under  the  lateral  eyes,  enclosing  each 
eye  within  a  bony  orbit  (Fig.  201,  D,  E),  another  beneath  the  median  eyes  (Fig.  201, 
B) ,  and  six  or  seven  pairs  of  irregular  patches  arranged  symetrically  on  the  dorsal 
wall  of  the  abdomen  along  the  median  margin  of  the  six  pairs  of  entapophyses. 
(Fig.  205.) 


FIG.  204. — Cross-section  of  the  dermal  skeleton  of  Limulus.     Margin  of  thoracic  shield. 

Minute  Structure. — The  chitenous  trabeculae  often  form  a  nearly  continuous 
sheet  spread  out  over  the  inner  surface  of  the  shell,  and  differing  from  it  to  a  very 
marked  degree,  in  color,  texture,  and  general  appearance.  (Figs.  201,  A  and  204.) 

The  shell,  in  such  places,  is  divided  into  several  layers: 

i.  The  outer  layer  is  strongly  laminated,  and  traversed  by  two  or  three  kinds 
of  rather  large  canals,  which  contain  ducts  of  mucous  glands,  or  nerve  fibers,  and 
other  tissue.  All  these  canals  reach  the  outer  surface,  and  either  open  freely  to 
the  exterior,  or  lead  into  spines  or  hairs  of  various  shapes.  Between  them  are 


298  THE    DERMAL    SKELETON. 

innumerable  canaliculae  (pore  canals  of  authors) .  They  are  extremely  minute  and 
extend  in  straight  lines  or  in  finely  wound  spirals  almost  to  the  outer  surf  ace,  p.c. 
The  most  superficial  layer,  e,  is  thin,  colorless,  vitreous,  and  devoid  of  canaliculi. 
Although  very  hard  and  polished,  it  is  easily  destroyed  under  some  conditions. 
In  surface  views  it  is  seen  to  be  divided  by  shallow  grooves  into  polygonal  facets 
or  zigzag  ridges.  (Fig.  200;  D.) 

2.  The  middle  layer  consists  of  broad  cancellous  spaces  separated  by  ir- 
regular vertical  partitions.     The  cancellae  are  filled  with  loose  connective  tissue, 
through  which  ramify  nerves  and  blood-vessels. 

3.  The  third,  or  inner,  layer  is  composed  of  trabeculae  arranged  parallel  to 
the  outer  surface.     It  is  horizontally  laminated,  and  is  pierced  with  large  irregular 
openings,  through  which  nerves  and  blood-vessels  pass  to  the  cancellated  layer. 

Each  bar,  or  trabecula,  is  covered  by  a  thin  layer  of  pigmented  ectoderm 
continuous  with  that  underlying  the  outer  layer,  and  that  secretes,  or  produces 
by  the  periodic  transformation  of  its  own  substance,  the  chitenous  lamellae  of  the 
shell.  The  lamellae  are  generally  grouped  in  bands  which  differ  in  their  color 
and  in  their  chemical  reaction.  The  older,  outer  layers  of  the  shield  are  dark 
brown,  and  a  band  of  this  colored  chiten  extends  into  the  axis  of  each  trabecula. 
The  deeper  lamellae  of  the  outer  layer,  and  the  outer  lamellae  of  the  trabeculae, 
are  transparent  and  nearly  colorless. 

The  axial  core  of  the  trabeculae  stains  deeply  in  haematoxylin  and  in  acetic 
acid  carmine,  the  outer  layers  remaining  colorless.  This  fact  is  important,  as  it 
indicates  some  preliminary  chemical  change  in  the  axis  of  the  bars,  where  the 
bone  cells  later  appear. 

In  the  oldest  crabs,  the  axis  of  each  bar  is  densely  crowded  with  spindle- 
shaped  cavities,  or  lacunae.  Their  long  axes  are  parallel  to  the  long  axis  of  the 
bar,  and,  under  favorable  conditions,  we  can  see  that  many  of  them  are  connected 
at  one  end  with  a  very  fine  tubule,  or  canaliculus,  which  runs  radially  toward  the 
periphery.  The  largest  lacunae  are  nearest  the  center  of  the  trabeculae.  (Fig.  206,  A .) 

If  a  section  of  the  bone  is  dried  in  the  air  and  then  mounted  in  balsam  or 
glycerine,  the  lacunae  and  canaliculi  appear  black  in  transmitted  light,  and 
silvery-white  in  reflected  light,  showing  that  by  the  drying  up  of  their  semi-fluid 
contents  they  have  become  filled  with  air.  Old  fragments  of  bone  that  have 
dried  in  the  sun  for  an  indefinite  period,  when  softened  and  sectioned,  show  the 
same  structure.  Caustic  potash  dissolves  the  granular  contents  of  the  lacunae, 
but  does  not  otherwise  affect  them. 

Stained  sections,  quickly  transferred  to  balsam,  have  many  of  the  lacunae 
and  canaliculi  injected  with  the  stain,  which  generally  disappears  when  the  sec- 
tions are  well  washed.  But  even  after  long  washing  with  acidulated  alcohol, 
some  of  the  larger  lacunae  show  a  characteristic  color  due  to  the  presence  of  a 
faintly  stained  granular  substance,  and  a  small  darker  colored  body. 

All  these  facts  show  that  we  are  dealing  with  actual  cavities  and  canals  in 
the  chiten,  some  of  which  are  filled  with  nucleated  protoplasm. 


LIMULUS. 


299 


The  lacunae  are  best  seen  in  old  crabs  where  several  trabeculae  unite.  At  such 
points  they  are  very  numerous  and  apparently  vary  a  good  deal  in  shape.  (Fig. 
206,  B.)  This  is  largely  due  to  the  fact  that  they  are  turned  in  various  directions, 


FIG.  205. — Inner  surface  of  the  shield  of  Limulus,  showing  the  muscle  markings  and  the  distribution  of  the 
bony  trabeculae,  the  entapophyses,  and  the  principal  muscle  markings. 

so  that  some  are  cut  crosswise,  others  lengthwise.  The  lacunae  are  usually  filled 
with  air  and  appear  jet  black.  As  the  canaliculi  enter  the  darker  axial  core 
(Fig.  206,  B.a.b.),  they  become  sinuous,  and  many  side  branches  arise  which 
terminate  in  minute  lacunae.  Some  of  the  latter  appear  to  increase  in  size  and 


300 


THE    DERMAL    SKELETON. 


to  separate  from   the  main  canal,  ultimately  opening  directly  to   the  exterior 
by  a  single  canaliculus. 

At  d,  large  elongated  lacunae  are  seen,  some  of  them  constricted  transversely 


FIG.  206. — Section  of  bony  trabeculas  from  the  shield  of  an  old  Limuius,  showing  the  shape  and  arrangement  of  the 

lacuna?  and  canaliculi;  highly  magnified. 


FIG.  207. — A,  Section  of  the  chitenous  investment  of  the  oesophagus  of  Limuius,  showing  the  deeply  stained 
chains  of  spindle-shaped  bodies  between  the  chitenous  lamellae.  Delafield's  haematoxylin.  B,  Section  of  the 
flexible  chiten  in  the  olfactory  region,  showing  the  minute,  deeply  stained,  nuclear-like  bodies  in  the  pire  canals. 
Haematoxylin. 

as  though  about  to  form  several  smaller  lacunae,  although  this  appearance  is  not 
as  common  as  the  branched  one  described  above.  At  e,  are  a  few  lacunae  belong- 
ing to  a  trabecula  running  at  right  angles  to  the  plane  of  the  paper. 


LIMULUS. 


3°  * 


The  dermal  bone  tissue  varies  considerably  in  extent  in  different  individuals. 
It  is  not  visible  in  specimens  less  than  eight  inches  long  and  probably  does  not 
make  its  appearance  till  the  animal  is  full  grown.  Even  when  it  is  largely  devel- 
oped, the  characteristic  lacunae  in  the  chitenous  bars  may  be  absent.  This 
shows,  I  believe,  that  the  lacunas  are  not  fully  developed  till  long  after  maturity 
is  reached,  for  it  was  in  the  very  oldest  individuals,  with  much  scarred  and  worn 
armor,  that  I  found  them  best  developed. 

It  is  difficult  to  determine  how  the  nuclei  get  into  the  lacunae.  They  appear 
to  migrate  into  them  from  the  surface  epithelium  through  the  canaliculi.  The 
following  observations  lend  some  support  to  this  view.  In  the  region  of  the  ol- 
factory organ,  the  chiten  is  soft  and  flexible  and  the  bone  cells  are  absent.  When 
stained  with  the  ordinary  nuclear  stains,  the  whole  thickness  of  the  wall  is  seen  to 
be  filled  with  minute  sharply  stained,  nuclear-like  bodies  of  uniform  size,  arranged 
with  considerable  regularity  in  the  pore  canals.  (Fig.  207,  B.) 

In  sections  of  the  adult  oesophagus  (Fig.  207,  A),  the  chiten  is  seen  to  be  filled 
with  bodies  that  also  take  a  nuclear  stain.  Here  they  are  of  varying  sizes;  some 
are  minute  dots,  others  much  larger,  and  spindle-shaped,  and  arranged  in  rows  that 
follow  the  undulations  of  the  lamellae.  The  rows  of  spindle-shaped,  or  bead- 
like  bodies  are  often  united  by  delicate  threads,  as  though  they  had  multiplied 
by  division  and  were  still  imperfectly  separated  from  each  other. 

It  is  possible  that  some  of  these  bodies  are  bacteria,  or  the  spores  of  a  parasitic 
fungus,  or  the  products  of  some  degenerative  process.  That  is  a  point  upon  which 
I  have  not  been  able  to  satisfy  myself,  one  way  or  the  other.  It  is  certain  that  a 
species  of  fungus,  Macrocystis,  does  grow  in  the  chiten  of  Limulus.  Fragments 
of  the  skin,  stained  with  methylene  blue,  often  show  the  deeply  stained  hyphae 
ramifying  in  all  directions  through  the  substance  of  the  chitenous  covering  of  the 
gills,  or  in  the  chiten  surrounding  the  olfactory  organ.  They  appear  to  dissolve 
out  channels  in  the  chiten,  which  are  then  completely  filled  by  the  growing 
hyphae.  The  characteristic  spores  of  Macrocystis  have  been  found  in  abundance 
on  the  outer  surface  of  the  chiten  in  the  olfactory  region  of  dried  shells.  There  is, 
however,  no  suggestion  of  any  connection  between  this  fungus  and  the  nuclear 
bodies  just  described. 


The  continuous. external  armor  of  arthropods  must  be  shed  at  regular  inter- 
vals, to  make  room  for  growth,  and  the  shedding  of  the  old  shell  is  always  a  diffi- 
cult and  dangerous  process.  In  Limulus  we  see  the  beginning  of  a  new  type  of 
exoskeleton,  one  that  is  subdermal  and  discontinuous,  that  need  not  be,  and  never 
is  cast  off  after  it  is  once  formed.  Indeed,  Limulus  could  no  more  shed  its  dermal 
bones  than  a  vertebrate  could  shed  its  cartilage  cranium  or  its  vertebral  column. 
Limulus  has  therefore  solved  the  problem  for  arthropods,  of  getting  rid  of  an 
impractical  external  covering,  a  covering  which  has  become  too  cumbersome 
and  too  impermeable  for  physiological  purposes,  which  prohibits  growth  if 


302  THE    DERMAL    SKELETON. 

retained,  and  which  it  is  a  menace  to  remove.  In  its  place,  it  is  producing  an 
armor  that  is  permeable,  capable  of  indefinite  expansion,  and  one  that  cannot, 
and  need  not,  be  shed  at  frequent  intervals. 

The  new  conditions  under  which  this  skeleton  is  developed,  and  especially 
its  permanent  retention  within  the  foreign  mesodermic  tissues,  were  no  doubt  im- 
portant factors  in  bringing  about  a  permanent  change  in  its  chemical  compo- 
sition. 

III.  SUMMARY  AND  COMPARISON. 

I.  We  have  shown  that  Limulus  possesses  a  remarkable  dermal  skeleton,  and 
that  in  either  coarse  or  minute  structure  there  is  nothing  resembling  it  known  in 
any  other  invertebrate.     The  nearest  approach  to  it  is  found  in  the  pteraspidian 
division  of  the  ostracoderms.     No  other  known  animal,  vertebrate  or  invertebrate, 
resembles  Pteraspis  in  the  structure  of  its  exoskeleton  so  closely  as  Limulus.     If 
Limulus  were  an  extinct  animal,  it  would  be  exceedingly  difficult  to  discover  any 
differences  between  the  minute  structure  of  its  deeper  lying  exoskeleton  and 
that  of  Pteraspis. 

II.  The  surface  ornamentation  of  the  exoskeleton  of  the  ostracoderms  may 
be  resolved  into  a  series  of  alternating  ridges  and  grooves  that  are  either  very 
uniform  in  size  and  nearly  parallel,  as  in  Pteraspis;  or  sinuous  and  with  a  tendency 
to  breakup  into  rows  of  tubercles,  as  in  Bothriolepis;  or  finally  forming  tubercu- 
late  polygonal  areas,  as  in  Cephalaspis. 

III.  The  outer  surface  is  also  marked  by  a  special  series  of  shallow  grooves 
in  Bothriolepis  (Fig.  247),  Ateleaspis  (Fig.  242),  and  Tremataspis  (Fig.  236),  that 
probably  mark  the  location  of  rows  of  sensory  organs. 

Another  set  of  deep  lying  canals,  probably  representing  enclosed  surface 
grooves,  are  present  in  Pteraspis  and  Tremataspis.  They  are  of  even  caliber  and 
open  outward  by  narrow  slits,  or  pores,  or  by  very  short,  chimney-like  canals, 
(Figs.  193,  196,  s.c.)  that  probably  contained  sensory  organs  or  mucous  glands. 
They  differ  from  the  vascular  canals  in  that  they  are  generally  filled  with  a  matrix 
like  that  outside  the  shell,  showing  that  they  communicated  freely  with  the  out- 
side, while  the  vascular  canals  and  even  the  cancellae,  may  be  quite  empty. 

IV.  The   surface   ornamentation   of   the   exoskeleton  of  the  ostracoderms 
may  be  regarded  as  a  further  specialization  of  the  ridges  and  grooves  seen  on  the 
outer  surface  of  the  exoskeleton  in  the  marine  arachnids.     It  would  require  but 
a  slight  modification  of  the  scale-like  markings  in  Pterygotus,  or  of  the  zigzag 
ridges  and  grooves,  or  the  polygonal  areas,  in  Limulus,  to  produce  the  character- 
istic markings  of  Pteraspis,  Bothriolepis,  or  Cephalaspis. 

V.  The  surface  ornamentation  of  the  cephalic  buckler  in  both  arachnids  and 
ostracoderms  may  be  regarded  as  the  external  expression  of  internal  irregulari- 
ties in  growth  which  ultimately  lead  to  the  breaking  up  of  the  continuous  shell  into 
separate  plates. 


SUMMARY. 


303 


VI.  From  a  vertebrate  standpoint,  the  continuous  dermal  armor  of  the  ostra- 
coderms  appears  to  be  a  very  primitive  exoskeleton,  and  one  from  which  that 
of  the  true  fishes  has  been  produced  by  breaking  up  the  surface  ornamentation 
into  isolated  dermal  denticles  and  bony  plates.     From  the  invertebrate  stand- 
point, the  ostracoderm  skeleton  is  a  highly  specialized  one,  produced  by  an  exag- 
geration of  the  type  of  exoskeleton  seen  in  Limulus. 

VII.  It  may  be  urged  that  the  bony  plates  of  the  ostracoderms  are  meso- 
dermal  in  origin,  while  those  of  Limulus  are  epidermal.     But  we  have  no  means 
of  knowing  whether  the  bony  plates  of  the  ostracoderms  were  developed  entirely 
inside,  or  outside   the   ectoderm,  and  we  cannot  class  them  as  subdermal,  or 
mesodermal,  unless  we  beg  the  whole  question  and  assume  that  the  ostracoderms 
are  typical  fishes. 

VIII.  The  structure  of  the  exoskeleton  of  the  ostracoderms  is  as  much  like 
that  of  Limulus  as  that  of  vertebrates.     The  dermal  skeleton  of  vertebrates  can 
be  derived  as  readily  from  that  of  Limulus  as  from  that  of  the  ostracoderms. 
To  do  this,  for  example,  we  need  only  to  assume  that  the  outer  layer  of  the  shell 
of  Limulus  has  been  reduced  to  a  thin  cuticular  layer  and  is  cut  off  entirely  from 
the  underlying  trabeculae.     The  skin  and  the  skeletal  parts  derived  from  it  would 
then  appear  to  be  formed  of  two  layers,  a  continuous  outer  layer,  and  an  inner 
one  composed  of  more  or  less  isolated  fragments  formed  by  local  ingrowths  from 
the  outer  layer. 

The  two  kinds  of  exoskeleton,  epidermal  and  subdermal,  would  then  be 
present  at  the  same  time,  as  indeed  they  are  in  Limulus.  But  in  Limulus  the  sub- 
dermal  skeleton  is  in  its  initial  stages,  and  the  epidermal  is  the  more  voluminous. 
In  primitive  vertebrates  the  epidermal  skeleton  is  about  to  disappear,  being  repre- 
sented solely  by  the  enamel  layer,  and  possibly  the  dentine,  while  the  subdermal 
layer  has  attained  its  maximum  development.  The  distinction  between  the 
epidermal  skeleton  of  the  arachnids,  and  the  dermal  one  in  vertebrates  is,  therefore, 
only  one  of  degree,  not  of  kind. 

In  the  embryonic  development  of  dermal  bones  in  vertebrates,  the  arachnid 
process  of  separating  bony  trabeculae  from  the  inner  surface  of  the  ectoderm  is 
apparently  greatly  abbreviated,  but  indications  of  it  have  been  observed  in  the 
development  of  certain  cranial  bones.  These  observations,  therefore,  need  not 
be  looked  upon  with  suspicion,  or,  if  accepted,  taken  as  evidence  of  the  untrust- 
worthiness  of  the  germ  layer  theories.  They  should  be  regarded  rather  as  the 
naturally  to  be  expected  embryological  indication  of  the  derivation  of  the  verte- 
brate dermal  skeleton  from  the  epidermal  armor  of  arachnid  ancestors. 

IX.  The  dermal  denticles  are  the  oldest  parts  of  the  dermal  skeleton  of  verte- 
brates, and  they  naturally  still  retain  the  clearest  indications  of  their  derivation 
from  an  arthropod  exoskeleton. 

The  chiten  of  arachnids,  and  the  enamel  and  dentine  of  the  dermal  bones  of 
primitive  vertebrates  have  essentially  the  same  structure  and  mode  of  growth,  as 
shown  by  their  pronounced  laminae  and  the  minute  parallel  canals  that  run  at 


3°4 


THE    DERMAL    SKELETON. 


right  angles  to  them.     The  differences  are  mainly  ones  of  degree,  i.e.,  the  number 
and  size  of  the  canals,  and  the  density  and  composition  of  the  matrix. 


-he.. 


1) 


M. 


FIG.  208— Semi-diagrammatic  sections  illustrating  the  evolution  of  the  chitenous  epidermal  exoskeleton  of  arach- 
nids into  the  bony  sub-dermal  exoskeleton  of  vertebrates.  A,  Exoskeleton  of  immature  Limulus  showing  the  begin- 
ning of  the  trabecular  ingrowths;  B,  mature  Limulus,  showing  the  cancellated  exoskeleton;  the  trabeculag,  with 
their  axial  lacunae;  and  the  dentine-like  chitenous  matrix,  with  its  parallel  pore  canals  and  concentric  lamellas;  C, 
the  exoskeleton  of  an  ostracoderm,  consisting  of  dentinal,  vascular,  cancellous,  and  basal  layers.  In  the  outer  layer 
are^shown  several  forms  of  tubercles,  denticles,  and  dentinal  ridges,  with  various  forms  of  dentinal  canals;  D,  the 
final  stages  showing  the  conversion  of  the  vascular,  cancellous,  and  basal  layers  into  subdermal  bone;  and  the  con- 
version of  the  isolated  remnants  of  the  primitive  epidermal  skeleton,  into  dermal  denticles. 

When  two  such  similar  structures  as  enamel  and  dentine  appear  to  arise,  one 
from  the  mesoderm,  the  other  from  the  ectoderm,  it  probably  means  that  either 


SUMMARY.  305 

there  is  no  real  distinction  between  the  two  layers,  or  else  that  both  structures  arise 
from  the  same  layer.  As  there  is  no  doubt  about  the  origin  of  the  enamel  layer, 
and  as  there  is  no  conclusive  evidence  that  the  odontoblasts  have  not  arisen  at  a 
very  early  period  from  the  ectoderm,  it  may  be  assumed  that  both  enamel  cells  and 
odontoblasts  were  primarily  derived  from  the  ectoderm. 

We  may,  therefore,  regard  the  dermal  skeleton  of  primitive  vertebrates  as 
consisting  of  two  parts,  viz.  i.  the  subdermal  trabeculae  that  develop  into  the  char- 
acteristic bony  plates,  and  that  consist  of  concentric  lamellae,  true  bone  corpuscles, 
and  vascular  chambers,  or  canals;  and  2.  the  more  superficial  skeleton,  that  arises 
either  from  the  outer  surface  of  the  ectoderm,  or  from  cells  in  intimate  relation 
with  it,  and  that  consists  of  parts  having  a  chitenoid,  enamel-like,  or  dentine-like 
structure. 

X.  The  general  nature  of  the  process  by  which  the  continuous  epidermal 
armor  of  the  arachnids  becomes  fragmented  and  divided  into  two  overlying 
systems,  and  the  method  of  substituting  the  inner  system  for  the  outer,  is  shown  in 
a  diagrammatic  way  in  Fig.  208. 

The  ostracoderm  skeleton,  C,  clearly  represents  a  transitional  stage  between 
the  vertebrate  and  the  arachnid  type.  In  the  vertebrates  themselves,  the  principal 
events  in  the  evolution  of  the  dermal  skeleton  are  the  gradual  elimination  of  the 
epidermal  structures,  except  those  retained  in  the  teeth;  the  reduction  of  the  sub- 
dermal  bones  to  relative  insignificance;  and  the  substitution  for  them  of  an  endo- 
skeleton  of  true  mesodermal  origin. 


CHAPTER  XVII. 
THE  ENDOCRANIUM,  BRANCHIAL  AND  NEURAL  CARTILAGES. 

I.  THE  ENDOSKELETON  OF  ARACHNIDS. 

Many  arthropods  are  provided  with  an  elaborate  system  of  ectodermic  in- 
foldings,  lined  with  chiten,  that  serve  for  the  attachment  of  muscles  and  in  some 
cases  as  a  supporting  framework  for  the  anterior  part  of  the  nervous  system. 
They  may  be  segmentally  arranged,  and  in  some  cases  appear  to  be  the  modified 
remains  of  duct-like  infoldings  that  originally  served  some  other  purpose  than  for 
the  attachment  of  muscles.  There  are  no  indications  that  these  structures  are 
represented  in  vertebrates,  and  we  merely  refer  to  them  here  in  order  to  emphasize 
the  distinction  between  them  and  the  true  endoskeleton  we  shall  describe  in  the 
following  pages. 

The  endoskeleton  of  arachnids  (Limulus)  consists  of  four  distinct  parts,  their 
relation  to  the  corresponding  parts  in  the  vertebrate  skeleton  being  sufficiently 
indicated  by  their  names.  They  are:  a.  the  neural  arches;  b.  the  branchial  car- 
tilages; c.  the  endocranium;  d.  the  notochord.  (Fig.  209.) 


eat. 


FIG.  209.  —  Diagram  of  the  endoskeleton  of  a  marine  arachnid  (based  on  Limulus)  showing  the  locations  of  the 
lemmatochord,  endocranium,  neural  arches  (dotted)  ,  branchial  cartilages  (black),  and  the  chitenous  entapophyses 
(shaded). 

With  the  exception  of  the  notochord,  these  structures  are  primarily  meso- 
dermic  in  origin;  they  serve  for  the  attachment  of  muscles,  and  with  the  possible 
exception  of  the  branchial  cartilages,  their  origin  and  development  was  determined 
by  the  development  of  the  muscles  now  associated  with  them.  They  have  the 
same  general  form,  location,  consistency,  and  chemical  reaction  that  the  corre- 
sponding cartilages  have  in  vertebrates. 

The  arthropod  notochord  is  a  modification  of  the  middle  cord  and  is  primar- 
ily ectodermic  in  origin.  The  median  nerve  derived  from  it  degenerates  and  the 
remnants  become  invested  with  a  thick  envelop  of  neuroglia-like  tissue  that  may 

306 


NEURAL  ARCHES   AND    BRANCHIAL   CARTILAGES.  307 

serve  for  the  attachment  of  segmental  muscles.  At  no  time  in  its  phyllogenetic 
history  has  it  any  functional  connection,  or  relation  to  the  endoderm,  or  to  the  ali- 
mentary canal. 

II.  THE  NEURAL  ARCHES.     (Figs.  70,  75,  78,  209.) 

The  neural  arches  (endochondrites  of  Lankester)  are  six  small  plates  of 
nbro-cartilage.  They  lie  on  the  neural  surface  of  the  cord  beneath  the  in  tegu- 
ment, to  which  they  are  attached  near  the  base  of  each  pair  of  gills.  Each  arch 
is  concave  on  its  inner  surface  and  partly  surrounds  the  nerve  cord,  holding  it 
firmly  in  place.  (Figs.  70,  75,  78.)  The  opercular  neural  plate  is  typical;  it  is 
rectangular,  and  its  flat  neural  surface  is  indented  by  two  pits,  from  which  arise 
a  pair  of  muscles  attached  to  the  inside  of  the  operculum.  A  pair  of  anterior 
and  posterior  processes  serve  for  the  attachment  of  strands  from  the  longitudinal 
muscles  of  the  abdomen.  On  the  sides  of  the  arch  is  a  pair  of  processes  which 
project  haemally  and  a  little  outward  and  backward,  one  on  each  side  of  the 
ventral  cord.  They  furnish  attachment  for  a  pair  of  haemo-neural  muscles  that 
are  inserted  on  the  haemal  side  of  the  carapace,  just  median  to  the  entopophyses. 

The  neural  arches  of  Limulus  may  be  regarded  as  the  precursors  of  the  neural 
arches  of  vertebrates,  with  which  they  agree  in  location,  and  in  their  general  form 
and  function.  No  other  invertebrate  is  known  to  have  neural  plates  of  this 
character. 

III.  BRANCHIAL  CARTILAGES.     (Figs.  78,  209,  210,  211.) 

There  are  seven  pairs  of  branchial  cartilages  in  Limulus.  The  most  anterior 
pair  are  two  small  bars  arising  from  the  inner  surface  of  the  chilaria,  and  attached 
to  the  posterior  margin  of  the  endocranium.  (Fig.  215.)  The  remaining  six 
pairs  arise  from  the  base  of  the  abdominal  appendages,  and  go  to  the  corresponding 
entopophyses.  (Fig.  209.) 

The  branchial  cartilages  arise  at  an  early  embryonic  period  as  clearly  de- 
fined outgrowths  of  the  walls  of  the  mesoblastic  somites.  (Fig.  210.)  Their 
union  with  the  epidermis  is  secondary,  and  they  are  in  nowise  derived  from 
chitenous  ingrowths  of  the  epidermis. 

The  branchial  bars  serve  for  the  attachment  of  the  flexor  and  extensor 
muscles  of  the  gills,  and  for  a  small  muscle,  the  internal  branchial,  arising  from 
the  corresponding  neural  plate.  The  opercular  bar  is  the  largest.  In  a  small 
male,  it  is  about  25  mm.  long,  oval  in  cross-section,  and  about  6  mm.  by  3  mm. 
in  diameter.  The  remaining  bars  decrease  gradually  in  size  to  the  posterior  end 
of  the  series. 

A  band  of  cartilage,  similar  histologically  to  that  of  the  branchial  cartilages, 
extends  from  the  distal  end  of  one  entopophysis  to  the  next,  uniting  the  haemal 
ends  of  the  gill  bars.  (Fig.  209.) 


3o8 


ENDOCRANIUM,    BRANCHIAL  AND    NEURAL    CARTILAGES. 


The  branchial  bars  are  hard  and  elastic,  and  have  the  general  appearance 
and  consistency  of  hyaline  vertebrate  cartilage.  Chemically  and  histologically, 
they  are  quite  different  from  the  fibre-cartilage  of  the  endocranium,  or  of  the 
neural  plates.  There  is  apparently  no  invertebrate,  outside  of  the  arachnids, 
that  has  a  tissue  comparable  with  it.  The  nearest  approach  to  it,  chemically 
and  histologically,  is  the  muco-cartilage  in  Petromyzon.  The  branchial  bars  of 
Limulus  correspond  to  the  gill  bars  of  the  post  auditory  region  of  vertebrates,  as 
we  first  indicated  in  1889.  In  1893,  we  called  attention  to  the  surprising  histo- 
logical  resemblance  between  these  cartilages  and  those  of  Petromyzon,  and  still 
later,  1896,  it  was  shown  that  in  abnormal  Limulus  embryos,  one  or  more  pairs  of 
appendages  were  invaginated,  forming  transverse  slits  along  the  sides  of  the  head, 
that  resemble  vertebrate  gill  slits  or  the  lung  books  of  the  arachnids.  (Fig. 
iB3t  A.) 


FIG.  210. — Sagittal   sections  of   Limulus  embryos-,  showing  successive  stages  in  the  development  of  the  branchial 

cartilages.    After  Patten  and  Hazen. 

Development  of  the  Branchial  Cartilages  in  Limulus. — The  following 
description  is  based,  in  the  main,  on  what  takes  place  in  the  operculum.  The 
cartilages  in  the  other  abdominal  appendages  develop  somewhat  later,  but  in  a 
very  similar  manner. 

In  an  embryo  of  three  abdominal  segments,  there  is  no  trace  of  the  opercular 
cartilage.  (Fig.  210,  A.)  By  the  time  five  abdominal  segments  are  developed, 
the  outer  wall  of  the  somite  forms  a  thick  ring  of  mesoderm  around  the  base  of 
the  appendage.  The  opercular  cartilage  makes  its  appearance  as  a  transverse 
plate  of  cells  subtending  the  ring,  with  its  distal  end  projecting  into  the  cavity  of 
the  appendage.  (Fig.  210,  B.) 

In  the  next  stage,  C,  where  one  gill  leaf  is  developed  on  the  first  branchial 
appendage,  the  cartilages  have  increased  in  size  and  now  show  the  features  that 
characterize  them  so  clearly  in  the  later  stages;  viz:  i,  the  cartilage  cells  are  larger 


BRANCHIAL    CARTILAGES    OF    LIMULUS.  309 

than  the  surrounding  mesodeim  cells,  and  have  distinct  cell  walls;  2,  they  are 
arranged  in  rather  regular  order;  and  3,  the  protoplasm  stains  very  lightly  in 
borax  carmine. 

In  the  next  stage,  with  three  gill  leaves  on  the  first  branchial  appendage  (Fig. 
210,  D),  the  cartilages  form  long  flat  plates  that  extend  some  distance  beyond 
the  ring  of  mesoderm  into  the  appendage.  The  cartilage  of  the  first  gill  is 
attached  to  the  anterior  wall  of  its  appendage,  and  extends  from  there  to  the 
corresponding  somite,  which  has  now  become  a  venous  sinus  bounded  by  a  thin 
membrane. 

A  perichondrium  is  now  visible,  composed  of  a  layer  of  spindle-shaped  cells, 
apparently  derived  from  the  breaking  up  of  the  mesodermic  ring,  and  not  from  a 
transformation  of  the  peripheral  cartilage  cells.  The  latter,  as  we  have  shown, 
are  formed  from  the  mesothelium  of  the  somatic  walls. 

The  ends  of  the  branchial  cartilages  finally  fuse  with  the  ectoderm  on  the 
anterior  wall  of  the  appendages  and  with  the  haemal  wall  of  the  abdomen.  At 
these  points  the  cartilage  and  ectoderm  are  so  completely  united  that  their  original 
boundaries  cannot  be  distinguished. 

The  spaces  in  the  distal  ends  of  the  gills  are  crossed  by  fibrous  columns 
arising  from  the  ectodermic  walls.  At  the  base  of  each  column  are  several  nuclei, 
as  though  the  columns  were  formed  by  the  union  of  several  cells.  At  this  stage 
no  mesoderm  extends  into  the  appendages  beyond  the  distal  ends  of  the  cartilages. 

In  the  early  trilobite  stage  the  branchial  cartilages  differ  but  little,  except 
in  size,  from  those  in  the  adult. 

Minute  Structure  of  Branchial  Cartilage. — During  the  late  larval  period, 
each  cartilage  cell  develops  on  its  outer  surface  a  cartilage-like  matrix  that  en- 
velops and  isolates  each  cell.  This  is  the  primary  capsule.  It  gradually  in- 
creases in  size,  forming  a  large  thin-walled  chamber,  within  which  the  cell  con- 
tinues to  divide  in  the  three  planes  of  space,  each  division  plane  being  generally 
at  right  angles  to  the  preceding  one.  After  each  division  the  daughter  cells  form 
new  capsules  inside  the  old  ones.  As  there  is  little  or  no  shifting  of  the  cells 
after  each  division,  the  shape  and  position  of  the  primary,  secondary  and  tertiary, 
etc.,  capsules  show  pretty  clearly  the  history  of  the  previous  divisions. 

The  cluster  of  cells  and  capsules  enclosed  in  the  primary  capsule  constitutes 
a  cartilage  nest.  The  nests  are  largest  in  the  axis  of  the  gill  bar;  the  periphery 
of  the  bar  consists  of  small  cells  not  clearly  grouped  into  nests.  The  cartilage 
tissue  terminates  abruptly  under  the  perichondrium.  The  two  tissues  are  dif- 
ferent chemically  and  ontogenetically  and  there  are  no  indications  of  intermediate 
stages  between  them,  although  occasionally  one  finds  a  small  cluster  of  cartilage 
cells  enclosed  like  a  foreign  body,  in  the  perichondrium,  or  in  the  fibrous  tissue 
connecting  the  inner  ends  of  the  entopophyses.  Similar  nests  of  capsuligenous 
cartilage  are  said  to  occur  in  the  endocranium  of  Mygale  (Lankester)  and  Tele- 
phonous  (Gaskell). 

When  the  gill  bars  with  their  adhering  fibrous  and  muscular  tissue  are  boiled 


3io 


ENDOCRANIUM,    BRANCHIAL   AND    NEURAL    CARTILAGES. 


for  a  short  time  in  caustic  potash,  the  perichondrium  and  muscles  are  at  once 
dissolved,  leaving  the  cartilage  bar  clean  and  free  from  all  other  tissues.  The 
cartilage  swells  and  turns  yellow  and  transparent,  but  otherwise  appears  to  be 
unchanged.  After  prolonged  boiling  it  breaks  down  and  disappears. 

Thin  sections  of  the  gill  bars,  boiled  in  potash  till  the  perichondrium  is  com- 
pletely dissolved,  show  under  the  microscope  irregular  crevasses  and  spaces 
around  the  central  cell  nests,  indicating  that  the  clear  matrix,  or  cement,  sur- 
rounding them  has  been  dissolved  out.  The  capsules  themselves  are  swollen, 
and  their  walls  appear  much  more  distinctly  laminated  than  before.  On  further 
boiling,  the  axial  portion  of  the  section  drops  out,  indicating  that  the  nests  have 
been  completely  isolated;  when  boiled  still  longer,  the  peripheral  portion  breaks 
down  also. 


FIG.  211. — Section  of  the  branchial  cartilage,  in  an  adult  Limulus,  highly  magnified,  showing  a  single  nest 
of  cartilage  cells.  The  arrangement  of  the  concentric  layers  of  differently  colored  chondrin  indicates  the  success- 
ive generations  of  cartilage  cells.  Stained  in  thionin  and  picric  acid. 

When  free  hand  sections  of  alcoholic  material  are  treated  with  thionin, 
complicated  color  reactions  take  place  that  vary  with  the  thinness  of  the  section, 
strength  of  the  solution,  and  apparently  with  the  exposure  to  air  and  duration  of 
the  staining.  The  reactions  may  be  studied  under  the  microscope  in  a  watch 
glass.  If  the  stained  sections  are  mounted  in  glycerine,  it  will  be  seen  that  the 
lacunae  are  lined  with  a  finely  granular  protoplasm,  with  one  or  more  small  nuclei 
on  the  periphery.  (Fig.  211^  n.)  The  center  is  filled  with  a  fluid,  containing  in 
some  cases  coarse  granules  of  a  deep  reddish-violet,  x'\  in  others,  large  spherules 
of  a  faint  yellow  color,  x.  In  most  cases  the  central  portion  appears  to  be  empty, 
and  if  the  sections  are  carelessly  handled  they  may  contain  large  air  bubbles. 

The  capsules  consist  of  alternating  red,  or  violet,  and  blue  laminae.  The  red 
bands  vary  greatly  in  thickness  and  in  the  intensity  of  the  color  in  different  cap- 
sules. The  innermost  one  is  deep  red,  with  a  rough  irregular  inner  surface,  the 
larger  protuberances  probably  marking  the  beginning  of  a  new  partition.  Just 


BRANCHIAL   CARTILAGES    OF    LIMULUS.  3!! 

outside  this  layer  there  may  be  a  very  sharp,  thin,  blue  band,  or  a  broad  red  band 
of  a  lighter  color.  The  most  intense  and  widest  blue  bands  form  the  middle 
layer  in  the  partitions  between  two  cells,  or  between  two  groups  of  cells,  or  in  the 
layers  surrounding  the  whole  nest. 

In  some  instances,  after  staining  a  short  time  only,  these  reactions  seem  to 
be  reversed,  the  violet  bands  appearing  blue  and  the  blue  ones  violet.  The 
violet  bands  are  the  first  to  stain,  the  blue  ones  appearing  much  later;  they  show 
most  clearly  after  the  sections  have  been  partially  decolorized  in  glycerine. 
Similar  color  bands  are  seen  after  staining  with  haematoxylin,  except  that  the 
bands  are  of  different  shades  of  purple. 

If  thionin  sections  are  treated  with  weak  picric  acid,  the  blue  bands  become 
intense  green,  and  later  bright  yellow,  while  the  violet  bands  are  affected  but  little. 
These  preparations  are  most  brilliant  and  are  the  ones  from  which  the  drawing 
was  made. 

In  the  axial  portions  of  the  gill  bars,  the  cell  nests  in  some  cases,  are  sepa- 
rated by  considerable  areas  of  a  nearly  homogeneous  matrix,  some  parts  being 
blue,  others  violet.  The  perichondrium  stains  a  bright  characteristic  violet, 
and  the  muscles  blue. 

The  probable  explanation  of  these  complicated  color  reactions  is  that  the 
walls  of  each  capsule  consist  of  a  graded  series  of  different  chemical  substances 
arranged  in  concentric  layers,  the  substance  giving  the  red  reaction  being  the 
most  abundant  in  the  inner  protoplasmic  layers  of  the  capsules,  and  that  giving 
the  blue  reaction  being  more  abundant  on  the  periphery,  with  intermediate 
transitional  compounds  between;  and  that  the  blue  material  is  formed  by  a  gradual 
modification  of  the  red. 

The  gill  bars  of  Limulus  are  exceedingly  interesting  histologically,  and  deserve 
a  more  careful  and  detailed  description  than  we  can  give  them  here.  The  main 
points  we  desire  to  establish  now  are  that  they  contain  true  cartilage  of  a  very 
primitive  type,  and  that  this  cartilage  is  quite  different,  chemically  and  histologi- 
cally, from  that  found  elsewhere  in  Limulus. 

The  reaction  of  the  gill  cartilages  to  thionin  and  haematoxylin  indicates  that 
they  contain,  besides  other  substances,  a  considerable  amount  of  mucin.  This 
fact,  together  with  their  remarkable  histological  structure,  emphasizes  still  more 
strongly  the  resemblance  between  the  gill  bars  of  Limulus  and  those  of  Petro- 
myzon.  Moreover,  the  extraordinary  difference  between  the  capsuligenous 
cartilage  and  the  fibro-cartilage  of  the  endocranium  in  Limulus  is  paralleled  by 
the  fact  that  there  is  a  corresponding  difference  between  gill  cartilage  and  cranial 
cartilage  in  the  cyclostomes. 

The  fibro-cartilage  of  the  arachnids  appears  to  have  arisen  by  the  gradual 
transformation  of  fixed  muscle  ends  into  sinews  or  tendons,  in  proportion  as  the 
corresponding  muscles  become  more  active  and  voluminous.  The  capsuligenous 
cartilage  appears  to  arise  "de  novo,"  making  its  appearance  as  an  entirely 
different  looking  material  from  that  in  which  it  is  imbedded.  The  gill  bars 


312  ENDOCRANIUM,    BRANCHIAL   AND    NEURAL    CARTILAGES. 

for  example  are  well  developed  at  a  very  early  embryonic  period,  and  appear 
to  be  quite  out  of  proportion  to  the  volume  and  functional  importance  of  the 
surrounding  muscles.  Moreover  isolated  and  highly  developed  cartilage  cells, 
or  cell  nest,  may  be  seen  in  adult  crabs  lodged  in  the  perichondrium  of  the  gill 
bars,  or  at  the  ends  of  the  entopophyses  and  in  other  places  where  they  appear 
quite  foreign  to  the  surrounding  tissues,  and  with  nothing  to  suggest  the  reason 
for  their  presence  in  such  unusual  surroundings. 

IV.  THE  ENDOCRANIUM. 

The  endocranium,  variously  named  prosomatic  endosternite,  cartilaginous 
sternum,  or  plastron,  has  been  found  in  many  of  the  arachnids  and  in  the 
phyllopods.  In  1889,  I  figured  and  described  the  endocranium  of  the  scorpion, 
and  compared  the  endocrania  of  the  scorpion  and  Limulus  with  the  primordial 
cranium  of  vertebrates.  In  1899,  in  collaboration  with  Mr.  Redenbaugh,  a 
graduate  student  in  Dartmouth  College,  the  endocrania  of  Apus,  Mygale,  and 
Limulus  were  described  and  illustrated  in  more  detail. 


The  endocranium  of  arachnids  is  a  broad  plate  of  nbro-cartilage  lying  on 
the  haemal  side  of  the  brain.  It  serves  primarily  for  the  attachment  of  the 
muscles  that  move  the  oral  appendages,  and  for  the  flexor  muscles  that  move  the 
cephalo-thorax  on  the  branchial  section  of  the  body.  In  scorpions  and  in  Limulus 
a  complete  occipital  ring  is  formed  about  the  spinal  cord,  near  its  union  with 
the  brain. 

The  floor  of  the  endocranium  is  a  continuous  structure  and  there  are  no 
indications  that  it  consists  of  originally  separate  pieces;  the  supra-occipital  plate, 
when  present,  may  be  regarded  as  a  modified  neural  arch  belonging  to  the 
vagus  segments.  We  recognize  in  the  higher  arachnids  three  principal  parts,  viz. 
two  lateral  bars;  a  broad  median  plate  that  unites  the  posterior  ends  of  the 
bars;  and  a  bridge  of  cartilage  on  the  neural  side  of  the  nervous  system,  which 
together  with  the  above  mentioned  parts  forms  a  closed  ring  about  the  anterior 
end  of  the  spinal  cord. 

The  endocranium  has  a  true  cartilaginous  consistency,  and  is  composed  of  a 
mass  of  interwoven  fibers  and  a  dense  matrix  containing  stellate  lacunae  united 
by  anastomosing  canaliculi.  In  the  living  cartilage,  the  canaliculi  contain 
minute  branching  processes  of  the  cartilage  cells  situated  in  the  lacunae. 

The  endocranium  of  arachnids,  as  I  first  pointed  out  in  1889,  represents 
the  ancestral  stage  in  the  evolution  of  the  primordial  cranium  of  vertebrates,  the 
lateral  bars,  the  transverse  plate,  and  the  neural  arch  of  the  arachnid  cranium 
corresponding  respectively  to  the  trabeculae,  the  parachordals,  and  the  occipital 
ring  of  the  vertebrates. 

The    Endocranium   of   Apus.    (Fig.    212.) — In   Apus   the   endocranium 


THE    ENDOCRANIUM    OF  APUS.  313 

is  a  thick  plate  of  fibre-cartilage,  without  an  occipital  ring,  located  just  behind 
the  mouth,  between  the  central  nervous  system  and  the  intestine.  The  body  of 
the  endocranium  is  elongated  in  a  transverse  direction,  its  flaring  ends  giving 
attachment  to  the  powerful  adductor  muscles  of  the  mandibles. 

A  pair  of  chitenous  apodemes,  apo.,  project  into  the  posterior  side  of  the 
endocranium.  They  are  ectodermic  invaginations  lined  with  chitin,  formed  be- 
tween the  bases  of  the  first  and  the  second  pair  of  appendages.  From  the  inner 
ends  of  the  apodemes,  a  pair  of  tendenous  cords  run  directly  through  the  body 
of  the  endocranium,  at  right  angles  to  its  fibers,  emerging  on  the  anterior  side  as 
the  anterior  cornua,  ac. 

The  endocranium  terminates  posteriorly  in  a  thin  membrane  wt  which  is 
attached  to  the  integument  between  the  nerve  cords. 

Longitudinal  muscles  of  the  abdomen  are  attached  to  the  posterior  sides  of 
the  apodemes  and  to  the  endocranium  itself.  A  process  on  the  neural  side  of 


I 


I  1 


,-• 


B 

FIG.  212. — Endocranium  of  Apus;  A,  Neural  surface;  B,  haemal  surface.  The  mandibular  muscles  are  at- 
tached to  the  large  transverse  processes,  m;  maxillary  muscles  to  the  small  processes,  x.  A  pair  of  chitenous  en- 
topophyses,  apa.,  are  imbedded  in  the  posterior  side  of  the  endocranium.  X  27  1/2.  After  Patten  and  Redenbaugh 

each  apodeme  serves  for  the  attachment  of  muscle  strands  going  to  the  inside 
of  the  second  pair  of  maxillae,  x.  Haemo-neural  muscles  are  attached  to  the  haemal 
sides  of  the  apodemes,  and  a  pair  of  muscles,  inserted  on  the  posterior  neural 
portion  of  the  endocranium,  y.,  pass  between  the  nerve  cords  to  the  integument 
just  back  of  the  first  cross  commissures. 

Endocranium  of  Mygale  (Fig.  213.) — The  endocranium  of  Mygale  is  a 
large  oval  plate  of  fibro-cartilage  with  crenate  margins;  like  that  of  Apus,  it  lies 
between  the  alimentary  canal  and  the  central  nervous  system. 

The  neural  surface  is  concave  and  provided  with  paired,  plate-shaped 
processes,  A,  n.pl.  From  about  the  middle  of  the  large  anterior  cornua,  arise  a 
pair  of  neural  processes,  n.pr' ',  which  bend  around  the  brain  and  attach  them- 
selves to  the  integument  close  together  on  the  neural  side.  They  probably 
represent  the  lateral  portion  of  the  occipital  ring  of  Limulus. 

On  the  haemal  side,  J5,  two  high,  flaring  ridges  converge  toward  the  posterior 
end  of  the  endocranium,  forming  a  deep  gully  in  which  lies  the  alimentary  tract. 


3*4 


ENDOCRANIUM,    BRANCHIAL   AND    NEURAL    CARTILAGES. 


The  haemal  ridges  are  split  up  into  five  pairs  of  haemal  processes,  h.pr.1  5,  of 
unequal  length.  The  endocranium  ends  posteriorly  in  a  short  median  process. 
From  nearly  the  whole  of  the  neural  surface  muscles  go  to  the  base  of  the  legs. 
Haemo-neural  muscles  are  attached  to  the  haemal  processes,  and  longitudinal 
muscles  to  the  posterior  process. 

The  oesophagus  passes  through  the  brain,  between  the  anterior  cornua, 
to  the  sucking  stomach  which  lies  in  the  groove  on  the  haemal  side  of  the  endocra- 
nium. Muscle  strands  run  from  the  stomach  to  the  walls  of  the  groove. 

Endocranium  of  Limulus  (Figs.  2,  70,  75,  78,  209,  214,  215). — The  endo- 
cranium of  Limulus  is  the  largest  of  any  living  arachnid.  It  is  a  rectangular 


I    C 

»  T  y 


ppr. Wj 


FIG.     213. — Endocranium    of    Mygale.    A,     Neural     surface;     B,     hasmal_*  surface.      Xs.      After    Patten    and 

Redenbaugh. 

plate  of  nbro-cartilage,  about  three  inches  long,  two  and  one-half  inches  wide, 
and  from  one-eighth  to  one-half  an  inch  thick  on  the  margins.  It  lies  near  the 
center  of  the  cephalo thorax,  with  its  anterior  margin  about  opposite  the  cheli- 
cerae,  and  its  posterior  one  opposite  the  chilaria.  It  serves  as  a  center  of  attach- 
ment for  the  more  important  muscles  of  the  thorax.  The  mouth  is  located  a 
little  anterior  to  the  center  of  the  endocranium.  From  it  the  oesophagus  passes 
forward,  through  the  brain,  and  between  the  anterior  cornua  to  the  mesenteron. 
(Figs.  78-209.)  The  latter  begins  a  short  distance  in  front  of  the  endocranium 
and  extends  straight  backward  close  to  its  haemal  side. 

The  neural  surface  of  the  endocranium  is  nearly  flat  and  bounded  on  either 
side  by  a  sharp  ridge  or  lateral  wall.  The  anterior  ends  of  the  wall  are  much 
higher  and  generally  slope  inward.  (Fig.  215,  A.)  The  posterior  ends  also  in- 
crease in  height,  turn  inward  and  become  continuous  with  the  plate  of  cartilage 
that  forms  the  roof  of  the  occipital  region.  The  endocranium  thus  forms  a  shal- 
low box,  with  low  vertical  walls  along  the  sides  and  along  a  part  of  its  posterior 
end.  There  is  no  transverse  anterior  wall  and  no  covering,  or  roof,  except  at  the 


THE    ENDOCRANIUM   OF   LIMULUS. 


315 


posterior  end.  The  brain  lies  on  the  floor  of  this  box,  the  ventral  cord  and  several 
pairs  of  nerves  extending  backward  through  the  large  occipital  foramen  in  the 
posterior  wall.  (Fig.  214.) 

We  may  recognize  the  following  parts,  viz.  The  anterior  cornua,  a.c.,  a  pair 
of  stout,  transversely  flattened  processes  formed  by  the  forward  prolongation  of 
the  thickened  lateral  margins.  From  each  process  arise  three  muscles,  the  oppo- 
site ends  of  which  are  attached  to  the  haemal  side  of  the  carapace  (Fig.  75);  one 
is  directed  forward  from  the  extremity  of  the  cornua,  one  perpendicularly  from 
its  inner  surface,  and  one  obliquely  forward  from  its  haemal  margin. 

The  neural  margins  of  the  cornua  and  the  entire  lateral  portions  of  the  endo- 
cranium,  including  the  posterior  lateral  processes,  give  attachment  to  the  plastro- 
coxal  muscles  of  the  second  to  the  sixth  pair  of  thoracic  appendages.  (Fig.  75.) 

Anteriorly,  the  muscles  do  not  cover  the 
neural  surface  of  the  endocranium,  Jbut  pos- 
teriorly the  muscles  increase  in  size  with  the 
increase  in  size  of  the  appendages,  and  encroach 
upon  the  neural  surface  even  to  the  median 
line.  There  is  therefore  on  the  anterior  neural 
surface  of  the  endocranium  a  triangular  space 
which,  except  for  a  few  loose  strands  (plastro- 
buccal  muscles  going  to  the  oesophagus)  is  free 
from  muscles  and  comparatively  smooth.  (Fig. 
215,  A) 

The  anterior  hamal  processes,  I.e.,  arise  from 
the  anterior  haemal  side  of  the  endocranium. 
They  consist  of  two  pairs  of  long  and  slender 
processes  each  one  attached  by  a  short  muscle 
to  the  haemal  side  of  the  carapace,  close  to  the 
origins  of  the  tergo-coxal  muscles. 

rj^i  ^  7    .  7     .          T  FIG.  214. — Endocranium  of  Limulus,  seen 

The  posterior  hamal  processes,  h.pr.,  lie  on  from  the  neurai  side,  with  the  brain  in  place. 
the  haemal  side  near  the  lateral  edge  of  the 

endocranium.  They  incline  slightly  outward,  and  each  gives  attachment  to  two 
muscles:  one  going  from  the  extremity  of  the  process  to  the  carapace,  and  the 
other  from  the  posterior  margin  of  the  process  to  the  first  entapophysis.  (Fig.  75.) 

The  posterior  lateral  processes,  Ip.pr.,  are  flattened  expansions  of  the  posterior 
portion  of  the  endocranium.  Along  the  posterior  margin  of  each  process,  on  the 
neural  side,  is  a  sharp  transverse  ridge  which,  toward  the  median  line,  unites  with 
the  lateral  ridge  and  with  the  base  of  the  occipital  ring.  The  posterior-lateral 
processes  give  attachment  to  some  of  the  plastro-coxal  muscles  of  the  sixth  pair 
of  legs,  which  are  the  most  powerful  appendages  of  the  animal. 

The  posterior  median  process,  p.pr.,  or  basioccipital,  begins  as  a  median  ridge 
on  the  haemal  side  of  the  endocranium,  between  the  haemal  processes.  It  in- 
creases in  thickness  posteriorly,  ending  in  a  bifid  process,  each  division  of  which  is 


3i6 


ENDOCRANIUM,    BRANCHIAL   AND    NEURAL    CARTILAGES. 


deeply  grooved  on  the  haemal  side.  Along  the  whole  haemal  side  of  the  process 
are  attached  two  large  muscles  that  go  to  the  first  pair  of  entapophyses.  To  the 
body  of  the  endocranium,  on  both  sides  of  the  basioccipital,  are  attached  longi- 
tudinal abdominal  muscles.  Their  attachments  extend  a  little  anterior  to  the 
haemal  processes.  In  front  of  this,  the  body  of  the  endocranium  is  destitute  of 
muscles  on  the  haemal  side.  A  pair  of  small  chilarial  muscles  are  attached  to 
the  haemal  side  of  the  extremity  of  the  basioccipital. 


I* 


? 


FIG.  215. — Endocranium  of  Limulus.     A,  Seen  from  the  neural  surface;  B,  from  the  haemal  surface;  C,  from  the 

caudal  end. 

The  occipital  ring,  oc.r.,  begins  at  the  points  where  the  marginal  walls  meet 
the  posterior-lateral  processes.  Here  two  vertical  outgrowths  are  formed  which 
unite  with  each  other  on  the  neural  side  of  the  ventral  cord.  At  their  bases  the 
processes  are  slender,  but  distally  they  enlarge  and  thicken,  forming  a  polygonal, 
supraoccipital  plate  that  is  joined  to  the  capsuliginous  bars,  B,  by  strands  of 
connective  tissue.  Upon  the  neural  surface  of  the  supraoccipital  plate  are  two 
depressions,  to  which  are  attached  a  pair  of  muscles  going  to  the  insides  of  the 
chilaria,  ch.m.  From  the  anterior  edge  of  the  plate,  muscle  strands  pass  for- 
ward to  the  integument  immediately  behind  the  mouth. 

The  capsuliginous  bars,  B,  arise  from  the  thin  posterior  margin  of  the  endo- 
cranium, bend  neurally  and  slightly  toward  the  median  line,  and  are  attached  to 


THE   ENDOCRANIUM   OF    THE    SCORPION.  317 

the  posterior  sides  of  the  bases  of  the  chilaria.  A  small  transverse  muscle  joins 
the  distal  ends  of  the  two  bars.  Two  other  small  muscles  run  to  the  chilaria 
from  the  thin  portions  of  the  endocranium,  near  the  bases  of  the  bars. 

It  is  especially  noteworthy  that  the  body  of  the  endocranium  is  composed  of 
fibroid  cartilage,  while  the  bars  just  described  are  of  capsuliginous  cartilage, 
exactly  like  that  found  in  the  abdominal  appendages.  The  development  of  the 
bars  shows  that  they  represent  a  pair  of  gill  bars  belonging  to  the  chilaria,  that 
have  been  secondarily  united  with  the  cranidium. 

Foramina. — There  are  two  pairs  of  foramina  for  the  passage  of  nerves.  One 
pair  lies  just  outside  the  marginal  wall,  appearing  on  the  haemal  side  of  the  endo- 
cranium, a  little  posterior  to  the  haemal  processes,  f1.  The  intestinal  branches 
of  the  haemal  nerves  belonging  to  the  sixth  thoracic  neuromere  pass  through  this 
pair.  (Fig.  214,  h.n6.)  The  intestinal  branch  of  the  chilarial  haemal  nerve  passes 
through  the  second  foramen,/2. 

The  occipital  foramen  is  the  large  canal  enclosed  by  the  basioccipital  and  the 
supraoccipital  plates.  Through  it  passes  the  spinal  cord,  the  chelarial  and 
opercular  nerves. 

Endocranium  of  the  Scorpion  (Figs.  43,  216,  217,  218). — The  endo- 
cranium of  a  small  American  scorpion,  probably  Buthus  carolinianus,  was  recon- 
structed, by  plotting  serial  sections  of  the  whole  thorax. 
The  endocranium  of  the  large  African  scorpion  was  dis- 
sected out  and  the  drawings  and  measurements  were 
made  under  the  simple  microscope. 

In  the  American  scorpion,  Fig.  216,  the  endocranium 
consists  of  two  nearly  parallel  plates  of  nbro-cartilage,  or 
trabeculae;  they  are  united  at  their  posterior  ends  by  a 
broad  basilar  plate,  which  extends  laterally  into  wing- 
like,  posterior-lateral  processes,  I. p.,  and  backward  in 
two  long  posterior  processes,  p.p.  The  thickened  median 
portion  forms  the  basioccipital.  The  trabeculae  are 
united  in  front  of  the  basioccipital  by  a  thick  membrane, 
on  the  neural  surface  of  which  lies  the  enlarged  anterior  FIG.  216. 

i  ^  •,      ,1  i        r  ^i          of    an    American    scorpion 

end  of  the  bothroidal  cord,  m.c.    Opposite  the  ends  of  the    (Buthus)  seen  from  the 


membrane,  are  two  plate-like  haemal  processes,  directed    surface-    Reconstructed  from 

sections  and  dissections. 

haemally   and    laterally.     The    middle   portion   of   the 

trabeculae  are  thin,  nearly  horizontal  plates.     The  anterior   ends  are  greatly 

thickened  and  end  in  two  pointed  processes. 

There  is  a  distinct  occipital  ring  enclosing  the  posterior  part  of  the  brain. 

In  the  large  African  scorpion  (Figs.  217,  218),  the  endocranium  is  heavy  and 
well  developed  and  in  its  general  form  resembles  the  one  just  described.  The 
neural  surface  of  the  diverging  trabeculae  is  flat  and  their  united  posterior  ends 
form  a  thick  basilar  plate  from  which  spring,  right  and  left,  the  broad  vertical 
plates  representing  the  posterior  lateral  processes,  1. p.  The  supraoccipital, 


ENDOCRANIUM,    BRANCHIAL   AND    NEURAL    CARTILAGES. 


oc.p.,  is  a  thick  triangular  plate  with  clean  cut,  beveled  edges.  The  apex  extends 
forward  as  two  diverging  tendons,  an.s.,  and  the  posterior  angles  extend  backward 
over  the  occipitals  as  diverging  crests  that  become  continuous  with  the  ragged 
flaring  plates  that  represent  the  posterior  processes,  p.p.  A  small  vertical  process 
arises  from  the  middle  of  the  posterior  margin  of  the  supraoccipital.  (Fig.  218,  n.p.) 

When  seen  from  behind,  the  occipital  ring  presents  a  very  striking  resemblance 
to  the  occipital  region  of  a  vertebrate  cranium.  (Fig.  218,  B.)  The  exoccipital 
region  is  heavily  reinforced,  and  on  each  side  a  ridge  extends  laterally  from  the 
posterior  face  of  the  exoccipitals  along  the  posterior  process. 

The  haemal  plates  are  two  deep,  flaring  plates  extending  along  the  haemal 
surface  of  the  trabeculae  and  basilar  plate.  The  posterior  haemal  processes,  h.p., 
may  be  regarded  as  a  local  specialization  of  the  haemal  plates,  corresponding  to 
the  posterior  pair  of  haemal  processes  in  the  endocranium  of  mygale.  They  are  long 


FIG.  217. — Endocranium  of  a  large 
African  scorpion,  seen  from  the  neural  surface. 
Cam.  outline.  X  7. 


FIG.  218. — Endocranium,  same  as  in  preceding 
figure.  A.  Seen  from  the  side;  B,  the  occipital  por- 
tion, seen  from  the  caudal  end.  X  7. 


narrow  plates  arising  about  opposite  the  anterior  margin  of  the  occipitals.  The 
anterior  haemal  process,  a.h.p.,  is  a  thin,  irregular  prolongation  of  the  anterior 
margin  of  the  haemal  plate.  It  is  of  uncertain  form,  since  its  margins  are  easily 
injured  in  the  dissection. 

The  haemal  plates  converge  posteriorly,  following  the  general  direction  of 
the  trabeculae.  The  anterior  ends  flare  outward;  the  posterior  ends  are  nearly 
vertical,  and  form  the  lateral  walls  of  a  deep  sub-cranial  channel  in  which  lies 
the  posterior  portion  of  the  stomodaeum  and  the  anterior  end  of  the  gut.  In  Fig. 
218,  A,  the  brain  and  endocranium  are  shown  from  the  side  in  their  proper  re- 
lations. The  anterior  ends  of  the  trabeculae  reach  almost  to  the  anterior  surface 
of  the  bent  over  forebrain.  The  neural  surface  of  the  trabeculae  forms  two 
horizontal  shelves  on  either  side  of  the  hindbrain,  the  four  posterior  thoracic 
neuromeres  sending  their  nerves  laterally  over  the  neural  surface  of  the  shelf. 


THE  ENDOCRANIUM;  SUMMARY. 

The  supraoccipital  forms  an  arching  roof  over  the  posterior  thoracic  and  vagus 
neuromeres.  The  thoracic  nerves  pass  out  of  the  endocranium  through  the  wide 
open  sides,  while  the  nerves  of  the  four  vagus  neuromeres,  together  with  the  nerve 
cords,  pass  backward  and  out  of  the  cranium  through  the  foramen  magnum. 

Telyphonus. — The  endocranium  of  Telyphonus  (Fig.  219),  can  be  readily 
reduced  to  the  type  of  those  described  above. 


.Joife 

FIG.  219. — Endocranium  of  Telyphonus.     A,  Neural  surface;   B,  haemal;  C,  side.       Xsi/3- 

V.  SUMMARY  AND  COMPARISON. 
I.  Endocranium. 

1.  A  cartilaginous  endocranium  is  eminently  characteristic  of  the  phyllopod- 
arachnid-vertebrate  stock.     It  appears  to  be  absent  in  the  insects,  myriapods  and 
Crustacea.     Its  simplest  adult  condition  is  seen  in  Apus  and  Branchipus  where 
it  is  an  unpaired,  unsegmented  basilar  plate  of  fibrocartilage,  with  thickened 
margins  and  projecting  cornua.     This  is  also  its  early  embryonic  condition  in 
Limulus. 

2.  The  evolution  of  the  form,  location,  and  mode  of  growth  of  the  endocra- 
nium was  determined  primarily  by  the  size  and  functional  activity  of  the  mus- 
cles and  appendages  with  which  it  was  associated. 

3.  The  basilar  plate  served  primarily  for  the  attachment  of  muscles  arising 
from  the  bases  of  the  mandibles,  or  of  the  anterior  thoracic  appendages.     It  is  first 
seen  in  the  anterior  midbrain  region  as  a  thickened  horizontal  membrane,  lying 
between  the  nerve  cord  and  the  alimentary  canal  and  extending  across  the  median 
line  from  the  base  of  one  appendage  to  its  mate. 

In  ADUS  and  Branchipus  it  is  confined  to  the  mandibular  metamere;  in 


320  ENDOCRANIUM,    BRANCHIAL   AND    NEURAL    CARTILAGES. 

Limulus  and  other  arachnids,  it  arises  from  all  the  thoracic  and  first  vagus 
metameres,  and  in  the  adult  it  may  cover  a  much  wider  territory  than  the  entire 
brain. 

4.  The  extension  backward  of  the  basilar  plate  in  the  arachnids,  and  the 
increased  volume  of  its  posterior  portion,  was  due  to  the  diminished  size  of  the 
anterior  thoracic  appendages  and  the  increase  in  size  of  the  posterior  ones.     Also 
to  the  fact  that  as  the  thoiacic  metameres  united  more  intimately  with  each  other 
and  with  the  forehead,  the  posterior  portion  of  the  basilar  plate  served  as  a  more 
and  more  important  point  of  attachment  for  those  voluminous  longitudinal  trunk 
muscles  that  helped  to  move  the  whole  cephalothorax  on  its  hinge-like  joint. 
Another  factor  that  aided  in  the  development  of  the  occipital  region  was  the  crowd- 
ing forward  of  the  neural  arches  of  the  vagus  segments  to  form  the  supraoccipital 
plate,  and  the  union  of  the  latter  with  the  underlying  basilar  plate  to  form  the 
occipital  ring.     The  occipital  region  was  also  reinforced  by  the  addition  of  the 
branchial  bars  of  the  rudimentary  vagus  appendages  to  the  posterior  part  of  the 
basilar  plate. 

5.  The  basilar  plate  is  anchored  to  the  haemal  wall  of  the  cranium  by  the 
powerful  plastro-tergal  muscles  that  arise  from  the  keel-like  haemal  plates.     The 
size  of  these  plates,  their  form,  and  the  direction  of  their  fibrous  constituents, 
is  determined  by  the  amount  and  direction  of  the  strain  on  them,  and  hence  is 
determined  indirectly  by  the  same  factors  that  control  the  form  and  development 
of  the  basilar  plate  and  the  occipital  ring. 

6.  Thus  the  evolution  of  the  arachnid  endocranium  becomes  intelligible. 
Its  four  main  axes,  or  planes,  of  growth  that  have  given  rise  to  the  transverse 
basilar  plate,  the  longitudinal  trabeculae,  the  vertical  haemal  plates,  and  the  heavy 
occipital  ring,  are  due  more  remotely  to  those  causes  controlling  the  concrescence 
of  the  cephalic  metameres,  and  more  directly  to  the  nature  of  the  three  axes  of 
muscular  strain,  transverse,  longitudinal,  and  vertical,  that  have  acted  upon  it. 
It  will  be  observed  that  owing  to  the  relation  of  the  endocranium  to  the  mouth, 
brain,  stomodaeum,  and  intestine,  the  framework  of  the  endocranium  could  not 
be  constructed  along  any  other  lines  than  those  indicated.     (Fig.  220.) 

7.  There  is  no  reason  to  doubt  that  the  conditions  prevailing  in  living  arach- 
nids, also  obtained  in  the  trilobites  and  merostomata.     In  the  gigantic  eurypter- 
ids,  with  powerful,  oar-like  appendages  and  movable  head,  the  endocranium 
probably  reached  a  higher  grade  of  development  than  in  any  of  their  living  repre- 
sentatives. 

8.  The  fully  developed  arachnid  endocranium  is  in  every  essential  respect  a 
duplicate  of  the  primordial  cranium  of  a  primitive  vertebrate  embryo.     They 
agree :    a.  In  their  position  relative  to  the  brain;  b.  in  their  general  form;  c.  in  their 
mesodermic  origin  and  histological  structure;  d.  in  their  absence  of  segmenta- 
tion, although  spreading  over  several  metameres;  e.  in  their  great  size  compared 
with  the  brain;  and  /.  which  is  the  most  important  agreement  of  all,  they  agree 
in  their  four  axes,  or  planes  of  growth;  namely,  the  transverse  growth  of  the  basilar 


THE  ENDOCRANIUM;  SUMMARY. 


321 


plate,  the  longitudinal  growth  of  the  trabeculae,  the  overarching  growth  of  the 
occipitals,  and  the  longitudinal  vertical  growth  of  the  haemal  plates  or  palato- 
pterygo-quadrate  arcade.  (Compare  Fig.  220.) 

9.  The  endocranium  has  developed  along  these  lines  primarily  in  response  to 
muscular  strain,  and  its  principal  axes  of  growth,  therefore,  coincide  with  the 
axes  of  strain.  But  after  it  has  become  established  phylogenetically,  as  in  the 
vertebrates,  it  acquires  a  new  moment  of  growth  that  is  independent  of  the 
direction  or  location  of  the  muscular  strains  brought  to  bear  upon  it. 


ac. 


P- 


FIG.  220. — Diagrams  illustrating  three  stages  in  the  evolution  of  the  arachnid  endocranium.  A,  Phyllopod 
stage,  seen  from  the  neural  surface,  and  in  transverse  section.  The  endocranium  consists  of  a  flattened  basilar 
plate,  located  in  the  region  of  the  oral  appendages  (mandibles  and  maxillse)  and  composed  of  a  thickened  trans- 
verse bar,  and  two  longitudinal  ones.  B,  Primitive  arachnid  stage;  the  endocranium  extends  over  the  whole 
of  the  thoracic  region,  and  shows  the  beginning  of  the  anterior  and  posterior  cornua,  a.c.  and  p.c.,  lateral  and 
haemal  processes,  l.p.  and  h.p.,  and  marginal  ridges,  m.r.  C,  Arachnid  stage,  showing  the  fully  developed  arachnid 
endocranium,  and  the  principal  axes  of  growth. 

10.  The  vertebrate  embryo  picks  up  the  growth  of  the  endocranium  at  about  the 
highest  stage  reached  in  the  arachnids.  If  the  growth  of  the  arachnid  endocranium 
advanced  still  farther  along  the  lines  it  has  already  established,  it  would  follow 
very  nearly  the  later  embryonic  development  of  the  vertebrate  cranium.  For 
example,  a.  the  upward  growth  of  the  lateral  walls  of  the  trabeculae;  b.  the  forward 
growth  of  the  supraoccipital ;  c.  the  forward  growth  of  the  trabeculae  and  the 


322  ENDOCRANIUM,    BRANCHIAL  AND    NEURAL    CARTILAGES. 

union  of  their  anterior  ends;  d.  the  closing  up  of  the  pituitary  foramen;  and  e.  the 
separation  of  a  part  of  the  haemal  arcade  from  the  basilar  plate  to  form  the  pterygo- 
quadrate  arcade. 

II.  Branchial  Skeleton. 

1.  a.  In  Limulus  the  first  six  pairs  of  appendages,  i.e.,  those  belonging  to  the 
same  metameres  as  the  endocranium,  never  develop  appendicular  cartilages.     In 
the  seven  following  appendages,  the  bars  are  present;  they  are  therefore  post- 
cranial  in  origin,  although  the  first  pair  (chilarial)  is  firmly  attached  to  the  pos- 
terior end  of  the  basilar  plate,     b.  The  haemal  ends  of  the  branchial  bars,  except 
the  first  pair,  are  united  by  a  longitudinal  band  of  cartilage,     c.  The  gill  bars  and 
the  longitudinal  band  are  composed  of  capsuliginous  mucoid  cartilage,  quite 
different  chemically  and  histologically  from  the  cartilage  of  the  endocranium; 
and  d.     The  three  pairs  of  haemal  processes  in  Limulus  have  a  superficial  resem- 
blance to  the  gill  bars  in  position  and  direction,  but  they  are  of  a  different  nature 
in  structure  and  origin,  and  belong  primarily  to  the  endocranium. 

2.  Similar  conditions  prevail  in  vertebrates:     a.  The  true  branchial  bars 
are  independent,  post-cranial  structures,  the  anterior  pairs  being  joined  second- 
arily with  the  cranium,     b.  The  segmentally  arranged  gill  bars  may  be  united  by 
continuous   longitudinal  bands,     c.  The  gill  bars  and  bands  are  composed  of 
muco-cartilage  differing  chemically  and  histologically  from  the  fibro-cartilage  of 
the  endocranium.     d.  The  four  or  five  preauditory  metameres   (excluding  the 
forebrain  region)   never  give  rise  either  to  typical  or  fully  developed  gills,  or  to 
gill   bars.     e.  The   proximal   portion   of   the   palato-pterygo-quadrate-hyoman- 
dibular  arcade  does  not  represent  modified  gill  arches.     It  belonged  originally 
to  the  primordial  cranium  and  represents  in  part  the  haemal  plate  arcade  of  the 
arachnid  endocranium.     (Fig.  220.) 

III.  Neural  Arches. 

The  neural  arches  of  Limulus  represent  the  initial  stages  in  the  formation  of 
a  vertebral  column.  Their  contour  is  due  to  the  direction  and  intensity  of  the 
muscular  strain  acting  on  them,  and  they  agree  in  general  form  and  in  the  direc- 
tion of  their  processes,  with  the  neural  arches  of  vertebrates. 


CHAPTER  XVIII. 
THE  MIDDLE  CORD  THE  LEMMATOCHORD  AND  THE  NOTOCHORD. 

The  failure  to  recognize  the  notochord  in  invertebrates  has  been  due  to  the 
prolonged  domination  of  the  annelid  and  gastrula  theories. 

It  has  been  generally  assumed  that  the  notochord  is  found  only  in  the  verte- 
brates because  no  one  could  find  one  in  the  invertebrate  midgut  where  the  gas- 
trula theory  proclaims  it  ought  to  be  located  if  present.  So  long  as-  it  was  con- 
fidently assumed  that  the  "  archenteron "  of  vertebrates  was  the  ontogenetic 
repetition  of  an  ancestral  midgut,  and  that  its  associated  parts,  mesoderm  and 
notochord,  were  necessarily  entodermic  in  origin,  no  one  ventured  to  look  for  the 
notochord  elsewhere  than  in  the  midgut  of  an  annelid,  or  in  some  other  worm- 
like  invertebrate.  It  is  not  surprising  then  that  almost  any  unpaired  enteric 
outgrowth  has  been  called,  at  one  time  or  another,  a  notochord,  thus  giving  a 
pseudo-respectability  to  such  morphological  curiosities  as  diplochorda,  adelo- 
chorda,  hemichordata^  etc.  On  the  other  hand,  any  organ  not  entodermic 
in  origin  was  thereby  branded  as  illegitimate  and  excluded  from  further  con- 
sideration. 

One  misinterpretation  led  to  another,  till  the  original  theory  became  so  deeply 
buried  that  embryologists  appeared  to  forget  that  the  whole  superstructure  rested 
on  the  extremely  doubtful  assumption  that  certain  stages  of  vertebrate  embryos 
were  to  be  interpreted  in  terms  of  adult  jelly-fishes. 

The  fact  that  there  was  little  or  no  evidence  that  the  archenteron  really 
represents  a  primitive  gut,  or  that  the  notochord  ever  had  any  other  function  than 
it  has  at  present,  was  persistently  ignored. 

As  I  stated  in  my  earliest  paper  on  this  subject,  1889,  p.  35 1 :  "  There  is  nothing 
in  the  embryology  of  the  vertebrates  to  show  to  what  germ  layer  the  notochord 
belongs.  It  is  never  continuous  with  functional  endoderm ;  there  is  no  evidence 
that  it  ever  exercised,  itself,  any  alimentary  functions;  or  that  it  is  ever  con- 
nected in  any  way  with  an  alimentary  canal."  The  only  thing  vertebrate  em- 
bryology tells  us  about  the  notochord  is  that  it  has  its  origin,  like  the  axial  meso- 
derm, the  nerve  cord,  and  the  entoderm,  in  the  common  mass  of  growing  cells  at 
the  tail  end  of  the  embryo.  As  to  the  original  function  of  the  notochord,  or  its 
relation  to  germ  layers,  vertebrate  morphology  has,  as  yet,  had  nothing  conclusive 
to  say. 

We  must,  therefore,  first  identify  the  representative  of  the  notochord  in  the 
invertebrates  before  we  can  safely  interpret  its  morphology  in  vertebrates.  This 
we  can  do  readily  enough  as  soon  as  we  eliminate  the  misconceptions  of  the 
gastrula  and  annelid  theories,  for  the  main  facts  in  the  development  of  the  verte- 

323 


3  24 


THE    MIDDLE    CORD,    THE    LEMMATOCHORD    AND    THE    NOTOCHORD. 


brate  and  invertebrate  notochord  are  sufficiently  clear.  The  beginning  of  the 
notochord  may  be  recognized  in  practically  all  segmented  invertebrates,  as  the 
so-called  middle  cord,  or  median  nerve,  and  in  its  derivative,  the  lemmatochord. 
This  structure  forms  a  fundamental  part  of  the  body  in  all  segmented  inverte- 
brates. It  undergoes  many  modifications,  but  its  location,  function,  mode  of 
growth,  and  its  development,  are  in  all  cases  essentially  the  same,  and  leave 
no  reasonable  doubt  that  it  is  indeed  the  long  looked  for  notochord  of  the  in- 
vertebrates. 

I.  THE  MIDDLE  CORD  OF  INSECTS. 

Acilius  (Figs.  221,  222). — The  middle  cord  arises  at  a  very  early  em- 
bryonic stage  as  a  median  longitudinal  groove  that  extends  from  the  posterior 
margin  of  the  stomodaeum  to  the  posterior  end  of  the  body.  It  should  not  be 
confused  with  the  so-called  primitive  groove  of  arthropods,  or  with  the  neural 
groove  of  vertebrates. 


rn.ck. 


FIG.  221. — The  second  pedal  neuromere  of  an  embryo  of  Acilius;   serial  sections  showing  the  development  of  the 
middle  cord,  and  its  relation  to  the  cross  commissures,  inner  neurilemma,  or  neuroglia,  and  median  nerve. 

The  walls  of  the  groove,  in  the  interganglionic  portions,  give  rise  either  to 
thickenings  to  which  muscles  are  attached  (thoracic  region)  or  to  the  median 
nerve  (abdominal  region).  The  walls  of  the  intra-ganglionic  groove  give  rise 
to  nerve  cells  that  form  a  part  of  the  ganglia,  and  to  the  neuroglia. 

The  early  conditions  are  shown  in  a  series  of  sections  of  the  nerve  cord  of 
Acilius  opposite  the  second  pair  of  legs,  during  the  formation  of  the  dorsal  oigan. 
(Fig.  221.) 

The  groove  deepens  in  front  of  the  second  pedal  neuromere  and  its  nuclei 
become  darker  colored,  A3.  In  the  middle  of  the  neuromere,  the  outer  walls  of 
the  groove  have  almost  disappeared,  while  the  floor  forms  a  solid  triangular  block 
of  cells  overlying  the  cross  commissures,  A*.  A  few  nuclei,  like  those  in  the 
middle  cord,  lie  underneath  the  lateral  ends  of  the  cross  commissures,  inl.  On 


THE  MIDDLE  CORD  OF  INSECTS. 


325 


the  posterior  margin  of  the  neuromere,  A5,  the  middle  cord  is  still  a  solid  tri- 
angular block  of  cells,  with  its  deep  lateral  angles  spreading  outward  underneath 
the  longitudinal  connectives.  Still  farther  back,  AQ,  the  middle  cord  is  again 
a  deep  groove,  open  at  the  surface,  and  with  thin  lateral  walls.  The  sections  show 
that  the  floor  of  the  intra-ganglionic  middle  cord  is  raised  over  the  cross  com- 
missures, and  that  the  cells  at  the  deep  angles  of  the  middle  cord  are  spreading 
forward  and  backward  around  the  cross  commissures  and  longitudinal  connec- 
tives. These  cells  form  the  so-called  inner  neurilemma,  or  neuroglia.  There  is 
nothing  to  indicate  that  the  cross  commissures  arise  from  the  intraganglionic  cells 
of  the  middle  cord,  as  is  claimed  by  many  authors.  They  appear  to  be  out- 
growths from  the  ganglion  cells  of  the  lateral  nerve  cords. 


m.eH. 


FIG.  222.  —  Nerve  cord  of  an  embryo  of  Acilius.  A  ,  '-fi,  serial  sections  of  an  abdominal  neuromere  showing  a  later 
stage  in  the  development  of  the  neuroglia  and  middle  cord  (median  nerve)  ;  Bl,  inter-  ganglionic  infolding  of  middle 
cord  of  the  thorax  in  a  newly  hatched  larva  of  Acilius,  showing  modifications  of  middle  cord  for  the  attachment 
of  muscles;  B-,  section  in  same  stage  of  an  inter-ganglionic  space  of  the  abdomen,  showing  the  middle  cord  as  the 
ganglion  of  the  median  nerve. 

In  one  of  the  next  stages,  a  series  of  sections  beginning  at  the  anterior  margin 
of  the  second  pedal  neuromere,  shows  that  the  middle  cord  has  now  lost  its  con- 
nection with  the  surface  ectoderm.  (Fig.  222.).  In  the  center  of  the  neuromere, 
A4,  its  cells  have  multiplied  and  form  a  thick  neuroglia  investment  around  the 
cross  and  longitudinal  connectives.  At  the  posterior  end  of  the  neuromere,  A6, 
the  middle  cord  assumes  its  characteristic  cell  structure,  and  gradually  merges 
into  the  interganglionic  cord. 

In  the  later  stages,  at  the  time  of  hatching,  the  interganglionic  segments  of 
the  thoracic  middle  cord  are  still  in  connection  with  the  ectoderm,  as  they  are 
through  life,  and  wing-like  bundles  of  muscles  are  attached  to  their  sides,  Bl. 
The  interganglionic  segments  in  the  abdomen  become  completely  separated  from 
the  ectoderm,  forming  a  cylinder  of  nerve  cells  like  those  of  the  lateral  cords,  B2. 
At  each  end  the  cord  merges  into  the  central  tissues  of  the  neuromeres.  The 
chain  of  abdominal  interganglionic  segments  of  the  middle  cord  may  now  be 
recognized  as  the  median  nerve  of  the  adult. 

Thus  the  thoracic  furcae  for  the  attachment  of  muscles,  the  median  nerve, 
and  the  neuroglia  are  but  different  stages,  or  modifications,  of  a  single  structure. 


326  THE    MIDDLE    CORD,    THE    LEMMATOCHORD   AND    THE    NOTOCHORD. 

We  may  conclude  from  the  wide  distribution  of  the  middle  cord  in  the  arthro- 
pods, and  from  the  important  part  it  plays  in  the  early  embryonic  stages:  i. 
That  the  central  nervous  system  of  primitive  arthropods  consisted  of  three  par- 
allel, longitudinal  bands  that  were  divided  into  similar  segments;  2.  that  the 
neuromeres  of  the  middle  cord  fused  with  those  of  the  lateral  cords,  and  that  the 
longitudinal  connectives  remained  separate;  and  3.  that  there  has  been  a  pro- 
gressive degeneration,  or  modification,  or  specialization,  of  the  intra-  and  inter- 
ganglionic  segments  of  the  middle  cord  from  the  head  end  backward  into 
non-nervous  structures;  while  the  lateral  cords  have  increased  in  the  specializa- 
tion of  nervous  tissues  from  behind  forward. 

The  Lemmatochord  of  Lepidoptera  (Figs.  223,  224). — The  lemmatochord 
of  lepidoptera,  or  Leydig's  cord,  is  a  large,  irregular,  cylindrical  rod  of  elastic, 
semi-gelatinous  tissue  extending  along  the  haemal  side  of  the  nerve  cord  from  the 
thoracic  neuromeres  to  the  posterior  end  of  the  cord.  It  serves  as  a  support  to 
the  nerve  cord  and  for  the  attachment  of  lateral  sheets  of  muscle  fibers,  the  ends 
of  which  are  imbedded  in  the  substance  of  the  cord.  (Fig.  224,  B.)  It  resembles 
the  notochord  of  vertebrates  in  its  position,  its  consistency  and  general  histological 
structure;  and  in  its  function.  Morphologically  it  represents  the  interganglionic 
segments  of  the  abdominal  middle  cord,  enveloped  in  the  thickened  neurilemma 
of  the  median  nerve  and  that  of  the  adjacent  lateral  cords.  It  makes  its  appear- 
ance during  the  metamorphosis  of  the  larvae  into  the  imago. 


The  following  observations  refer  to  the  development  of  the  lemmatochord  of 
Cecropia  and  Sphinx. 

At  the  close  of  the  larval  period,  the  lateral  and  median  cords  are  surrounded 
by  two  membranes,  an  inner,  distinctly  cellular  layer,  i.sh.,  and  an  outer  one,  o.sh., 
that  forms  a  thick  hyaline  membrane.  During  the  early  pupal  stages,  large 
polygonal,  or  oval  cells,  with  clear  protoplasm  and  small  nuclei,  make  their  ap- 
pearance between  the  inner  membrane  and  the  nerve  cord.  (Fig.  223,  A.)  In 
some  places  they  are  isolated  and  imbedded  in  a  darker  plasma.  In  others  they 
are  crowded  together  and  appear  to  have  thick,  but  not  sharply  defined  walls. 

At  this  time  the  outer  hyaline  membrane,  o.sh.,  also  increases  greatly  in  thick- 
ness and  becomes  distinctly  laminated.  Here  and  there  small  flattened  nuclei  are 
seen,  and  in  some  places  clusters  of  thick-walled  chorda  cells,  B.  These  changes 
take  place  in  the  membranes  surrounding  the  ganglia,  the  longitudinal  connectives, 
and  the  median  nerve. 

During  the  subsequent  period,  the  distinction  between  the  two  investing  mem- 
branes disappears,  owing  to  the  conversion  of  the  flattened  cells  of  the  laminated 
membrane  into  the  characteristic,  thick-walled  chorda  cells,  and  to  its  invasion 
by  chorda  cells  formed  from  the  inner  membrane.  The  chorda  cells  develop 
most  rapidly  along  the  haemal  surface  of  the  lateral  cords,  and  around  the  median 
nerve;  they  thus  form  three  large  irregular  bands  of  chorda  cells,  roughly  tri- 


THE    LEMMATOCHORD    OF    LEPIDOPTERA. 


327 


angular  in  cross-section.  The  two  lateral  bands  finally  crowd  the  median  one 
inward,  and  unite  with  it  to  form  a  single  cylindrical  cord,  in  which  the  arrange- 
ment of  the  chorda  cells  may  still  indicate  the  separate  origin  of  peripheral  and 
axial  cells.  (Fig.  224,  A.  s.mn.  and  s.l.c.}  Meantime  the  median  nerve  has  dis- 
appeared, or  at  least  no  traces  of  it  can  be  seen,  except  where  it  rises  to  the  surface 
of  the  chorda  to  enter  the  ganglion,  m.n. 


^Ssfe- 


5  nuv 


-    /; 


FIG.  223.  —  Sections  through  the  abdominal  nerve  cord  of  Cecropia,  early  pupal  stage,  showing  the  greatly 
thickened  sheaths  of  the  median  and  lateral  nerve  cords.  A,  Midway  between  two  neuromeres;  B,  near  the  pos- 
terior side  of  a  neuromere. 

At  the  close  of  the  metamorphosis,  the  cord  has  become  irregularly  oval,  the 
walls  of  the  chorda  cells  have  disappeared,  and  then  nuclei  have  become  iriegular 
in  shape  and  size.  (Fig.  224,  B.)  As  the  chorda  assumes  its  final  form,  a  thick 


FIG  .  2  2  4 . — Nerve  cords  and  lemmatochord  of  Cecropia.  A ,  Late  pupal  stage  showing  the  f u  lly "formed  lemmato- 
chord,  derived  from  the  condensed  sheaths  of  the  median  and  lateral  cords;  also  remnants  of  the  median  nerve;  B, 
adult  Cecropia.  The  tissue  of  the  lemmatochord  has  undergone  degenerative  changes,  and  at  this  point  is  invaded 
by  the  ends  of  the  attached  muscle  cells. 

hyaline  sheath  forms  around  it,  except  along  its  haemal  side  where  the  muscle  cells 
are  attached. 

At  the  beginning  of  the  pupal  period  the  thickened  neurilemma  on  the  neural 
side  of  the  lateral  cords  becomes  inflated  with  a  network  of  trachea,  tr.,  which 
later  disappears,  leaving  the  cords  again  surrounded  by  a  thin  double  layered 
membrane. 

The  chorda  muscles  appear  at  the  close  of  the  larval  period,  just  in  front  of 


328 


THE    MIDDLE    CORD,    THE    LEMMATOCHORD   AND    THE    NOTOCHORD. 


each  neuromere,  and  close  to  the  lateral  branches  of  the  median  nerve,  as  a  thin, 
dark,  nucleated  band,  extending  across  the  haemal  surface  of  the  median  nerve. 

In  the  young  pupae  the  band  expands  very  rapidly,  forming  in  front  of  each 
neuromere  a  transverse  muscular  sheet,  broad  over  the  median  line  and  tapering 
to  a  point  at  either  side.  They  ultimately  form  a  continuous  sheet  beneath  the 
cord.  In  the  adult,  the  ends  of  many  of  the  muscle  fibers  spread  out  in  fanshaped 
masses  of  fibers  that  penetrate  the  substance  of  the  cord  in  all  directions.  (Fig. 
224,  B,  m.) 
II.  THE  MIDDLE  CORD  OF  THE  SCORPION.  (Figs.  15,  16,  43,  71,  225  to  230.) 

The  middle  cord  and  associated  parts  are  in  some  respects  imperfectly  de- 
veloped in  the  scorpion,  so  that  it  is  difficult  to  follow  their  local  modifications; 
but  the  conditions  appear  to  be  essentially  the  same  as  those  we  have  described  in 
embryos  of  Acilius,  and  in  the  larvae  of  Cecropia. 

In  the  scorpion,  the  median  nerve  itself  is  hardly  recognizable;  its  neurilemma 
forms  in  part  the  walls  of  a  blood  sinus.  The  neurilemmas  of  the  median  and 
lateral  cords  form  the  bothroidal  cord  of  the  abdomen  and  the  merochord  of  the 
posterior  thoracic  neuromeres.  Chiten-lined  neural  apodemes  are  absent. 

A.  Neural  Sinus,  Merochord  and  Bothroidal  Cord  of  the  Adult. — In  the 
adult  scorpion  a  large  blood  vessel  extends  along  the  haemal  side  of  the  nerve  cord, 
exclusive  of  the  brain.  In  the  abdomen  and  tail  it  opens  into  vertical  channels 


inl 


FIG.  225. — Cross-sections  of  the  nerve  cord  of  an  adult  scorpion.  No.  i,  Section  through  the  posterior  portion 
of  the  first  free  abdominal  neuromere;  No.  2,  section  between  the  vagus  and  abdominal  neuromeres,  showing  the 
very  thick  walls  and  small  lumen  to  the  neural  sinus;  No.  3,  section  just  in  front  of  the  second  free  abdominal 
neuromere,  showing  the  conspicuous  hamal  tracts;  No.  4,  section  through  the  anterior  half  of  the  last  caudal 
neuromere,  showing  the  solid  cord  of  cells  derived  from  the  middle  cord  and  continuous  with  the  neural  artery; 
No.  5,  section  just  in  front  of  the  anterior  vagus  neuromeres  showing  the  merochord. 

which,  behind  each  neuromere,  pass  between  the  connectives  to  the  skin  on  the 
neural  surface  of  the  body.  There  are  also  in  each  segment  two  lateral  branches 
which  follow  pretty  closely  the  course  of  the  spinal  nerves.  Between  the  succes- 
sive neuromeres  the  vessel  is  much  enlarged,  and  is  either  round  or  triangular  in 
section.  (Fig.  22 53.)  Beneath  each  neuromere  it  is  much  flattened  and  in  some 
cases  hardly  visible.  (Fig.  2252.)  The  walls  of  the  sinus  consist  of  an  inner  epithe- 
lial layer  of  clear  cells,  sharply  contrasted  with  the  dark  coagulum  in  the  sinus. 


THE  MIDDLE  CORD  OF  THE  SCORPION. 


329 


The  outer  layer  is  denser,  contains  a  few  small  dark  nuclei  and  scattering 
bundles  of  longitudinal  and  circular  fibers;  it  appears  to  be  continuous  with  the 
inner  fibrous  layer  of  the  neurilemma.  The  sinus  and  the  whole  spinal  cord  is 
surrounded  by  a  layer  of  large  granular  cells  which  vary  greatly  in  number  and 
arrangement  in  different  parts  of  the  body. 

The  Bothroidal  Cord  or  Lemmatochord. — Along  the  haemal  surface  of  the  neural 
sinus  is  an  elongated,  lobular  organ  extending  the  whole  length  of  the  abdomen. 
At  irregular  intervals  it  forms  large,  spindle-shaped  bothroidal  masses  that  are 
united  with  each  other  by  a  very  delicate  hyaline  fiber.  (Figs.  71,  72).  The  masses 
vary  in  number  and  size;  in  three  different  specimens  I  have  counted  4,  7,  and  9 
of  them.  In  sections  they  have  a  lymphoid  appearance,  and  are  seen  to  consist  of 
dense,  indistinctly  fibrous  masses  crowded  with  minute,  deeply  stained  nuclei. 
(Fig.  22$,l.ch.)  They  are  united  here  and  there  with  the  neural  sinus  by  short 
stalks.  In  one  specimen  there  were  ten  attachments  of  the  cord  to  the  sinus,  six 
of  which  were  hollow  and  opened  into  the  neural  sinus. 

In  the  embryos  the  tissue  from  which  the  bothroidal  cord  arises  extends 
forward  into  the  thorax,  where  it  forms  segmental  thickenings  between  the  succes- 
sive neuromeres.  All  these  thoracic  thickenings  disappear,  with  the  exception  of 
the  one  beneath  the  connectives  of  the  fifth  and  sixth  thoracic  neuromeres.  This 
one  becomes  the  merochord.  It  lies  on  the  neural  surface  of  the  endocranium 
near  the  anterior  edge  of  the  cross  bar.  (Fig.  71.)  The  adult  merochord  (Fig. 
2255)  is  a  large  rounded  body  containing  a  few  clear  cells,  many  small  dark  nuclei, 
and  irregularly  coiled  muscle  strands. 

The  overlying  interganglionic  space  is  closed  and  is  filled  with  a  darkcoagulum 
connected  with  the  merochord  by  an  irregular  reticulum. 

B.  Development  of  the  Lemmatochord. — The  lemmatochord  arises,  in 
part,  as  an  axial  cord  of  cells  extending  forward  from  the  primitive  streak.  In 
stage  A,  Fig.  15,  the  primitive  streak  is  seen  as  a  large  median  mass  of 
polygonal  cells  near  the  posterior  end  of  the  embryo.  In  sections  the  cord  appears 
to  form  as  an  inward  proliferation  of  the  surface  cells,  but  without  the  surface 
infolding  seen  in  Limulus.  (Fig.  2261.)  From  the  point  of  proliferation,  covering 
but  one  or  two  sections,  the  cord  extends  forward  a  short  distance  as  a  well  de- 
fined cylinder.  In  stage  B,  it  is  greatly  reduced  in  thickness  and  forms  a  broad 
lenticular  band  with  the  edges  thinned  out  to  a  single  layer  of  cells,  not  sharply 
marked  off  from  either  mesoderm  or  endoderm.  It  is  largest  just  in  front  of  the 
base  of  the  tail  lobe,  and  extends  forward,  becoming  less  and  less  distinct,  as  far 
as  the  third  abdominal  neuromere.  It  does  not  extend  into  the  tail  lobe. 

In  the  following  stages,  up  to  stage  G,  the  primitive  streak  forms  lens-shaped 
thickenings  beneath  the  3,  4,  5,  6,  and  7  interganglionic  spaces  of  the  abdomen. 
It  is  easy  enough  to  distinguish  these  thickenings  at  this  stage,  but  difficult  to 
determine  their  lateral  boundaries.  A  fairly  defined  layer  of  flattened  cells 
separates  them  from  the  yolk.  (Fig.  228.) 

In  stage  K,  Fig.  229,  the  primitive  streak  has  definitely  split  along  its  whole 


330 


THE    MIDDLE    CORD,    THE    LEMMATOCHORD   AND    THE    NOTOCHORD. 


length  into  two  layers,  the  under  one  being  the  anlage  of  the  sexual  organs,  g.c., 
the  upper  one,  the  lemmatochord,  l.ch.  In  a  series  of  cross-sections  of  this  stage, 
beginning  at  the  posterior  end  of  the  abdomen,  the  lemmatochord  is  first  seen  just 
back  of  the  sixth  abdominal  ganglion,  as  a  dark  lenticular  thickening.  (Fig.  2301.) 
The  cord  has  the  same  flattened  appearance  in  all  the  sections  until  we  reach  the 
vagus  neuromeres,  when  it  suddenly  enlarges  and  assumes  more  of  its  future 
appearance.  (Fig.  229,4"7.)  The  anterior  end  of  the  lemmatochord  is  seen 
between  the  second  and  third  vagus  neuromeres.  (Fig.  22g2.) 

From  this  stage  up  to  the  time  of  hatching,  the  genital  cells  gradually  sepa- 
rate from  the  lemmatochord,  and  the  latter  separates  from  the  neural  sinus,  except 
at  certain  places  where  it  remains  permanently  attached  to  the  neurilemma  of  the 


FIG.  226. — Embryo  scorpion  (Buthus  carolinianus).  No.  i,  2,  3,  Sections  through  the  posterior  end  of  the 
embryo,  stage  A;  No.  4,  5,  6,  7,  sections  through  the  interganglionic  spaces  of  the  nerve  cord,  stage  B.C;  No.  8, 
section  through  the  middle  of  a  terminal  neuromere,  stage  C. 

middle  cord.  By  the  time  the  body  pigment  is  well  developed,  the  lemmato- 
chord of  the  first  free  abdominal  ganglion  appears  as  in  Fig.  230*.  In  this 
figure  we  can  see  indications  of  the  passageway  into  the  sinus.  The  lemmato- 
chord, just  behind  the  third  vagus  neuromere,  is  reduced  to  a  slender  fiber.  (Fig. 
2306.)  At  the  posterior  end  of  the  abdomen  it  decreases  in  size  and  disappears, 
apparently  running  directly  into  the  thickened  wall  of  the  neural  sinus.  (Fig. 

2305.) 

In  embryos  just  hatched  we  may  obtain  good  surface  views  of  the  lemmato- 
chord by  dissecting  out  the  entire  nervous  system.  (Fig.  71.) 

In  half  grown  scorpions  the  lemmatochord  is  in  about  the  same  condition  as 
in  the  adult. 

Merochord. — The  lemmatochord  tissue  extends  into  the  thoracic  region,  giving 
rise  to  the  merochord  and  to  two  adjacent,  parallel  cords  of  dense  connective  tissue. 
(Fig.  71.)  The  first  traces  of  these  structures  are  a  few  isolated  cells,  lying  be- 
neath the  interganglionic  spaces  of  the  thoracic  and  the  first  two  or  three  ab- 
dominal segments.  They  form  lenticular  thickenings,  which  vary  in  size  and 


THE   NEURAL    SINUS,    NEUROGLIA  AND    CANALIS    CENTRALIS. 


331 


appearance  in  the  different  segments,  the  ones  between  the  fifth  and  sixth  neuro- 
meres  being  the  largest.  In  stage  K,  one  of  these  thickenings  forms  a  disc-like 
mass  of  dense  tissue,  with  deeply  stained  nuclei,  lying  on  the  ventral  surface  of  the 
sternum  beneath  the  sixth  interganglionic  space  of  the  thorax.  (Fig.  229,  m.l.ch.) 
This  body  is  the  merochord.  It  appears  to  be  merely  an  isolated  local  enlarge- 
ment of  the  thoracic  portion  of  the  lemmatochord. 

C.  Development  of  the  Neural  Sinus,  Neuroglia  and  Canalis  Centralis. 
-The  middle  cord  groove  in  the  scorpion  develops  in  essentially  the  same  way  as 
in  Acilius. 

From  the  walls  of  the  intraganglionic  portions  are  formed  a.  ganglion  cells; 


^ 

5riSaHHS^^*^Hi 


ec 


FIG.  2  2  7 .—Scorpion  embryo.  No.  i ,  Section  of  an  abdominal  segment  in  front  of  the  primitive  streak,  stage  A  ; 
No.  2,  abdominal  neuromere,  showing  the  breaking  up  of  the  medullary  plate  into  primitive  .sense  organs,  stage  C; 
No.  3,  abdominal  region,  between  two  neuromeres,  stage  E;  No,  4,  abdominal  region  through  the  middle  of  a  neuro- 
mere, stage  E. 

b.  the  epithelium  of  a  central  canal;  c.  neuroglia  cells.  From  the  interganglionic 
portions  arise  a.  transient  nerve  fibers  of  the  median  nerve  connectives;  b.  walls 
of  the  neural  sinus;  c.  blood  corpuscles. 

In  the  adult  scorpion,  the  walls  of  the  neural  sinus  consist  of  an  endothelial 
layer  of  clear  cells,  often  in  sharp  contrast  with  the  enclosed  coagulum.  They 
pass  without  perceptible  break  into  confused  masses  of  tissue,  and  these  into 
blood  corpuscles,  which  seems  to  indicate  that  the  latter  are  formed  from  the  walls 
of  the  sinus  during  adult  life. 

It  is  difficult  to  define  exactly  where  the  sinus  terminates  anteriorly.  In  the 
thoracic  region  of  the  adult,  the  interganglionic  spaces  may  be  quite  large  and 
filled  with  the  gelatinous  substance  and  free  cells,  but  the  spaces  do  not  seem  to 
communicate  with  each  other  freely  or  to  be  directly  connected  with  the  abdominal 


snus. 


332 


THE    MIDDLE    CORD,    THE    LEMMATOCHORD   AND    THE    NOTOCHORD. 


In  surface  views  of  young  embryos,  the  lateral  cords  are  seen  to  be  separated 
by  a  continuous  shallow  groove.  (Fig.  15,  A.)  As  the  cords  break  up  into  neuro- 
meres,  the  median  groove  narrows  and  deepens  and  becomes  distinctly  divided 
into  a  succession  of  oval  pits,  one  between  each  half  neuromere.  (Fig.  15,  B.) 
The  anterior  pits  become  more  distinct  for  a  while;  their  walls  are  then  incor- 
porated into  the  body  of  the  neuromere  at  the  points  where  the  transverse  com- 
missures are  formed.  The  posterior  pits  are  formed  between  the  future  longi- 
tudinal connectives;  they  gradually  flatten  out  and  become  less  distinct  in  sur- 
face views.  Sections  from  this  stage  show  that  the  ectoderm  of  the  posterior 
pits  thins  out  and  draws  away  from  the  underlying  basement  membrane,  forming 
clear  areas,  or  interganglionic  spaces.  (Fig.  2264,  i.g.s.)  They  may  contain  a 


c.c 


U       ,*  faaSSss    -^L"- 


FIG.  228. — Scorpion  embryo,  stage  G;  cross-sections  of  the  nerve  cord,  illustrating  the  development  of  the  middle 
cord,  neuroglia,  neural  blood-vessel,  lemmatochord,  etc.  No.  i,  Section  through  the  posterior  margin  of  the  third 
vagus  neuromere  (comb  segment) ;  No.  2,  section  through  the  middle  of  the  third  abdominal  interganglionic  space, 
or  the  one  next  behind  the  comb  neuromere;  No.  3,  section  through  the  posterior  margin  of  the  same  space;  No.  4, 
section  through  the  interganglionic  space  between  two  thoracic  neuromeres,  showing  the  continuity  of  the  outer 
portion  of  the  middle  cord  from  one  neuromere  to  the  other,  and  also  the  deeper  lying  cells  derived  from  the  inner 
portion  of  the  middle  cord;  No.  5,  section  between  the  fifth  and  sixth  thoracic  neuromeres;  No.  6,  section  through 
the  middle  of  the  fourth  thoracic  neuromere;  No.  7,  section  through  the  middle  of  the  second  thoracic  neuromere, 
showing  the  middle  cord  as  a  prominent  mass  of  cells  on  the  bottom  of  the  neural  canal.  The  middle  cord  is  not 
connected  with  the  ectoderm  at  this  point.  No.  8,  Section  through  the  space  between  the  second  and  third  thor- 
acic neuromeres. 


finely  granular  substance,  a  few  fibers,  and  an  occasional  nucleus,  the  latter  lying 
just  above  the  basement  membrane,  or  among  the  fibers  extending  downward 
from  the  roof.  In  stages  E  and  F,  the  abdominal  spaces  contain  a  few  free  cells 
which  are  undoubtedly  blood  corpuscles.  As  the  spaces  at  this  period  are  com- 
pletely closed,  the  blood  corpuscles  were  evidently  formed  by  a  modification  of 
the  ectodermic  cells  of  the  middle  cord. 

In  stage  F,  a  few  light  coloied  cells  appear  on  the  periphery  of  the  interspaces 
that  mark  the  beginning  of  the  neuroglia,  or  the  inner  neurilemma.  (Fig.  22j,i.n.L) 

In  stage  F,  the  spaces  are  small  and  shut  off  from  the  ectoderm  by  the  union 
of  the  outer  parts  of  the  lateral  nerve  cords.  (Fig.  2273.)  The  intraganglionic 


THE    NEURAL    SINUS,    NEUROGLIA   AND    CANALIS    CENTRALIS. 


333 


cord  is  distinctly  tubular.  (Fig.  227'.)  Owing  to  the  crowding  of  the  thoracic 
neuromeres,  these  middle  cord  tubes  almost  unite  above  the  interganglionic  spaces 
thus  forming  a  nearly  continuous  canal. 

In  the  next  stage,  sections  through  the  middle  of  each  abdominal  neuromere 
show  that  the  invaginated  portion  of  the  median  furrow  is  losing  its  central  cavity, 
and  now  lies  in  the  heart  of  the  neuromere  as  a  great  cluster  of  cells  difficult  to 
distinguish  from  the  surrounding  nerve  cells.  At  the  anterior  and  posterior  ends 
of  the  neuromere,  the  tissues  of  the  middle  cord  are  continuous  with  the  neuroglia 
layer  separating  the  medulla  from  the  cortex.  (Fig.  228',  i.n.l.) 


rn.lek. 
Lick  era.  UK 


FIG.  229. — Scorpion  embryos,  stage  K,  showing  the  later  stages  in  the  development  of  the  middle  cord,  neural 
canal,  neural  blood-vessel,  genital  cells  and  lemmatochord.  No.  i,  Section  through  the  posterior  vagus  neuromere 
showing  the  transverse  commissural  fibers  above  and  below  the  remnants  of  the  middle  cord  (neural  canal) ;  No.  2 
section  through  the  anterior  vagus  neuromere,  showing  median  and  lateral  divisions  of  the  lemmatochord;  No.  3, 
section  just  in  front  of  the  sixth  thoracic  neuromere,  showing  the  local  enlargement  of  the  lemmatochord;  No.  4, 
section  between  the  second  and  third  vagus  neuromeres,  showing  the  enlarged  lemmatochord  and  the  anterior  end 
of  the  germ-cell  cord;  No.  5,  section  through  the  middle  of  the  third  free  abdominal  neuromere;  No.  6,  section  about 
midway  between  the  third  and  fourth  vagus  neuromeres;  No.  7,  section  through  the  anterior  margin  of  the  fourth 
vagus  neuromere;  No.  8,  section  through  the  posterior  portion  of  the  third  free  abdominal  neuromere. 

The  'nterganglionic  spaces  in  the  thorax  contain,  besides  a  few  scattered 
nuclei,  a  loose  fibrous  cord  that  appears  to  run  from  one  neuromere  to  another. 
It  probably  represents  the  remnants  of  a  median  nerve. 

In  stages  H  and  K,  remarkable  changes  have  taken  place.  All  the  inter- 
ganglionic spaces  are  crowded  with  rounded  cells.  They  fill  the  interganglionic 
spaces  and  push  their  way  forward  and  backward  under  the  ganglia  till  they  form 
a  continuous  cord.  (Fig.  2297~8.)  They  are  everywhere  shut  off  from  the  sur- 
rounding tissues  by  the  inner  neurilemma  and  by  the  basement  membrane. 
At  the  anterior  and  posterior  ends  of  each  neuromere  they  are  continuous  with 
the  intraganglionic  portion  of  the  middle  cord.  The  latter  is  now  reduced  to  a 
small  but  well  defined  cord  of  cells  in  the  middle  of  each  neuromere  just  above 
the  medulla.  (Fig.  2294~5.)  It  is  probable  that  the  cells  filling  the  interspaces 
arose  from  a  rapid  proliferation  of  the  ends  of  the  ganglionic  portion  of  the 


334 


THE    MIDDLE    CORD,    THE    LEMMATOCHORD   AND    THE    NOTOCHORD. 


middle  cord,  as  well  as  from  the  division  of  the  scattering  cells  seen  in  these 
spaces  at  an  earlier  period.  The  same  kind  of  cells  arise  in  those  parts  of  the 
middle  cord  that  remain  united  with  the  ectoderm,  forming  there  masses  of  cells 
continuous  with  those  in  the  underlying  interspaces.  (Fig.  229*,  b.c.)  The 
wedges  are  best  developed  just  back  of  each  free  abdominal  ganglion;  their 
central  cells  become  free  blood  corpuscles,  and  the  walls  form  the  vertical 
vessels  arising  from  the  neural  sinus. 

Sections  through  the  vagus  neuromeres  show  that  the  cells  filling  the  in- 
terganglionic  spaces  are  united  with  those  in  the  spaces  in  front  and  behind 
by  two  cell  cords;  one  is  the  intraganglionic  portion  of  the  middle  cord,  and  lies 
in  the  medulla  between  the  neural  and  haemal  commissures;  the  other  consists 
of  cells  that  have  pushed  their  way  beneath  the  medulla.  (Fig.  229 \  m.ch,  Ic.h.) 


FIG.  230. — Scorpion  embryo,  about  ready  to  hatch.  No.  i,  Section  through  the  space  between  the  fifth  and 
sixth  thoracic  neuromeres;  No.  2,  section  through  space  between  sixth  and  seventh  thoracic  neuromeres;  No.  3, 
section  through  posterior  portion  of  the  second  vagus  neuromere;  No.  4,  section  through  the  first  free  abdominal 
neuromere;  No.  5,  section  between  the  second  and  third  free  abdominal  neuromeres;  No.  6,  section  through  the 
posterior  margin  of  the  third  vagus  neuromere;  No.  7,  section  just  back  of  the  third  free  abdominal  neuromere; 
No.  8,  section  between  the  second  and  third  thoracic  neuromeres,  showing  the  remnants  of  the  anterior  end  of  the 
middle  cord. 

Soon  after  stage  H,  the  cell  cord  that  filled  the  interganglionic  spaces,  and 
that  extended  beneath  the  medulla  of  each  ganglion,  is  replaced  by  a  thin-walled 
tube,  the  neural  sinus. 


III.  MIDDLE  CORD  OF  LIMULUS. 

It  is  difficult  to  follow  the  middle  cord  in  Limulus.  In  the  adult  it  lies  inside 
the  tough  outer  sheath  of  the  nerve  cord,  and  consists  of  irregular  masses  of  matted 
tissues  resembling  that  in  the  bothroidal  cord  of  the  scorpion.  (Figs.  55,  67,  68.) 
It  is  arranged  in  two  main  lateral  cords,  one  on  either  side  of  the  neuromere,  l.l.ch. 
In  the  vagus  region,  a  conspicuous  median  mass  is  present.  (Fig.  55,  m.lch.) 
The  lemmatochord  tissue  is  continuous,  by  means  of  fine  fibers,  with  the  neuroglia 
that  everywhere  permeates  the  nerve  cord  and  forms  an  envelop  for  the  ganglion 
cells  and  the  bundles  of  nerve  fibers. 


SUMMARY. 

The  condition  in  Limulus  may  be  compared  with  the  early  larval  stages  in 
the  development  of  the  lemmatochord  in  Cecropia,  where  the  lemmatochord  tissue 
completely  surrounds  the  cord. 

IV.  SUMMARY  AND    COMPARISON. 

1.  The  middle  cord  of  arthropods  is  of  the  same  fundamental  importance 
in  the  morphology  of  segmented  animals  as  the  lateral  nerve  cord. 

2.  In  the  arthropods  we  may  recognize  two  parts  in  the  middle  cord,  viz. 
the   intra-  and   interganglionic   segments.     The    intraganglionic   segments  are 
located  in  the  central  portion  of  the  neuromeres,  on  the  neural  side  of  the  haemal 
cross  commissures.     They  may  give  rise  in  each  neuromere  to  both  ganglion 
cells  and  neuroglia  cells,  and  they  may  persist  a  longer  or  shorter  period  as  solid 
cords,  or  as  epithelial  canals  representing  the  walls  of  the  original  median  groove. 


be.-" 


FIG.  231. — Diagrams  to  illustrate  the  mode  of  growth  of  the  axial  organs  in  the  arachnids;  such  as  the  lateral 
nerve  cords,  their  infolding,  the  overgrowth  of  the  ectoderm,  and  the  formation  of  the  cross  commissures;  the  local 
modifications  of  the  middle  chord  to  form  the  neuroglia,  the  lining  of  the  central  canal,  blood  cells,  and  lemmato- 
chord; and  the  origin  of  nerve  cords,  ectoderm,  and  germ  cells  from  the  primitive  streak.  A,  Sagittal  section  of 
the  axial  oigans;  B,  C,  and  D,  cross-sections  in  front  of  the  apex  of  the  caudal  lobe,  showing  the  separation  of  the 
nerve  cords  and  germ  cells  from  the  tissue  of  the  primitive  streak;  E,  section  farther  forward,  through  the  middle 
of  a  neuromere;  F,  between  two  neuromeres;  G,  still  farther  forward,  through  the  middle  of  a  neuromere;  H.  same, 
between  two  neuromeres. 

The  interganglionic  segments  of  the  middle  cord  gave  rise  primarily  to 
the  median  nerve  and  its  neurilemma.  They  undergo  various  local  modifications. 
In  the  insects  (Acilius),  the  oral  segments  disappear;  those  in  the  'eg  segments  may 
produce  the  chiten-lined  supports,  or  furcae,  which  are  permanently  connected 
with  the  ectoderm,  and  which  serve  for  the  attachment  of  muscles;  in  the  abdom- 
inal region,  the  same  kind  of  infoldings  are  formed,  but  they  separate  from  the 
ectoderm  at  an  early  period  and  are  converted  bodily  into  the  median  nerve. 
In  lepidoptera,  during  the  metamorphosis,  the  neurilemmas  of  the  median  and 
of  the  lateral  nerve  cords  become  enormously  enlarged  and  form  a  semi-cylin- 


336  THE    MIDDLE    CORD,    THE    LEMMATOCHORD    AND    THE    NOTOCHORD. 

drical,  unsegmented  cord,  lying  on  the  haemal  side  of  the  nervous  system,  and 
enclosing  the  remnants  of  the  median  nerve. 

3.  In  the  scorpion,  the  interganglionic  pits  of  the  median  groove,  in  the 
abdominal  region,  give  rise  to  groups  of  cells,  some  of  which  become  free  blood 
corpuscles,  others  neuroglia  cells,  and  others  form  the  walls  of  separate  sinuses, 
comparable  with  the  neurilemma  of  a  segment  of  a  median  nerve.     The  inter- 
ganglionic spaces  finally  extend  forward   and   backward  on  the  haemal  side  of 
the  neuromeres,  forming  a  continuous  neural  sinus. 

4.  The  bothroidal  cord,  or  lemmatochord,  appears  to  develop  from  the  primi- 
tive streak.     Later  it  unites  with  the  wall  of  the  neural  sinus.     It  probably 
represents  the  enlarged  neurilemma  of  the  lateral  and  median  cords,  and  to- 
gether with  the  neural  sinus,  corresponds  approximately  to  the  lemmatochord 
of  lepidoptera.     The  merochord  represents  a  local  enlargement  of  the  lemmato- 
chord in  the  sixth  thoracic  segment. 

5.  With  the  crowding  together  of  neuromeres,  the  interganglionic  and  in- 
traganglionic  segments  of  the  middle  cord  tend  to  unite  and  to  form  two  con- 
tinuous cords,  or  canals,  which  stand  on  different  levels,  and  which  have  different 
functions.     One  forms  an  internal  cord,  or  canal,  which  lies  in  the  central  portion 
of  the  neuromeres  between  the  neural  and  haemal  cross  commissures;  it  gives  rise 
to  neuroglia  cells,  nerve  cells,  and  the  epithelium  of  the  canal.     The  other  forms 
an  external  cord,  strengthened  by  a  heavy  investment  of  neurilemma,  and  serves 
for  the  attachment  of  muscles.     Both  cords  may  contain  a  longitudinal  canal, 
and  each  canal  may  open  into  the  other  through  segmentally  arranged  openings 
between  the  cross  commissures  of  successive  neuromeres.     These  conditions  are 
represented  diagrammatically  in  Fig.  231. 

6.  These  two  cords  are  represented  in  vertebrates  by  the  epithelium  of 
the  canalis  centralis,  with  its  adjacent  neuroglia  tissue,  and  by  the  notochord. 
The  embryonic  origin  of  both  cords,  in  vertebrates  as  in  arthropods,  may  be 
traced  to  the  growing  apex  of  the  embryo,  and  the  method  of  growth  of  these  organs 
appears  to  be  essentially  the  same  in  both  classes. 

Whether  the  median  groove  is  differentiated  into  its  component  parts  grad- 
ually, so  that  all  the  intermediate  stages  are  seen  at  the  same  time,  arranged 
in  superficial  linear  order  from  one  end  of  the  embryo  to  the  other,  as  in  the 
scorpion;  or  whether  the  differentiation  takes  place  rapidly,  at  some  deep-lying 
point  in  the  primitive  streak,  as  for  example  in  Amphioxus,  is  merely  an  ontogenetic 
variation  due  to  the  amount  of  detail  in  the  process  of  recapitulation  and  to  the 
relative  time  at  which  the  details  appear. 

In  the  vertebrates,  the  early  stages  in  the  differentiation  of  the  middle  cord 
are  passed  through  rapidly,  forcing  the  anterior  end  of  the  notochord  below 
the  surface  as  fast  as  it  is  formed  at  the  primitive  streak.  Its  primitive  relation 
to  the  tissues  lining  the  floor  of  the  neural  canal  is  indicated  by  the  temporary 
communications  that  obtain  between  the  cavity  of  the  notochord  and  that  of  the 
canalis  centralis  at  the  posterior  end  of  vertebrate  embryos. 


CHAPTER  XIX. 
THE  OSTRACODERMS  AND  THE  MARINE  ARACHNIDS. 

In  the  preceding  chapters,  we  have  shown  that  there  is  essential  agreement 
in  the  structure  and  mode  of  growth  of  the  corresponding  systems  of  organs  in 
the  vertebrates  and  arthropods.  This  agreement  is  so  intricate  and  all-pervading 
that  it  is  intelligible  only  on  the  assumption  that  the  arthropods  represent  the 
ancient  stock  from  which  the  vertebrates  arose. 

But  in  spite  of  this  underlying  agreement  in  structure,  there  is  a  wide  dif- 
ference in  outward  appearance  between  any  living  arthropod  and  any  living 
vertebrate.  To  demonstrate  a  direct  genetic  relationship  between  them,  we  must 
fill  this  apparent  gap  with  real  animals  that  are  intermediate  in  character,  or 
account  in  some  other  way  for  the  abrupt  transition. 

In  either  case,  we  must  determine  what  are  the  highest  arthropods  and  what 
are  the  lowest  vertebrates,  and  when  and  how  the  transition  from  one  to  the  other 
took  place.  Unfortunately  there  is  no  a  priori  way  of  deciding,  in  a  phylogenetic 
sense,  what  is  "high"  and  what  is  "low,"  until  we  have  found  out  what  are  the 
main  lines  of  progressive  evolution,  and  the  directions  in  which  they  lead.  In  an 
inquiry  of  this  nature  there  is  only  one  method  that  can  be  used,  namely  the 
picture-puzzle  method,  whereby  we  aim  through  repeated  trials  to  fit  all  the  facts 
into  a  complete  and  intelligible  picture,  knowing  full  well  that  there  is  but  one  way 
for  them  to  fit,  and  that  when  they  do,  we  shall  have  an  accurate  picture  of  the 
truth. 

If  on  consulting  the  geological  record  we  find  in  the  remote  past  some  period 
toward  which  the  genetic  lines  in  question  converge,  blending  there  with  a  group 
of  animals  having  some  characteristics  of  each,  then  there  will  be  a  very  strong 
presumption  that  we  have  correctly  identified  the  upper  and  lower  ends  of  the 
break  in  the  series;  that  that  class  of  animals  was  the  connecting  link  between 
them;  and  that  the  actual  transition  took  place  at,  or  before,  that  period. 

If  on  further  examination  it  can  be  shown  that  the  hypothetical  connecting 
link  resembles  in  several  different  ways  the  upper  end  of  the  lower  series  and  the 
lower  end  of  the  upper  one,  and  forms  with  them  a  continuous  graded  series,  with 
a  tendency  on  one  side  of  the  connecting  link  to  produce  special  structures,  or 
special  methods  of  growth  that  are  either  anticipated  or  find  fuller  expression  on 
the  other,  then  our  previous  assumption  of  genetic  relationship  attains  thereby 
the  rank  of  actual  demonstration,  and  it  will  not  be  shaken  by  any  amount  of 
negative  evidence,  or  by  the  threatened  collapse  of  cherished  convictions  that  the 
picture  was  going  to  be  something  very  different  from  what  it  actually  turns  out 
to  be. 

22  337 


338  THE    OSTRACODERMS   AND    THE    MARINE   ARACHNIDS. 

Such  is  the  nature  of  the  problem  we  have  before  us  in  the  present  chapter, 
and  such  is  the  method  we  have  used  in  seeking  an  answer  to  it.  Fortunately  we 
are  dealing  with  animals  whose  size,  mode  of  life,  and  abundant  skeletal  struc- 
tures are  highly  favorable  to  their  preservation  as  fossils,  and  paleontology  should, 
and  in  my  judgment  does,  give  us  the  materials  with  which  this  part  of  our  prob- 
lem may  be  solved. 

We  may  state  at  once  that  the  conclusion  we  have  reached  is  as  follows: 
The  giant  sea  scorpions,  or  merostomata,  as  shown  by  their  living  representatives, 
Limulus  and  other  arachnids,  may  be  regarded  in  a  phylogenetic  sense  as  the 
highest  arthropods,  not  because  they  now  are  the  most  highly  organized,  or  the 
most  specialized,  or  the  most  efficient,  because  they  are  not  entitled  to  any  of  these 
distinctions,  but  because  of  all  the  invertebrates  of  their  time  they  had  made  the 
greatest  progress  in  the  attainment  of  that  particular  plan  of  structure  that  was 
later  to  be  so  fully  elaborated  in  the  vertebrates.  They  were  already  in  existence 
in  very  remote  pre-cambrian,  or  proterozoic  times,  and  had  reached  a  high  stage 
of  development  in  the  lower  Silurian,  when  the  ostracoderms  were  making  their 
first  appearance,  and  long  before  any  true  vertebrates  were  known  to  exist.  The 
newly  arrived  ostracoderms  had  all  the  characteristics  of  an  annectant  class,  for 
as  all  authors  admit,  they  bore  a  superficial  resemblance,  at  least,  in  form  and 
mode  of  life  to  their  arachnid  contemporaries  and  associates,  and  at  the  same 
time  they  unquestionably  resembled  the  true  fishes  that  were  soon  to  appear  on 
the  geological  horizon.  Paleontology,  therefore,  points  very  clearly  toward  the 
marine  arachnids  as  the  historic  predecessors  of  the  ostracoderms,  and  to  the 
ostracoderms  as  the  probable  connecting  link  between  them  and  the  first  true 
vertebrates,  such  as  the  early  dipnoi  and  the  crossopterygians. 

I.  THE  MARINE  ARACHNIDS  AND  THEIR  ORIGIN. 

The  most  primitive  arthropods  were  undoubtedly  marine  animals  of  the 
short-bodied  phyllopod  type,  consisting  of  a  relatively  large  forehead,  or  pro- 
cephalon,  and  a  small  body  composed  of  a  small  number  of  ill  defined  metameres. 

We  have  seen  that  the  foundation  of  segmented  animals  is  the  primitive  head, 
and  that  the  primitive  head  represents  the  body  of  their  remote  ccelenterate  an- 
cestors. The  ccelenterate,  or  radiate,  stage  is  probably  indicated,  more  or  less 
clearly,  in  the  embryonic  or  larval  stages  of  all  ccelenterate  derivatives.  In  the 
annelids  and  molluscs,  it  is  seen  in  the  trochosphere  larvae.  In  the  arthropods 
and  vertebrates,  the  corresponding  stages  are  heavily  provisioned  with  yolk,  and 
a  free  larval  form  for  this  stage  does  not  exist;  but  remnants  of  it  may  be  recog- 
nized in  the  procephalic  lobes,  with  the  included  gastrula  and  stomodaeum. 

From  the  trochozoa,  the  hypothetical,  phylogenetic  antecedents  of  these 
larval  stages,  evolution  proceeds  along  two,  possibly  three,  main  lines.  In  the 
molluscan  phylum,  the  main  theme  consists  in  variations  and  specializations 
arising  in  the  primitive  head,  and  in  the  incipient,  but  still  unsegmented 
trunk.  In  the  annelids  and  arthropods  the  most  important  variations  and 
the  most  significant  new  characters  appear  in  the  growing  trunk,  while 


THE    MARINE   ARACHNIDS   AND    THEIR    ORIGIN.  339 

the  primitive  head  from  which  it  arose,  gradually  dwindles  into  structural 
insignificance. 

The  subdivision  of  the  trunk  into  a  linear  series  of  like  parts  or  metameres 
was  coincident  with  its  elongation  and  increase  in  size,  and  there  was  probably 
little  difference  between  annelids  and  arthropods  during  the  earliest  stages  of 
this  process.  But  in  the  former,  the  production  of  new  metameres  by  apical 
growth  frequently  persists  for  an  indefinite  post-embryonic  period,  and  special 
groups  of  metameres  may  separate  from  the  parent  stock  by  transverse  fission, 
giving  rise  to  a  succession  of  new  individuals. 

In  the  primitive  arthropods,  the  increase  in  the  size  of  the  body  and  in 
the  number  of  metameres  proceeded  very  slowly.  In  the  primitive  arachnids, 
crustaceans,  and  insects,  the  number  of  metameres  produced  was  small,  and 
precisely  limited;  in  no  case  was  there  a  persistent  production  of  new  metameres, 
either  following  normal,  fission  or  otherwise. 

The  production  of  new  metameres  did  not  take  place  at  a  uniform  rate,  but 
in  a  spasmodic,  or  interrupted,  manner.  In  the  arthropods  each  new  generation 
of  metameres  consisted  of  a  relatively  small  number,  frequently  in  threes  or  sixes, 
followed,  after  a  recognizable  pause,  by  a  new  generation,  and  so  on.  Thus  the 
primitive  arthropod  trunk  consists  of  a  small  number  of  metameres  divided  into 
groups,  each  group  sharply  distinguished  by  the  size  and  the  degree  of  special- 
ization of  its  organs  from  the  group  in  front  or  behind.  The  general  appearance 
is  that  of  an  annelid  undergoing  transverse  fission,  and  consisting  of  a  chain  of 
incompletely  separated  individuals. 

Various  manifestations  of  this  condition  are  seen  in  the  nauplius,  meta- 
nauplius,  and  other  larval  stages;  in  the  successive  addition  of  metameres  in 
larval  trilobites,  and  in  the  persistent  subdivisions  of  the  body  of  many  other 
arthropods  into  tagmata,  or  groups  of  like  metameres,  such  as  the  oral,  thoracic, 
vagal,  abdominal  and  caudal.  In  these  cases  each  subdivision  is  produced  more 
or  less  clearly  by  a  spasmodic  generation  of  metameres,  each  group  arising  behind 
the  one  previously  produced. 

This  phenomenon  is  not  confined  to  the  arthropods;  it  is  still  recognizable 
in  the  subdivision  of  the  vertebrate  head,  and  in  the  successive  generations  of 
nephric  tubules  and  other  organs  in  the  post-cephalic  region.  One  of  the  most 
important  events  in  the  evolution  of  the  ostracoderms  was  the  addition,  probably 
during  the  Ordovician  period,  of  a  new  generation  of  caudal  metameres  to  the 
twenty  odd  that  constituted  the  sum  total  of  their  inheritance  from  the  arachnids. 

The  main  difference  between  annelids  and  arthropods,  besides  the  method 
of  apical  growth,  lies  in  the  extraordinary  development  of  the  chitenous  mantle  or 
exoskeleton  of  the  latter,  the  presence  of  which  no  doubt  exercised  profound  in- 
fluence over  the  whole  course  of  their  evolution.  Primitive  arthropods,  on  the 
other  hand,  appear  to  resemble  the  molluscan  type  in  their  restricted  apical 
growth  and  in  the  presence  of  the  membranous  folds  arising  from  the  aboral 
region  of  the  head,  and  which  give  rise  in  a  suggestively  persistent  manner,  either 


340  THE    OSTRACODERMS   AND    THE    MARINE   ARACHNIDS. 

to  the  shell  gland,  or  mantle,  or  to  embryonic  membranes,  or  branchiocephalic 
folds. 

The  earlier  stages  in  the  evolution  of  arthropods  can  only  be  inferred,  as 
indicated  above,  from  the  records  of  comparative  anatomy  and  embryology. 
The  original  phylogenetic  records  are  lost  beyond  recovery,  for  in  the  oldest 
rocks  in  which  any  formal  organic  remains  aie  found,  the  eurypterid  type  appears 
to  be  already  present.  These  fragmentary  remains,  Beltina  danai,  consisting 
of  tracks,  outlines  of  heads  and  appendages,  were  found  by  Walcott1  nine 
thousand  feet  below  the  uncomformity  between  the  Proterozoic  (Algonkian)  and 
the  Cambrian. 

During  the  long  subsequent  period,  including  the  Cambrian  and  Ordovician, 
the  familiar  types  of  marine  arthropods,  such  as  the  short-bodied  bivalve  ostra- 
codes  and  the  shield-covered  phyllopods  and  phyllocarids,  make  their  appearance 
in  increasing  numbers.  Trilobites  and  merostomes  are  likewise  found  in  great 
abundance  and  variety,  the  former  reaching  their  climax  in  the  Ordovician,  the 
latter  in  the  Silurian  and  early  Devonian. 

During  this  long  period,  organic  evolution  proceeded  very  slowly  and  there 
are  no  indications  that  any  single  event  took  place  of  exceptional  importance 
morphologically.  Nevertheless  important  progress  was  made  in  the  arthropods 
in  those  complex  processes  of  local  suppression,  union,  and  enlargement  of  mul- 
tiple organs,  that  are  such  essential  features  of  all  progressive  organizations,  and 
which  alone  could  make  a  more  active,  varied,  and  efficient  mode  of  life  in  seg- 
mental  animals  a  possibility.  This  kind  of  reorganization  tends  to  convert  the 
linear  succession  of  like  metameres,  each  complete  in  itself  and  independent  of 
the  others,  into  a  linear  succession  of  unlike  organs,  each  subordinate  to  all  the 
others  and  all  together  forming  a  new  organism  of  a  much  higher  order.  See 
Chapters  I  and  II. 

While  the  most  important  event  of  the  Proterozoic  period  was  no  doubt  the 
outgrowth  from  the  radially  symmetrical  ccelenterate  of  a  new  bilaterally  symmet- 
rical trunk,  and  the  perfecting  in  it  of  a  high  degree  of  metamerism,  in  the  Cam- 
brian and  Ordovician  the  important  events  were  the  breaking  down  of  this  meta- 
merism, especially  in  the  older  and  more  anterior  metameres,  and  the  successive 
merging  of  its  various  parts  into  a  more  efficient  aggregate,  the  cephalo-thorax, 
that  at  a  much  later  period  was  to  become  an  important  part  of  the  complex 
vertebrate  "head." 

The  trilobites,  judging  from  strategraphical  evidence  and  from  their  ex- 
ternal form,  remained  what  they  had  been  almost  from  the  outset,  slow-moving, 
sea-bottom  feeders,  like  Limulus  to-day,  making  only  occasional,  apparently  aim- 
less excursions,  as  free  swimmers,  into  the  water  above. 

But  most  of  the  merostomes  in  the  manner  above  indicated  had  acquired, 
at  a  very  early  period,  a  superior  organization  in  the  head  region  that  enabled 
them  to  leave  the  bottom  and  swim  at  large  with  purpose  and  effect.  So  far  as 

1  Bull.  Geol.  Soc.  Am.,  Vol.  X. 


THE    MARINE   ARACHNIDS   AND    THEIR    ORIGIN.  341 

we  know  they  were  the  first  arthropods,  or  for  that  matter  the  very  first  animals 
of  any  kind,  endowed  with  the  form  and  mechanical  structure,  with  the  sensory 
and  neuro-muscular  system,  adequate  to  perceive  at  a  distance,  to  pursue,  and  to 
capture  living  prey  with  measurable  vigor  and  skill,  and  they  finally  became 
the  most  rapacious  and  effective  organisms  of  their  time. 

But  the  very  events  that  were  necessary  preliminaries  to  their  active  life 
were  in  the  end  the  cause  of  their  decline.  The  reduction  of  the  oral  appendages, 
the  fusion  of  thoracic  metameres,  the  enlarged  and  condensed  thoracic  neuromeres, 
gradually  led,  in  the  manner  fully  explained  under  their  appropriate  headings, 
to  the  closing  of  the  mouth,  the  inclusion  of  the  lateral  eyes  in  the  neural  tube, 
and  to  other  important  changes  that  constitute  the  most  complete  metamorphosis 
in  the  history  of  organic  evolution. 

The  ostracoderms,  and  their  allies,  were  the  products  of  this  metamorphosis, 
and  formed  the  most  characteristic  animals  of  the  upper  Silurian.  Not  till 
toward  the  beginning  of  the  Devonian  had  they  become  sufficiently  readjusted 
to  their  new  conditions  to  again  form  active  and  dominant  organisms,  or  to  give 
rise  to  the  true  vertebrates. 

All  the  available  evidence  points  to  the  conclusion  that  the  merostomes 
gave  rise  to  the  ostracoderms  in  the  Ordovician  or  early  Silurian  period. 
The  constant  association  of  merostomes  and  ostracoderms  in  the  Silurian  shows 
that  they  lived  together  and  were  preserved  under  similar  conditions;  moreover 
the  similarity  in  structure  and  in  general  appearance  between  these  two  types; 
the  sudden  appearance  of  the  ostracoderms  in  the  Silurian  and  their  absence  in  the' 
preceding  periods  under  conditions  that  are  known  to  be  favorable  to  their  pre- 
servation, as  shown  by  the  well  preserved  remains  of  their  thinner  skinned  asso- 
ciates of  the  Silurian;  the  culmination  of  the  merostomes  at  about  this  period, 
and  their  subsequent  decline  and  extinction;  and  finally  the  absence  of  any 
positive  evidence  to  the  contrary,  admits  of  no  other  conclusion. 

II.  THE  OSTRACODERMS. 

The  ostracoderms  are  so  unlike  any  known  vertebrates  or  arthropods  that 
very  different  opinions  have  been  expressed,  and  are  still  held,  as  to  their  structure 
and  relations.  It  is  a  remarkable  fact  that  in  spite  of  their  great  antiquity  and 
simple  structure,  the  liveliest  discussions  concerning  them  were  on  the  question : 
Are  they  vertebrates  or  arthropods  ?  When  the  uncompromising  verdict  of  Hux- 
ley, and  later  of  Lankester,  was  delivered  in  favor  of  the  vertebrates,  indeed  defi- 
nitely locating  them  in  a  highly  specialized  group  of  comparatively  modern 
teleosts,  that  verdict  was  generally  accepted  as  final,  and  the  morphological 
significance  that  clearly  belongs  to  them  was  nullified  or  ignored. 

Apparently  no  one  seriously  considered  the  possibility  that  the  ostracoderms 
might  be  very  primitive  vertebrates  that  would  shed  a  greatly  needed  light  on 
the  character  of  their  remote  ancestors,  or  that  they  might  be  annectant  forms, 
standing  midway  between  the  arthropods  on  one  side,  and  the  true  vertebrates  on 
the  other. 


342  THE    OSTRACODERMS   AND    THE    MARINE   ARACHNIDS. 

No  doubt  this  was  due,  in  part,  to  the  remarkable  development  of  their  der- 
mal skeleton,  because,  for  a  long  time,  it  had  been  very  generally  assumed  that  an 
animal  with  a  continuous  dermal  armor  could  not  be  a  primitive  vertebrate,  for 
in  the  elasmobranchs,  which  were  supposed  to  be  the  most  primitive,  the  body 
was  covered  with  minute,  isolated,  dermal  ossicles.  It  was  also  implicitly  be- 
lieved at  that  time,  as  it  is  to-day,  with  a  conviction  born  of  constant  repetition, 
that  the  immediate  ancestors  of  the  vertebrates  were  animals  like  either  Amphi- 
oxus,  Balanoglossus,  the  tunicates,  or  the  annelids,  which  had  no  dermal  skeleton 
whatever. 

The  author  (Patten,  1889)  was  the  first  one  to  claim  for  the  ostracoderms  an 
important  place  in  the  phylogeny  of  the  vertebrates,  basing  this  claim  in  part 
on  the  very  characters  which  were  regarded  by  others  as  evidence  of  their  high 
degree  of  specialization. 

At  that  time  our  knowledge  of  the  ostracoderms  was  very  imperfect  and  the 
anatomical  foundation  for  any  inference  in  regard  to  them  was  exceedingly  in- 
secure. Since  then,  our  knowledge  has  greatly  increased  and  we  now  possess  in 
Bothriolepis,  one  of  the  higher  representatives  of  the  group,  an  unprecedented 
wealth  of  material,  which  in  spite  of  its  great  age  is  in  an  ideal  state  of  preserva- 
tion. Indeed,  I  know  of  no  other  extinct  animal  that  has  been  so  abundantly  and 
perfectly  preserved  in  its  original  form,  attitudes,  and  surroundings. 

This  new  material  for  the  first  time  enables  us  to  identify  with  certainty  the 
neural  and  haemal  surface  of  an  ostracoderm;  it  furnishes  us  the  first  precise  in- 
formation concerning  the  nature  and  location  of  the  sense  organs,  the  jaws,  mouth, 
gills,  and  other  viscera,  and  as  to  their  mode  of  life;  and  for  the  first  time  it  affords 
a  secure  basis  of  fact  for  the  interpretation  of  other  representatives  of  the  group 
that  are  not  so  well  preserved.  In  the  light  of  this  evidence,  we  may  now  confidently 
affirm  that  the  ostracoderms  belong  neither  to  the  arthropods  nor  to  the  vertebrates, 
but  constitute  a  new  class  standing  midway  between  them,  the  ancestors  of  the 
one  and  the  descendants  of  the  other,  the  long  sought  missing  link  between  the 
vertebrates  and  the  invertebrates. 

From  the  geological  record  we  may  conclude  that  the  true  vertebrates  arose 
from  the  ostracoderms  not  later  than  the  Silurian;  the  ostracoderms  from  the 
marine  arachnids  not  later  than  the  Ordovician;  while  the  marine  arachnids  had 
their  origin  in  the  immense,  unfathomable  periods  during,  or  preceding,  the 
Proterozoic. 


Historical  Review. 

Before  proceeding  to  a  fuller  description  of  the  ostracoderms,  we  may  to 
advantage  review  the  earlier  literature  on  their  structure  and  systematic  posi- 
tion, for  it  brings  out  the  most  striking  features  of  such  important  genera  as 
Pterichthys,  Pteraspis,  and  Cephalaspis. 


HISTORICAL   REVIEW.  343 

Hugh  Miller,  the  discoverer  of  Pterichthys,  says  (Old  Red  Sandstone,  p.  50),  in 
comparing  a  trilobite  with  Cephalaspis,  "  The  fish  and  the  crustacean  are  won- 
derfully alike."  "They  exhibit  the  points  at  which  the  plated  fish  is  linked  to 
the  shelled  crustacean."  Agassiz  was  at  first  in  doubt  as  to  whether  Pterichthys 
was  a  fish  or  a  crustacean. 

Sir  Roderick  Murchison,  when  first  shown  specimens  of  Pterichthys  wrote 
regarding  them  that,  "If  not  fishes,  they  more  clearly  approach  to  crustaceans 
than  to  any  other  class."  Again,  "They  (Cephalaspis  and  Pterichthys)  form  the 
connecting  links  between  crustaceans  and  fishes." 

InSiluria  (London,  1854,  p.  252),  speaking  of  Cephalaspis  agassizii,  he  says: 
"This  fish  with  its  large  buckler-shaped  head  and  its  thin  body,  jointed  somewhat 
like  a  lobster,  is  perhaps  the  most  remarkable  example  of  a  fish  of  apparently  so 
intermediate  a  character  that  the  detached  portions  of  its  head  when  first  found 
were  supposed  to  belong  to  Crustacea."  In  a  footnote  Murchison  adds:  "Mr. 
Miller  has  requested  his  readers  to  compare  the  head  of  Asaphus  (now  Phacops) 
caudatus,  a  well-known  silurian  trilobite,  with  that  of  C.  lyellii,  to  illustrate  how 
the  two  orders  of  crustaceans  and  fishes  seem  here  to  meet — in  the  view  of  persons 
who  have  not  mastered  the  subject." 

Eichwald  says  (1854,  page  105):  "It  is  very  remarkable  that  this  colossal 
crab  (Pterygotus)  formerly  regarded  by  L.  Agassiz  as  a  fish,  occurs  in  the 
dolomitic  chalk  of  Rootzikiill  in  Oesel,  together  with  another  genus,  Thyestes, 
standing  between  crabs  and  fishes  and  resembling  Bunodes  and  Cephalaspis." 

The  genus  Pteraspis  was  first  proposed  by  Rudolph  Kner,  in  1847,  to  include 
the  forms  described  in  1835  by  Agassiz  as  Cephalaspis  lewisii,  and  C.  lloydii. 
Their  appearance  was  so  unlike  the  ordinary  fish  remains  that  for  a  long  time 
Kner  did  not  suspect  that  they  had  been  already  described  by  Agassiz  in  his 
Poissons  Fossils.  From  a  study  of  their  minute  structure  Kner  believed  them  to 
be  the  internal  shells  of  cephalopods  allied  to  Sepia. 

In  1856,  F.  Roemer  described  a  form  closely  related  to  C.  lloydii  as  Palaeoteu- 
this,  and  referred  it  to  the  sepiidae,  but  suggested  that  the  forms  described  by 
Kner  were  crustaceans  related  to  Dithyrocaris  or  Pterygotus. 

In  1855,  R.  W.  Banks  in  his  paper  on  the  Downton  Sandstones,  after  com- 
menting on  the  association  in  these  beds  of  Lingula  cornea,  Pterygotus  and  Pter- 
aspis (Cyathaspis),  made  the  following  observation,  page  98,  "On  the  underside 
of  the  sharp  projections  before  referred  to  (on  either  side  of  the  rounded  snout) 
are  protuberances  which  seem  to  be  projecting  horny  eyes  similar  to  those  of 
crustaceans." 

He  remarks  further  on,  that  doubtful  as  it  is  whether  the  buckler-like  fossil 
remains  above  referred  to  belong  to  fishes  or  to  crustaceans,  it  is  certain  that  they 
are  closely  allied  to  Cephalaspis  lloydii  and  C.  lewisii.  In  a  final  note,  it  is  an- 
nounced that  Professor  Huxley  is  now  minutely  examining  their  structure  to 
determine  their  true  relationship  either  to  the  crustaceans  or  to  the  fishes.  When 
Huxley's  paper  appeared,  although  he  gave  a  very  good  description  of  the  minute 


344  THE    OSTRACODERMS   AND    THE    MARINE   ARACHNIDS. 

structure  of  the  shell  of  these  animals,  and  concluded  that  they  were  not  crusta- 
ceans, he  entirely  ignored  the  existence  of  the  eye  tubercles,  although  their  pres- 
ence afforded  very  weighty  evidence  against  his  conclusion. 

Huxley  (1858,  page  277)  in  reply  to  Agassiz,  who  had  remarked  on  the 
singular  resemblance  between  the  shell  of  C.  lloydii  and  that  of  crustaceans,  and 
to  Roemer's  and  Kunth's  opinion  that  Pteraspis  was  a  crustacean,  seems  to  have 
closed  the  discussion  for  the  time  with  his  oft-quoted  statement  that  "No  one  can, 
I  think,  hesitate  in  placing  Pteraspis  among  fishes.  So  far  from  its  structure 
having  'no  parallel  among  fishes,'  it  has  absolutely  no  parallel  in  any  other  divi- 
sion of  the  animal  kingdom.  I  have  never  seen  any  molluscan  or  crustacean 
structure  with  which  it  could  be  for  a  moment  confounded." 

Roemer  accepts  these  statements  apparently  because  they  came  from  Huxley, 
although  he  does  not  make  an  unconditional  surrender  of  his  opinion,  for  he  says 
"Allerdings  manche  Analogic  der  aiisseren  Form  mit  Crustacean-Formen  dar 
bieten  wurde." 

In  1864,  Lankester  divided  the  pteraspidae  into  the  three  genera,  Pteraspis, 
Cyathaspis  and  Scaphaspis.  But  in  1872,  Kunth  described  a  shield  of  Cyathaspis, 
below  which  he  found  one  belonging  to  Lankester's  genus  Scaphaspis,  and  he 
rightly  concluded  that  the  two  shields  belonged  to  the  same  animal.  He  main- 
tained that  the  lower  shield  bore  the  same  relation  to  the  upper  one  that  the  tail 
plate  of  a  rolled  up  trilobite  does  to  its  head  shield,  and  that  between  the  two  were 
a  number  of  pieces  comparable  with  the  segmental  trunk  plates  of  a  trilobite. 
Other  plates  were  present  which  Kunth  regarded  as  locomotor  organs,  or  foot- 
jaws.  From  the  above  facts  Kunth  concluded  that  these  remains  were  not  those 
of  a  fish,  but  of  an  arthropod.  In  referring  to  Huxley's  statement  that  there  is  no 
molluscan  or  crustacean  structure  with  which  such  remains  could  be  for  a  moment 
confounded,  and  to  Kner's  belief  that  Scaphaspis  was  the  shell  of  Sepia  officinalis, 
Kunth  adds  "  so  schienen  mir  diese  Ansichten  in  verein  mit  unserem  vorliegenden 
Stiicks  mir  zu  beweisen  dass  wir  es  mit  einer  Crustacean  Abtheilung  von  ganz 
eigenthiimlicher  Schalstructur  zu  thun  haben.  Denn  jedenfalls  giebt  es  weder 
einen  Fisch  noch  eine  Sepien  Schulpe,  die  eine  ahnliche  Structur  wie  die  Schilder 
zeigte;  wohl  aber  ist  die  Organization  des  ganzen  Stiickes  beweisend  fur  Crus- 
taceen  Character"  (page  6). 

Both  Schmidt  (1873,  Pa&e  33°)  and  von  Alth  (page  47)  agree  with  Kunth 
that  Scaphaspis  is  the  ventral  shield  of  Pteraspis,  but  they  deny  that  any  of  the 
remains  described  as  Pteraspis,  Cyathaspis  or  Scaphaspis  are  crustaceans, 
although  no  valid  reasons  are  given  for  doing  so. 

Lankester  (1868,  page  26)  admitted  the  presence  in  Cyathaspis  of  tubercles 
corresponding  with  similar  tubercles  in  Pteraspis,  which  are  "produced  by  the 
supposed  orbits;"  but  how  a  vertebrate  eye,  or  an  "orbit,"  could  be  preserved  as 
a  beautifully  rounded  protuberance  when  all  the  other  soft  parts  are  completely 
destroyed,  is  not  discussed. 

Lankester  attached  much  importance  to  the  presence  of  scales  on  the  anterior 


HISTORICAL    REVIEW. 


345 


trunk  region  of  Pteraspis,  for  these  scale-like  structures  were  regarded  as  conclu- 
sive proof  that  the  pteraspidae  belong  to  the  vertebrates.  He  says  (1868,  page 
1 8)  "All  that  is  known  as  regards  the  scales  of  these  fishes  is  from  a  single 
specimen  found  in  the  cornstones  of  Herefordshire."  This  specimen,  he  says 
elsewhere  (1873,  Page  I9I)  "Shows  seven  rows  of  rhomboidal  scales  attached 
(not  merely  adjacent  to)  to  a  portion  of  the  head  shield  of  Pteraspis.  That  these 
are  true  scales,  or  lozenges  of  sculptured  calcareous  matter  is  absolutely  certain. 
It  is  also  absolutely  certain  that  the  shield  is  pteraspidian  and  that  the  scales  and 
shield  belong  to  the  same  individual  organism.  The  scales  are  fish-like.  I  know 
no  arthropod,  nor  any  other  organism  except  a  fish  which  possesses  any  structure 
even  remotely  representing  them."  "The  shields  of  the  chitonidae  and  cerripedae 
are  the  only  animal  structures,  except  the  scales  of  a  ganoid  fish  (with  which  they 
agree  exactly)  which  they  could  even  vaguely  suggest."  "The  form  of  this 
shield,  and  its  details  as  to  apertures,  processes,  etc.,  agrees  with  the  view  that  it 
belongs  to  a  fish  most  fully.  It  has  not  the  remotest  suggestion  of  crustacean 
affinities  about  it." 

After  commenting  on  the  fact  that  the  fossil  in  question  was  marked  with 
long  parallel  striae,  and  that  the  middle  layer  contained  the  polygonal  cavities,  he 
adds  (1864,  page  195),  "This  structure,  which  has  no  parallel  among  fishes,  or, 
indeed,  any  group  of  the  animal  kingdom,  leaves  no  possibility  of  a  doubt  that 
the  specimen  is  a  fragment  of  Pteraspis."  Lankester  further  maintains  (1868, 
page  4)  that  by  the  discovery  of  these  scales  "  the  piscine  nature  of  these  fossils 
was  definitely  set  at  rest." 

These  positive  statements  are  contradictory  and  are  hardly  warranted  by 
the  facts,  for  the  crustacean  character  of  the  shields  had  been  repeatedly  com- 
mented on  by  competent  observers,  and  in  his  own  monograph  (page  61)  he  has 
described  a  fragment,  possibly  connected  with  Cephalaspis  which  he  names 
Kallostrakon  podura  (Tolypaspi  ?)  "on  account  of  the  resemblance  to  the  well- 
known  microscopic  markings  of  the  scales  of  the  insect  Podura." 

But  all  recent  students  of  the  shell  of  Pteraspis  are  agreed  that  it  is  not  "  ex- 
actly" like  that  of  a  ganoid  fish,  in  fact  its  microscopic  structure  is  altogether  of  a 
different  character,  and  it  is  not  true  that  there  are  no  arthropods  with  structures 
even  remotely  resembling  the  scales  of  Pteraspis,  because  in  Pterygotus  the  entire 
body  is  covered  with  an  ornamentation  astonishingly  like  fish-scales  in  outward 
appearance,  so  much  so  as  to  deceive  such  a  keen  observer  of  fishes  as  Louis 
Agassiz.  Moreover,  in  many  trilobites,  in  the  ceratiocarina,  and  in  arachnids, 
(Phrynus),  the  surface  of  the  shell  is  ornamented  with  ridges  and  grooves  not 
unlike  those  of  Pteraspis  in  external  appearance. 

Probably  neither  Huxley  nor  Lankester  would  have  made  the  above  state- 
ments had  they  kept  Pterygotus  in  mind,  or  had  they  been  acquainted  with  the 
structure  of  the  shield  of  Limulus. 

In  1889,  the  author  compared  the  arrangement  of  plates  and  sense  organs 
in  the  cephalic  buckler  of  Cephalaspis  and  Pterichthys  with  those  on  the  cephalo- 


346  THE    OSTRACODERMS   AND    THE    MARINE   ARACHNIDS. 

thorax  of  certain  trilobites,  and  contended  that  the  ostracoderms  were  the  connect- 
ing links  between  the  arachnids  and  true  vertebrates;  and  in  1894  he  pointed  out 
the  extraordinary  resemblance  in  the  microscopic  structure  of  the  shield  of  Pteras- 
pis  and  other  ostracoderms  and  that  of  Limulus.  These  resemblances  in  the  min- 
ute structure  of  the  shields  were  either  ignored  by  later  writers,  or  regarded  as 
mere  coincidences,  or  as  the  results  of  mimicry  or  of  " parallelism." 

Lankester,  Woodward,  Traquair,  Rohon  and  others,  agree  in  denying  the 
existence  of  arthropod  characters  to  the  pteraspids,  apparently  because  of  the 
abundant  evidence  now  available  that  Pteraspis  is  related  to  Cephalaspis,  whose 
ichthyic  affinities  have  rarely  been  questioned,  rather  than  because  the  arthropod 
features  of  Pteraspis  have  been  dispassionately  considered  and  found  wanting. 

But  within  recent  years  there  seems  to  be  a  growing  tendency  to  doubt  the 
affinity  between  Pteraspis  and  Cephalaspis.  Reis  protests  against  their  union, 
and  apparently  Traquair  is  in  doubt,  treating  them  together  largely  as  a  matter  of 
convenience.  Lankester  in  his  earlier  monograph  states  that  "The  heterostraci 
are  associated  at  present  with  the  osteostraci  because  they  are  found  in  the  same 
beds,  because  they  have,  like  Cephalaspis  a  large  head  shield,  and  because  there  is 
nothing  else  with  which  to  associate  them."  More  recently  he  has  said  (1897) 
"There  is  absolutely  no  reason  for  regarding  Cephalaspis  as  allied  to  Pteraspis 
beyond  that  the  two  genera  occur  in  the  same  rocks,  and  still  less  for  concluding 
that  either  has  any  connection  with  Pterichthys."  Zittel  says,  Vol.  Ill,  page  147, 
"Mir  scheinen  die  Beziehungen  der  Pteraspiden  und  Cephalaspiden  nach  Form 
und  Structur  so  entfernt  dass  beide  besser  als  besondere  Ordnungen  betrachtet 
werden."  He  remarks  further  on  that  while  the  cephalaspidae  certainly  appear  to 
be  ganoids,  the  position  of  the  pteraspidae  is  very  doubtful. 

Muscle  Markings. — In  1872,  A.  Kunth  described  in  Cyathaspis  integer  a 
series  of  six  "flache  Hocher,"  situated  on  the  under  surface  of  the  neural  shield, 
which  he  regarded  as  indications  of  segmentation.  Lankester  (1873),  describes 
similar  impressions  on  the  shield  of  Cyathapis  banksii  and  believes  that  in  both 
cases  they  indicate  the  position  of  a  series  of  branchial  chambers.  In  Pteraspis 
also,  Lankester  has  described  five  narrow  ridges,  with  four  broad  shallow 
depressions  between  them,  which  radiate  from  the  center  of  the  inner  surface 
of  the  neural  shield.  They  are  perhaps  best  marked  in  Pteraspis  crouchii  and 
P.  rostratus.  These  markings  I  have  explained  as  indications  of  the  original 
segmentation  of  the  mesocephalon,  produced  in  part  by  the  attachment  of  strong 
segmental  muscles  extending  vertically  from  the  inner  surface  of  the  neural  shield, 
either  to  a  cartilaginous  cranium,  or  to  a  series  of  gill-like,  or  jaw-like,  segmental 
appendages  on  the  haemal  side.  (Fig.  244,  M.) 

The  following  quotation  illustrates  the  attitude  of  modern  paleontologists 
toward  the  ostracoderms.  A.  S.  Woodward,  in  his  text-book  of  Paleontology 
(1898,  page  5)  states  that  "Nearly  all  the  genera  mimic  in  a  curious  manner 
the  contemporaneous  eurypterids;"  and  on  page  24  of  the  Introduction,  that 
"The  oldest  ostracoderms,  sometimes  claimed  as  the  immediate  allies  of  the 


HISTORICAL    REVIEW. 


347 


crustacean  or  arachnid  merostomata  of  the  same  period,  are  fundamentally  differ- 
ent from  the  latter  in  every  character  which  admits  of  detailed  comparison;  they 
are  to  be  regarded  merely  as  an  interesting  example  of  mimetic  resemblance  be- 
tween organisms  of  two  different  grades  adapted  to  live  in  the  same  way  and 
under  precisely  similar  conditions." 

Surely  no  one  knows  the  precise  "way"  or  the  precise  conditions  under 
which  these  forms  lived,  or  any  probable  advantage  to  be  gained  by  one  mimicing 
the  other.  It  would  certainly  be  very  remarkable  if  many  members  of  one  class 
should  mimic  those  of  another,  when  the  two  classes  were  as  fundamentally  unlike 
as  the  arthropods  and  vertebrates  are  supposed  to  be. 

All  the  ostracoderms  are  said  to  "mimic"  the  eurypterids,  because  they 
have  a  similar  shape,  similar  cephalic  appendages,  shell  covered  orbits  and  mouth 
parts,  and  a  similar  minute  structure  of  the  dermal  armor.  But  such  a  resem- 
blance is  too  intricate  and  far-reaching  to  be  accounted  for  on  the  ground  of 
mimicry,  or  functional  parallelism,  or  mere  coincidence;  it  can  only  be  explained 
on  the  ground  of  genetic  relationship. 

Chamberlin  and  Salisbury  have  taken  a  less  conservative  position  on  this 
question.  In  their  recent  text-book  of  Geology,  it  is  stated,  Vol.  II,  page  482 : 

No  more  suggestive  combination  of  ancient  life  is  presented  by  the  geological 
record  than  that  which  is  found  in  these  supposed  fresh  water  deposits.  The 
type  was  foreshadowed  by  the  eurypterids  and  fishes,  of  fish-like  forms,  that 
appeared  in  the  closing  stages  of  the  Silurian,  but  the  record  of  that  time  is  too 
imperfect  to  disclose  its  deeper  significance.  Even  with  the  much  superior 
material  of  the  Devonian  period,  the  more  profound  significance  is  only  just 
beginning  to  be  realized.  The  center  of  interest  is  a  unique  group  of  ostracoderms 
which  were  at  first  interpreted  as  placoderm  fishes,  and  later  classed  with  the 
jawless  fishes  (agnatha,  lampreys,  etc.),  but  which  seem  now  to  be  clearly  proven 
to  be  an  entirely  distinct  class  lying  between  the  arthropods  and  vertebrates,  and 
having  some  of  the  characteristics  of  each,  but  not  truly  belonging  to  either.  Their 
supreme  interest  lies  in  the  force  they  give  to  the  suggestion  that  the  vertebrates 
sprang  from  the  arthropods. 


CHAPTER  XX. 
THE  OSTRACODERMS. 

The  ostracoderms,  as  we  have  seen,  constitute  a  class  of  animals  standing 
midway  between  the  most  primitive  vertebrates  and  the  merostome-like  arachnids. 
They  are  now  entirely  extinct  and  only  a  few  representatives  have  come  down 
to  us  in  the  form  of  fossils  that  are  well  enough  preserved  to  afford  either  full  or 
precise  information  about  them.  The  recognizable  remains  usually  agree  in  the 
minute  structure  of  their  exoskeleton,  but  there  is  a  great  diversity  in  the  form 
and  general  anatomy  of  the  better  known  representatives  of  the  class,  showing 
that  it  must  have  been  a  very  large  one. 

The  ostracoderms  were  not  a  dominant  class.  Some  of  them  were  nearly 
or  quite  blind,  their  powers  of  locomotion  were  limited,  and  their  mouth  parts 
were  feeble  and  ill  adapted  for  attack  or  defense.  In  all  that  constitutes  active 
resourceful  animals  they  were  less  effective,  and  estimated  for  that  alone,  were 
less  highly  organized  than  their  immediate  predecessors.  The  reason  for  this  is 
obvious  enough  if  we  accept  the  conclusions  in  the  preceding 'chapters,  and  the 
fact  that  they  do  present  this  condition  is,  in  itself,  important  evidence  that  those 
conclusions  are  correct.  The  ostracoderms,  as  our  theory  demands,  were  transi- 
tional forms;  they  were  in  a  phase  of  structural  readjustment  that  had  a  definite 
course  to  run  before  either  a  condition  of  organic  stability  could  be  attained  or  a 
high  degree  of  functional  adaptation  to  external  conditions  could  be  acquired. 
They  were  in,  as  it  were,  the  pupal  period  in  the  phylogeny  of  the  vertebrates. 
The  period  was  one  of  suspended  efficiency,  because  great  internal  changes  were 
taking  place  and  the  functional  relations  of  the  whole  organism  to  the  outer  world 
were  necessarily  reduced  to  a  minimum.  A  new  type  of  exoskeleton  was  form- 
ing; the  lateral  eyes,  but  newly  transferred  to  the  walls  of  a  hollow  brain,  had  not 
fully  regained  their  relations  to  the  outer  world;  paired  enteric  diverticula  opened 
to  the  exterior  by  newly  formed  visceral  clefts;  the  oral  arches  for  the  first  time  had 
been  transferred  to  the  haemal  surface  of  the  head,  a  new  mouth  formed;  and  the 
old  one  closed;  and  the  locomotor  functions  were  about  to  be  shifted  from  the 
cephalic  appendages  to  the  newly  acquired  flexible  trunk,  with  its  greatly  in- 
creased number  of  segments.  At  no  other  period  of  organic  evolution  were  so 
many  important  internal  readjustments  taking  place  at  the  same  time,  and  no  other 
great  class  of  animals  has  so  quickly  run  its  gamut  of  changes  and  so  completely 
disappeared  in  the  process  of  giving  birth  to  a  new  race. 

This  is  as  it  should  be,  for  it  will  be  seen  that  the  most  important  events  were 
of  the  open  or  shut  variety  that  permit  no  intermediate  stages;  when  they  do  occur 
they  at  once  create  totally  different  conditions,  to  which  the  organism  must 
respond  by  a  correspondingly  rapid  readjustment  elsewhere,  or  go  out  of  existence. 

348 


SUBDIVISIONS    OF   THE    BODY.  349 

•  The  ostracoderms  probably  rose  from  the  merostomes  during  the  Ordovician, 
and  reached  their  highest  development  in  the  upper  Silurian,  after  which  they 
rapidly  declined,  disappearing  at  the  close  of  the  Devonian. 

The  first  recognizable  ostracoderm  to  appear  in  America  is  Palaeaspis,  from 
the  lower  Silurian  of  Perry  Co.,  Penn.  (Fig.  244,  B  and  C.) 

Walcott  has  described  fragments  of  bony  plates  from  the  lower  Trenton 
horizon  of  the  Ordovician,  Colorado,  the  primitive  character  of  which  is  shown  by 
the  pronounced  lamination  of  the  outer  derrtinal  layers. 1  But  evidence  based  on 
such  fragments,  however  well  preserved,  is  inconclusive  since,  as  we  have  seen  in 
Chapter  XVI,  there  is  no  way  to  distinguish  fragments  of  the  exoskeleton  of  a 
primitive  ostracoderm  from  those  of  the  higher  marine  arachnids. 

The  ostracoderms,  like  their  arachnid  ancestors,  are  small,  usually  a  few 
inches  long.  Two  isolated  species  only  attain  a  length  of  one  and  a  half  or 
possibly  two  feet.  They  inhabited  shallow  waters,  and  crawled  clumsily,  oral 
side  down,  over  or  through  soft  muddy  bottoms,  or  swam  heavily,  oral  side 
up,  with  spasmodic  strokes  of  their  oar-like  cephalic  appendages,  aided  by 
the  more  flexible  posterior  portions  of  the  slender  trunk  and  tail.  They  probably 
fed  on  minute  organisms  sifted  out  of  the  mud  or  water,  or  on  the  soft  parts  of 
plants,  or  on  decomposing  organic  matter. 

Most  ostracoderms  have  large,  rounded,  or  pointed  heads,  a  small  trunk,  and 
a  tail  consisting  of  a  narrow  terminal  ribbon,  or  filament,  with  a  ventral  lobe  some 
distance  in  front  of  the  end. 


Subdivisions  of  the  Body. — The  body  may  be  divided  into  a  procephalon, 
mesocephalon,  branchiocephalon,  trunk  and  tail. 

The  procephalon,  which  may  form  a  narrow  projecting  rostrum,  contains  on 
its  neural  surface  the  median  and  lateral  eyes  and  the  olfactory  organs.  It 
is  intimately  united  with  the  mesocephalon,  to  which  belong  the  oral  arches,  or 
jaws,  and  the  oar-like  cephalic  appendages. 

The  branchiocephalon  may  be  separated  from  the  mesocephalon  by  a  distinct 
hinge  joint  (antiarcha),  or  by  a  transverse  furrow  or  scar  (cephalaspidae).  It 
contains  six  to  eight  pairs  of  gills,  usually  enclosed  in  a  large  peribranchial,  or 
atrial  chamber,  that  is  covered  on  all  sides,  except  the  posterior,  by  large 
dermal  plates.  The  right  and  left  sides  of  the  chamber  are  continuous  on  the 
ventral  side,  but  are  separated  along  the  mid  dorsal  line  by  the  tough  tissues  that 
suspend  the  branchial  portion  of  the  head  to  the  inner  surface  of  the  branchial 
shield.  The  cloaca  may  open  into  the  posterior  part  of  the  peribranchial  chamber, 
its  materials  being  discharged  with  the  water  of  respiration  from  the  posterior 
opening.  (BothriMepis.)  The  viscera,  stomach,  intestines  and  reproductive 
organs  lie  anterior  to  the  cloacal  opening,  in  that  part  of  the  head  and  trunk  en- 
closed within  the  atrial  chamber.  (Bothriolepis).  In  the  cephalaspidae  there  is 

1  Walcott,  Bull.  Geol.  Soc.,    Vol.  III. 


350  THE    OSTRACODERMS. 

no  closed  peribranchial  chamber,  the  gills  probably  lying  on  the  oral  surface, 
beneath  the  posterior  part  of  the  mesocephalon.  (Fig.  232.) 

The  trunk  is  short  and  slender,  generally  triangular  in  cross-section.  It  may 
be  practically  naked,  or  provided  with  minute,  scattered  tubercles  only  (Bothrio- 
lepis) ;  or  covered  with  rounded,  overlapping  scales  (Pterichthys) ;  or  with  large 
segmentally  arranged  oblong  plates  on  the  neural  surface,  and  small  irregular 
ones  on  the  haemal  side  (Cephalaspis) ;  or  with  shagreen-like  denticles  (ccelolepidae). 
There  are  one,  or  two,  unpaired  dorsal  fins,  stiffened  by  delicate  internal  rays 
(Bothriolepis),  or  by  minute  oblong  dermal  plates  (Cephalaspis).  Pectoral  and 
pelvic  fins  are  absent. 

Lateral  Fold. — A  narrow  fold  extends  along  the  ventro-lateral  margins  of  the 
trunk.  It  may  be  entirely  membranous  (Bothriolepis);  or  supported  by  minute 
rays  (Pterichthys  and  one  species  of  Cephalaspis) ;  or  it  may  be  formed  by  the  pro- 
jecting ends  of  segmental  trunk  plates  (Tremataspis) ;  or  it  may  consist  of  a  series 
of  segmentally  arranged,  separately  movable,  appendage-like  plates,  or  fringing 
processes  (Cephalaspis). 

The  Cephalic  Appendages. — Large,  oar-like  cephalic  appendages  form  one 
of  the  most  striking  features  of  the  ostracoderms.  In  Bothriolepis  they  are  at- 
tached to  the  posterior  haemal  margin  of  the  mesocephalon,  in  front  of  the  gills, 
and  consist  of  two  joints,  or  segments,  covered  with  bony  plates.  They  are  hollow, 
triangular  in  cross-section,  and  contain  indications  of  a  cartilage  axis.  (Fig.  257.) 
An  opening  on  the  posterior  proximal  end  of  the  arm,  and  an  adjacent  one  on  the 
side  of  the  branchiocephalon,  serve  for  the  passage  of  nerves,  blood-vessels,  and 
other  tissues. 

In  Cephalaspis  the  cephalic  appendages  are  covered  with  minute,  semi-isolated 
dermal  plates,  and  their  broad  distal  ends,  of  undetermined  contour,  are  horizon- 
tally flattened.  Parts  of  armored  cephalic  appendages  similar  to  those  of  Bothrio- 
lepis have  been  found  in  Palaeaspis,  Cyathaspis,  Tremataspis,  and  Psamosteus.  In 
Pteraspis,  Drepanaspis,  and  Berkenia,  there  are  certain  marginal  notches,  or  open- 
ings, that  have  been  considered  as  lateral  eye  orbits,  but  which  may  possibly 
represent  the  points  of  attachment  of  cephalic  appendages.  In  these  genera  they 
were  smaller,  less  heavily  armored,  if  they  were  armored  at  all,  and  were  not  used 
as  swimming  oars. 

The  cephalic  appendages  of  the  ostracoderms  are  not  comparable  with  the 
pectoral  fins  of  vertebrates,  but  with  one  of  the  pairs  of  thoracic  swimming  legs 
of  the  merostomata.  They  are  represented  in  vertebrates  by  the  so-called  "  bal- 
ancers," and  the  cephalic  tentacles  of  amphibian  larvae,  and  by  similar  processes 
in  certain  fishes,  i.e.,  Protopterus,  Accipenser  and  Bdellostoma. 

Jaws. — The  mouth  lies  in  a  membranous  portion  of  the  haemal  surface,  cau- 
dad  to  the  projecting  rostrum,  or  to  the  anterior  margin  of  the  procephalon.  In 
Bothriolepis,  there  are  three  pairs  of  bony  plates,  which  represent,  in  part,  the 
premaxillary,  maxillary,  and  mandibular  arches  of  vertebrates. 

The  maxillary  arch  is  probably  represented  by  the  small  movable  plates  (Figs. 


JAWS.      SKELETON.  351 

254,  259,  mx.)  The  broad  premaxillae,/>.w#.,  have  heavy  crushing  margins  on  their 
median  ends,  and  move  laterally  to  and  from  the  mouth,  which  lies  in  the  median 
line  between  them.  The  mandibles  (Fig.  254,  md.),  are  narrow,  curved,  and 
pointed  at  their  median  ends,  and  form  a  thick  bony  covering  to  a  hollow  axial  por- 
tion that  probably  consisted  of  cartilages.  They  move  diagonally  forward  and  in- 
ward, and  backward  and  outward,  pushing  or  scooping  the  food  forward  between 
the  premaxillae  and  toward  the  mouth,  m.  Behind  the  mandibles,  om.,  the  cir- 
cumoral  membrane  is  stiffened  by  two  thin  bands  of  bone  which  probably  rep- 
resent the  dermal  armor  of  the  hyoid  arches. 

In  Tremataspis  (Fig.  237),  the  oral  region  is  occupied  by  flat  polygonal  plates, 
showing  little  resemblance  to  jaws.  They  are  arranged  in  four  transverse  rows, 
which  possibly  represent  the  premaxillary,  maxillary,  mandibular,  and  two  rows 
of  hyoid  plates  seen  in  Bothriolepis. 

In  Cephalaspis  there  are  indications  of  one  pair  of  large  crushing  jaws.  The 
jaws  of  other  members  of  the  ostracoderms  probably  resemble  those  of  Bothrio- 
lepis or  Tremataspis,  but  no  traces  of  them  have  as  yet  been  found. 

The  Skeleton. — The  ostracoderms  were  no  doubt  provided  with  cranial 
cartilages,  including  an  endocranium  and  a  notochord,  but  they  were  not  volumi- 
nous or  resistant,  for  in  Bothriolepis  no  certain  traces  of  them  can  be  seen,  although 
the  skin  and  other  soft  parts  are  clearly  indicated.  In  sections  of  the  branchio- 
cephalon  small  black  rings  are  sometimes  seen,  mingled  with  the  blackened  rem- 
nants of  the  gills  and  viscera,  that  may  be  fragments  of  the  notochord  sheath. 

The  chief  interest  lies  in  the  bony  exoskeleton,  which  presents  a  structure  in- 
termediate between  the  chitenous  epidermal  skeleton  of  arthropods  and  the  der- 
mal skeleton  of  vertebrates. 

In  a  primitive  ostracoderm,  the  general  character  of  the  external  armor  is 
similar  to  that  of  a  trilobite,  or  a  merostome,  in  that  it  may  consist  of  an  almost 
continuous  shell,  or  buckler,  for  the  broad  cephalic  and  branchial  regions,  and  seg- 
mentally  arranged  plates,  corresponding  with  the  pleural  and  tergal  plates,  on 
the  flanks  and  dorsal  surface  of  the  trunk.  The  dorsal  fin,  all  but  the  terminal 
part  of  the  tail,  and  the  ventral  surface  of  the  trunk,  may  be  covered  with 
minute  oblong  plates  similar  in  structure  and  surface  ornament  to  the  larger  ones. 
(Cephalaspis.) 

In  its  simplest  condition,  the  matrix  of  the  dermal  skeleton  consisted  of  paral- 
lel, or  concentric  lamellae,  that  apparently  have  been  formed  in  the  same  manner  as 
the  characteristic  lamellae  in  chitenous  exoskeletons.  It  is  unlike  chiten  chemically, 
but  resembles  it  in  the  varying  degree  of  hardness,  color,  and  other  optical  proper- 
ties of  the  lamellae,  and  in  the  presence  of  innumerable,  parallel,  unbranched 
canals  (pore  canals,  or  primitive  dentinal  tubules)  which  everywhere  penetrate  the 
matrix  at  right  angles  to  the  lamellae. 

The  exoskeleton  usually  consists  of  three  principal  layers  consisting  of  a  bony 
or  dentine-like  substance:  a.  an  inner  one  of  horizontal  lamellae;  b.  a  middle  one 
of  large  polygonal  spaces,  or  cancel lae,  and  c.  an  outer  layer  consisting  mainly  of 


352  THE    OSTRACODERMS. 

dentine,  with  an  underlying  stratum  of  Haversian,  or  other,  canals.  The  lamellae 
are  always  parallel  to,  or  concentric  with,  the  walls  of  the  cancellae,  or  with  those 
of  the  larger  spaces  they  enclose.  Openings  through  the  inner  layer  serve  for  the 
passage  of  blood-vessels  and  other  tissues  into  the  cancellae,  and  hence  to  the  canals 
of  the  outer  layer. 

The  outer  surface  is  denser  and  harder  than  the  rest,  and  may  be  without 
distinct  dentinal  tubules  or  lamellae,  thus  forming  a  thin  enamel,  or  ganoin  layer. 
It  is  often  divided  into  small  polygonal  areas,  and  is  variously  ornamented  with 
tubercles  or  ridges. 

The  most  primitive  lacunae  are  unipolar,  or  bipolar,  and  arise  as  dilata- 
tions of  the  inner  ends  of  primitive  pore  canals  (Pteraspis) ;  or  a  linear  series  of 
lacunae  may  be  formed  from  local  enlargements  of  a  single  canal  (Tremataspis). 
The  more  highly  developed,  or  typical  bone  lacunae,  arise  from  the  unipolar 
lacunae  through  the  formation  of  secondary  lateral  canals,  or  canaliculi.  The 
primitive  lacunae  are  located  mainly,  and  primarily,  in  the  deeper  lamellae;  that  is, 
in  the  axial  portions  of  the  trabeculae,  and  in  the  partitions  separating  the  cancellae 
and  larger  canals.  As  the  lacunae  develop  in  size  and  complexity,  they  lose  their 
original  arrangement  and  their  relation  to  pore  canals,  taking  up  their  position 
between  the  lamellae,  with  their  long  axes  parallel  with  one  another  and  with  the 
plane  of  the  lamellae,  those  in  one  layer  often  standing  at  right  angles  to  those  in 
the  adjacent  layers.  The  lacunae  of  the  same  and  of  the  adjacent  layers  are  then 
united  by  many  branching  canaliculi,  As  the  latter  increase  in  numbers,  they 
form  an  anastomosing  network  that  takes  the  place  of  the  primitive  unbranched  and 
parallel  pore  canals.  The  primitive  condition  of  the  pore  canals  is,  however, 
largely  retained  in  the  outermost  layers  of  the  shell,  forming  the  dentine  layers 
characteristic  of  the  surface  ridges,  spines,  and  tubercles. 


The  general  trend  of  development  in  the  exoskeleton  of  the  ostracoderms 
is  as  follows: 

1.  The  lacunae  become  parallel  with  the  lamellae,  instead  of  with  the  pore 
canals. 

2.  They  increase  in  number,  and  their  numerous  canaliculi  replace  the 
primitive  pore  canals. 

3.  The  cancelli  break  down,  owing  largely  to  the  increasing  number  of  vascu- 
lar channels  (Haversian  canals)  and  their  more  uniform  distribution  throughout 
the  various  layers,  this  process  gradually  producing  a  condition  similar  to  that 
in  the  typical  dermal  bones  of  vertebrates. 

4.  The  armor  breaks  up  into  separate  plates  or  "bones"  of  various  sizes, 
which  may  or  may  not  be  movably  articulated,  and  which  may  bear  some  defi- 
nite relation  to  the  underlying  organs,  such  as  the  primitive  subdivisions  of  the 
head,  or  the  arrangement  of  segmental  muscles,  appendages,  or  jaws;  or  they 


THE    EVOLUTION    OF    THE    EXOSKELETON. 


353 


may  possibly  have  some  relation  to  the  plates  that  are  present  in  the  cephalothorax 
and  branchial  regions  in  their  trilobite-  or  merostome-like  ancestors. 

5.  The  armor  may  also  break  up  into  small  polygonal  platelets,  of  uniform 
size,  that  are  quite  separate  from  one  another,  the  lines  of  fragmentation  following 
the  " ornamental"  polygonal  markings  visible  on  the  outer  surface  of  the  shield 
in  Limulus,  (Fig.  200,  A.C.D.),  in  Cephalaspis,  or  in  Ateleaspis.     (Fig.  200,  B.) 

6.  With  this  process   of  fragmentation  there  is  a  tendency  to  accentuate 
the  difference  between  the  inner  layers  of  more  highly  developed  bony  tissue, 
and  the  outer  layers  that  still  retain  their  primitive  stratification  and  parallel  pore 
canals,  and  into  which  neither  the  bone  cells  nor  the  vascular  canals  have  pene- 
trated to  any  great  extent.     The  isolated  ridges,  spines,  or  tubercles  of  the  outer 
layer,  initiate  the  dermal  denticles  of  the  vertebrates,  and  represent  their  enamel 
and  dentinal  caps;  the  lower  layers  initiate  the  basilar  plates  composed  of  true 
dermal  bone.     (Fig.  208.) 

7.  During  the  phylogenetic  process  of  fragmentation  the  two  layers  may 
undergo  unequal  development.     In  such  forms  as  the  ccelolepidae  (Thelodus  and 
Lanakia)  apparently  only  the  isolated  epidermal  denticles  are  retained,  while 
the  underlying  network  of  bony  plates  and  trabeculae  has  largely,  or  wholly, 
disappeared.     In  Bothriolepis  and  related  forms,  the  dentinal  layer  is  scanty, 
or  for  the  most  part  absent,  while  the  large  bony  lamellae  of  the  inner  and  middle 
layers  are  highly  developed.     In  the  pteraspidian  section  all   three  layers  are 
present,  but  the  underlying  ones  have  no  true  bone  cells,  only  the  spindle-like 
dilatations  of  the  pore  canals. 

8.  In  the  antiarcha,  as  in  the  vertebrates,  the  placoid  bones  were  extensively, 
if  not  entirely,  covered  with  a  layer  of  epidermal  cells.     This  is  indicated  not 
only  by  their  general  structure,  but  is  conclusively  demonstrated  by  the  faint 
impressions  on  their  outer  surface  left  there  by  the  nuclei. 

9.  The  dentinal  layer  of  the  unfragmented  buckler  and  of  the  isolated  den- 
ticles, like  the  exoskeleton  of  arthropods  and  the  enamel  of  vertebrate  teeth,  was 
probably  the  product  solely  of  underlying  epidermal  cells.     Their  outer  surface 
was   probably  never   covered  with  epithelial  cells,  except  by  secondary  over- 
growths resulting  from  infoldings  like  those  in  the  developing  teeth  of  vertebrates. 

10.  A  remarkable  feature  of  the  cephalaspidae  is  the  union  of  the  margins  of 
the  upper  and  lower  shields  by  anastomosing  bony  trabeculae  which,  like  those  in 
Limulus,  form  the  solid,  or  cancellous  cornua,  and  the  heavy  hoop-like  margin 
along  the  front  and  sides  of  the  cephalic  shield. 

In  Eukeraspis  (Fig.  235,  D)  there  are  peculiar  chambers  (marginal  cells  of 
Lankester),  in  the  bony  tissue  on  the  anterior  margin  of  the  shield.  They  may 
be  merely  enlarged  cancellae,  or  possibly,  enclosures  formed  by  a  deeply  serrated 
or  scalloped  margin  like  that  in  Thyestes,  A,  or  like  the  enclosures  formed  by 
marginal  infoldings  in  the  trilobites,  B. 

11.  In  Cephalaspis  (Fig.  232),  there  are  conspicuous  oval  areas  on  the  lateral 
margins,  and  behind  the  orbits,  that  are  formed  by  thickened  patches  of  bony 

23 


354 


THE    OSTRACODERMS. 


trabeculae.     They  correspond  roughly  with  the  areas  in  Limulus  where  the  bony 
trabeculae  are  most  highly  developed.      (Fig.  205.)     In  Tremataspis,  Cephalaspis, 


•-d.o.t. 


-en. 


FIG.  232. — Restoration  of  Cephalaspis.     A,  From  the  side;  B,  from  the  haemal  surface.     Based  mainly  on 

C.  murchisoni. 

and  Thyestes,  these  areas  are  covered  with  loose,  superficial,  polygonal  plates 
which  may,  and  usually  do,  fall  out,  leaving  sharply  defined,  shallow  openings 
in  the  outer  layers.  (Figs.  235,  A,  236,  B.)  They  have  a  floor  consisting  of 


THE    EYES.  355 

bony  trabeculae  belonging  to  the  inner  layer.  There  are  no  indications  that 
special  organs,  sensory  or  otherwise,  were  located  in  these  openings. 

The  Eyes.— The  parietal  and  lateral  eyes,  whenever  their  position  can  be 
certainly  determined,  are  located,  with  the  olfactory  organs,  on  the  median  dorsal 
portion  of  the  cephalic  buckler,  forming  in  the  antiarcha  and  aspidocephali  a 
very  characteristic  group. 

The  lateral  eyes  in  these  families  are  remarkable.  They  are  contained  in 
spherical  chambers,  the  floor  and  sides  of  which  are  formed  of  a  basket  work  of 
bony  trabeculae  similar  to  those  in  Limulus.  (Tremataspis,  Cephalaspis  Figs. 
233,  A,  B,  239,  A.)  They  are  situated  on  short  bony  stalks  that  could  be  lowered 
into  the  chamber,  or  the  distal  ends  of  the  stalks  could  be  raised,  exposing  the 
convex  surface  of  the  oval  or  kidney-shaped  cornea.  (Fig.  239.)  The  latter  is 
covered  by  a  thin  shell  that  appears  to  be  an  extension  of  the  dermal  armor; 
it  is  convex  and  smooth,  and  in  life  no  doubt  it  was  transparent  (Tremataspis 
and  Cephalaspis). 


KL 


FIG.  233 . — A ,  Cross-section  through  the  thoracic  shield  of  Limulus,  showing  the  location  of  the  principal  patches 
of  bony  trabeculae ;  B,  section  through  the  head  of  Cephalaspis  showing  the  bony  trabeculae  encasing  the  lateral 
eyes,  the  thickened  margin,  and  a  part  of  the  haemal  wall  of  the  shield;  C,  fringing  processes  of  C.  lyelli;  D,  same 
of  C.  pagei;  F,  cross-section  of  the  trunk  of  C.  lyelli. 

In  the  pteraspidian  and  anaspidfan  sections,  and  in  Ateleaspis,  the  lateral 
eyes  are  apparently  absent,  or  covered  by  a  thick  skeletal  layer  that,  as  in  young 
lampreys,  effectively  conceals  their  location. 

The  apparent  absence  of  the  lateral  eyes  in  certain  ostracoderms  is  very 
significant.  There  is  no  reason  to  suppose  that  the  forms  without  lateral  eyes 
were  cave  animals,  or  deep  sea  animals,  or  that  they  belonged  to  an  eyeless  stock. 
The  unusually  large  size  of  the  median  eye  tubercle,  or  fossa,  and  the  well  devel- 
oped lateral  eyes  in  their  immediate  relatives,  is  sufficient  evidence  to  the  contrary. 
The  temporary  suppression  of  such  ancient  organs  as  the  lateral  eyes  is  best  ex- 
plained on  the  assumption  that  they  are  in  a  metamorphic,  or  transitional,  condi- 
tion, midway  between  the  lateral  eyes  of  arthropods,  which  remain  outside  the 


356  THE    OSTRACODERMS. 

medullary  tube,  and  the  cerebral  eyes  of  vertebrates  that  have  been  carried  into 
it  during  the  early  stages  of  development.  Those  ostracoderms  in  which  the 
lateral  eyes  appear  to  be  absent  are  to  be  regarded  as  the  ones  whose  newly- 
formed  cerebral  eyes  (phylogenetically  speaking)  have  not  become  functionally 
adjusted  to  their  new  environment.  The  concealed  lateral  eyes  of  larval  cyclo- 
stomes  are  in  a  similar  condition,  and  they  are  to  be  explained  in  a  similar  manner. 
See  Chapter  IX. 

The  parietal  eye  was  relatively  large,  and  was  lodged  in  a  deep  pit  on  the 
under  side  of  a  projecting  tubercle  of  the  cephalic  buckler  (pteraspidians) ,  or  on 
the  under  side  of  a  small  movable  plate,  lying  between  the  lateral  eyes,  (Tremataspis, 
Cephalaspis,  and  Bothriolepis).  In  Bothriolepis  there  are  two  additional  pits  on 
the  inner  surface  of  the  postorbital  plate,  that  probably  contained  another  pair  of 
parietal  ocelli.  (Figs.  252,  253,  255.) 

The  olfactory  organs  were  probably  located  in  a  hypophysis-like  median  sac, 
situated  just  in  front  of  the  orbits.  In  Tremataspis,  Cephalaspis,  and  Thyestes, 
the  oval  opening  to  the  sac  lies  at  the  bottom  of  a  shallow  depression,  that  may  be 
a  little  deeper  on  either  side.  In  Bothriolepis  there  is  a  small  movable,  T-shaped 
plate  (Fig.  255,  e),  that  stands  nearly  vertically  in  the  large  opening  common  to  the 
median  and  lateral  eyes.  To  the  outer  ends  of  the  plate  are  attached  two  con- 
cave, lateral  wings,  I.e.,  that  appear  to  have  partly  enclosed  the  olfactory  organs. 
A  narrow  canal  leads  outward  from  each  chamber,  opening  to  the  exterior  just 
in  front  of  the  top  of  the  plate. 

Auditory  Organs. — In  Tremataspis  and  Bothriolepis  there  are  two  small, 
sharply  defined  openings,  situated  close  together  in  the  occipital  region,  which 
probably  represent  the  outer  ends  of  endolymphatic  ducts. 

In  Bothriolepis,  when  seen  either  in  sections  or  dissections,  they  lose  their 
sharply  denned  walls  just  below  the  outer  surface,  and  open  into  irregular  cham- 
bers that  may  lead  either  into  the  cancellous  tissue  or  into  the  interior.  There  are 
no  definite  openings  corresponding  to  them  on  the  inner  surface  of  the  shell,  but 
in  etched  heads  there  may  be  present  a  conspicuous  spur  representing  the  cast 
of  the  inner  opening.  (Fig.  252.)  In  Tremataspis.  the  canals  lead  into  small  bony 
tubes  that  project  some  distance  from  the  inner  surface  of  the  shield.  In  Cyathas- 
pis  there  are  two  V-shaped  ridges  in  this  region,  that  have  been  regarded  as  the 
surface  indications  of  semi-circular  canals. 

The  ducts  are  in  some  way  related  to  the  lateral  line  organs,  for  in  young 
specimens  of  Bothriolepis  they  mark  the  median  termination  of  the  orbital  lines. 

Cutaneous  Sense  Organs. — A  system  of  cutaneous  sense  organs  is  fully  and 
clearly  mapped  out  in  Tremataspis  and  Bothriolepis.  In  the  former  (Fig.  236), 
each  line  consists  of  a  series  of  short,  shallow  grooves  on  the  smooth  outer  surface 
of  the  shell.  A  circumorbital  line,  a.m,  postorbital,  p.o,  occipital  oc,  a  posterior 
branchial,  p.b,  and  a  lateral  line  p.m,  are  represented.  The  occipital  lines  lead 
toward  the  endolymphatic  pores  described  above.  .  No  grooves  occur  on  the  ven- 


THE  CUTANEOUS  SENSE  ORGANS. 


357 
(Fig. 


tral  side,  except  for  a  short  line,  or  dash,  on  one  or  two  of  the  oral  plates. 

237^0 

In  Bothriolepis  the  lines  form  continuous  shallow  grooves.  (Figs.  247,  248.) 
One  line  extends  from  the  infra-orbital  across  the  front  of  the  head;  there  is  a 
V-shaped  orbital  line  connected  with  the  occipital  pores,  and  one  on  the  dorsal  sur- 
face of  the  branchial  shield;  another  extends  onto  the  ventral  surface,  across  the 
maxillary  plates,  probably  connecting  with  the  line  on  the  premaxillary  plates; 
while  a  lateral  line  extends  along  the  sides  of  the  branchial  shield  and  appears  to  be 
continuous  with  a  furrow  extending  along  the  sides  of  the  fleshy  trunk. 


FIG.  234. — A  small  species  of  Cephalaspis,  sp.f  associated  with  Bothriolepis,  from  Miguasha,  Bay  Chaleur,  P.  Q. 
About  natural  size.  A,  From  side;  B,  haemal  surface;  C,  cross-section  of  the  trunk;  D,  sagittal  section  of  the  head 
E,  cross-section  of  the  head.  In  B,  a  heavy  bony  ridge  is  seen,  b,  that  probably  divides  the  oral  from  the  bron- 
chial chamber.  From  specimens  in  the  author's  collection. 

In  Pteraspis  and  Palaeaspis  there  are  scattered  dash-like  markings  on  the 
outer  surface  similar  to  those  of  Tremataspis,  but  less  regularly  arranged.  There 
is  also  a  special  system  of  canals  lying  within  the  shell,  the  distribution  of  which 
is  imperfectly  known;  they  may  perhaps  represent  closed  lateral  line  canals 
(Tremataspis,  Pteraspis,  and  Palaeaspis) 

In  Ateleaspis,  nov.sp,  from  Dalhousie,  sensory  grooves  are  present  similar  to 
those  in  Tremataspis,  but  located  on  a  narrow  ridge.  The  infra-orbital,  the  ant- 
orbital,  and  the  anterior  end  of  the  lateral  line,  are  indicated.  (Fig.  242.) 


358 


THE    OSTRACODERMS. 


The  prevailing  position  of  the  lateral  line  organs  on  the  neural  surface  of  the 
head,  where  they  are  out  of  touch  with  the  food  or  surrounding  objects,  is  only 
intelligible  on  the  assumption  that  they  represent  the  remnants  of  the  gustatory 
and  tactile  organs  that  were  located  on  the  neural  surface  of  the  head,  in  their 
arachnid-like  ancestors.  See  Chapter  VII,  p.  121. 

The  ostracoderms  may  be  divided  into  the  following  orders: 


FIG.  235. — Cephalic  bucklers  of  Ostrocoderms  and  marine  arachnids.     A,  Head  of  Thyestes;  B,  Odonto- 
cephalus;  C,  Corycephalus;  D,  Eukeraspis.     Silurian. 

I.    ASPIDOCEPHALI. 

Head,  thin,  broad.  Trunk  and  tail,  narrow  and  small.  Exoskeleton  form- 
ing a  continuous  cephalic  buckler;  trunk  plates  separate,  segmentally  arranged. 
Outer  surface  of  dermal  armor,  smooth,  or  with  low  rounded  tubercles  on  poly- 
gonal areas,  that  may  separate  into  small,  five-  or  six-sided  plates.  Marginal  and 
central  openings  on  the  dorsal  mesocephalic  shield,  filled  with  loose  polygonal 
plates  belonging  to  the  outer  layer  only.  Olfactory,  or  hypophyseal  opening,  not 
enclosed  in  the  orbital  foramen. 

Cephalaspidae. — The  head  is  shield-shaped,  rounded  or  pointed  in  front,  and 
with  thickened  margin;  posterior  margin  expanded,  cornuate.  Oral  region  is  a 
small  membranous  area  in  the  center  of  the  thin,  convex,  haemal  wall.  The 
branchiocephalic  shield,  small,  indistinctly  segmented;  haemal  branchial  shield 
absent.  Gills  not  enclosed  in  an  atrial  chamber.  Large  cephalic  appendages 


THE  ASPIDOCEPHALI. 


359 


attached  to  the  under  side  of  mesocephalon,  median  to  the  cornua;  flexible,  with 
terminal  horizontal  expansions.  Trunk  membranous,  or  covered  with  segment- 
ally  arranged  plates,  with  twenty-five  to  thirty  pairs  of  separately  movable,  appen- 
dage-like, fringing  processes.  Lateral  eyes,  prominent,  median.  Caudal  axis 
straight,  ending  in  slender  filament.  Ventral  lobe  of  caudal  fin,  narrow  and  sub- 
terminal.  Upper  Silurian  to  upper  Devonian. 

Cephalaspis  (Figs.  232,  233,  234);  Eukeraspis  (Fig.  235,  D)-y  Thyestis  (Fig. 
235,  A)-,  fringing  plates  (Fig.  233,  C,  D,  E). 


..a.m.. 


FIG.  236. — Restoration  of  Tremataspis,  seen  from  the  neural  surface,  and  showing  the  location  of  the  principal  sense 
organs,  canal  organs,  and  appendages.      X  about  i  1/2. 

Tremataspidae. — Dorsal  and  ventral  shields  of  the  mesocephalon  and  branch- 
iocephalon  united  to  form  an  oblong,  lenticular  buckler.  Parietal  eye  plate  free. 
Small,  2-3  in.  long.  The  best  known  form  is  Tremataspis  Schmidti,  whose 
polished  yellow  shields  are  beautifully  preserved  in  fine  grained  chalky  rocks  of 
the  upper  Silurian  in  the  island  of  Oesel,  Baltic  sea. 

Exoskeleton. — Divided  into  the  usual  three  layers;  outer  surface  nearly  smooth, 
polished.  Cancellae,  somewhat  irregular,  small.  Two  sets  of  horizontal  canals, 
forming  networks  just  below  the  outer  layer.  In  one  set,  the  canals  are  of  varying 


36° 


THE    OSTRACODERMS. 


caliber,  and  terminate  in  coarse,  dentinal  canals  that  dilate  at  intervals,  forming 
in  the  outer  shell  layer  vertical  rows  of  multipolar  lacunae;  inwardly  they  lead  into 
the  cancellae,  and  hence  via  coarse  vertical  canals,  through  the  basal  layer  into 
the  head.  The  second  set  are  uniform  in  diameter  and  open  outwardly  by  clear- 
cut  conical  chimneys.  (Fig.  205.) 


t 
V/ 


FIG.  237. — Restoration  of  Tremataspis,  seen  from  the  haemal  surface,  and  showing  the  arrangement  of  the  circum- 

oral  plates,   X  about  i  1/2. 

Oral  Region, — -There  is  a  large  opening  on  the  anterior  haemal  part  of  the 
head,  near  the  center  of  which  lies  the  mouth.  (Fig.  237.)  It  is  surrounded  by 
close-fitting  plates,  seldom  preserved  in  place.  The  opening  in  which  these 
plates  belong  is  bounded  in  front  and  on  the  sides  by  the  narrow,  overturned  rim  of 
the  neural  shield.  On  the  anterior  rim  is  a  triangular,  ill  defined  plate,  adhering 


FIG.  238. — Photograph  of  the  orbital  region  of  the  shield  of  Tremataspis.      X  about  3. 

to  the  inner  surface  of  the  neural  shield.  It  has  a  median,  oval  ridge  b,  with  a 
roughened  or  porous  texture.  On  either  side  is  a  polished,  conical  projection 
of  the  narrow  rim,  a.  On  the  lateral  and  posterior  margins  of  the  opening  are 
eight  or  nine  pairs  of  thick-lipped,  semi-circular  incisions,  whose  concave  surface 
is  conspicuously  porous.  There  are  corresponding  incisions  on  the  adjacent 


THE    TREMATASPID^. 


36l 


oral  plates,  thus  forming  a  regular  series  of  openings  leading  into  the  interior 
and  increasing  in  size  from  behind  forward. 

The  oral  plates  form  four  or  five  transverse  rows,  the  form  of  the  smallest  and 
most  anterior  ones  being  imperfectly  known.  In  the  only  known  specimen  that 
has  the  plates  in  position,  the  large  anterior  pair  were  crushed  and  broken. 
What  appears  to  be  one  of  the  same  plates  has  been  found  isolated  and  intact. 


FIG.  239. — Cross-section  of  the  orbits  of  Tremataspis,  showing  the  movable  lateral  eye  stalks,  the  parietal  eye, 
the  network  of  bony  trabeculae  forming  the  floor  of  the  median  and  lateral  eye  chambers,  and  the  extension  of  the 
dermal  armor  over  the  corneal  surface  of  the  lateral  eyes.  Semi-diagrammatic.  X  about  7  1/2. 

It  has  a  small  rounded  notch  at  one  end,  and  appears  to  represent  the  premaxil- 
lary  of  Bothriolepis,  although  its  form  does  not  suggest  a  jaw  plate.  None  of  the 
remaining  plates  resemble  those  of  Bothriolepis. 

The  exact  location  of  the  mouth  is  uncertain.  It  was  probably  in  the  ill-de- 
fined triangular  depression  between  the  premaxillary  plates.  In  any  case  it 
must  have  been  very  small  and  narrow. 

The  lateral  eyes  were  on  short  stalks  that  could  be  raised  above  the  level  of  the 


FIG.  240.  —  Bony  plate  from  the  neural  surface  of  the  basal  joint  of  a  cephalic  appendage  of  Tremataspis.  A, 
External  surface,  showing  a  circular  groove  in  the  polished  outer  layer,  made  by  striking  the  edge  of  the  shield 
in  the  swimming  movements  of  the  appendages;  d,  neck  of  the  plate,  attached  by  flexible  membranes  to  one  of  the 
openings  on  the  haemal  margin  of  the  shield;  B,  inner  surface  of  the  plate.  X  ,  10. 

shell,  or  lowered  into  the  large  spherical  orbits,  the  floor  of  which  consisted  of  a 
basket  work  of  bony  trabeculae.  The  entire  outer  surface  of  the  eye  stalk  and 
the  front  of  the  eye  itself  was  covered  with  a  thin  layer  of  the  exoskeleton.  (Figs. 


The  marginal  and  post-orbital  openings  were  originally  filled  with  thin  poly- 
gonal plates;  but  the  latter  are  usually  absent,  leaving  shallow  depressions  with 
scalloped  margins,  the  floor  consisting  of  the  modified  inner  layer  of  the  shell. 


362 


THE    OSTRACODERMS. 


Fig.  238  shows  a  photograph  of  a  head  in  which  the  plates  are  retained  in  their 
proper  place. 

Sections  of  the  whole  buckler  show  that  the  lateral  margins  of  the  upper  and 
lower  shields  were  united  by  bony  trabeculae,  and  that  two  plate-like  entapophyses 
projected  from  the  inner  surface  of  the  dorsal  shield,  that  probably  served  for  the 
attachment  of  muscles.  (Fig.  236,  r.) 

The  lateral  line  organs  consist  of  short,  shallow  grooves  separate  in  the  younger 
specimens,  but  united  into  longer  grooves  in  the  older  ones.  There  is  a  circum- 
orbital,  lateral,  occipital,  and  a  posterior  dorsal  line.  Short,  sensory  grooves 
occur  on  some  of  the  isolated  oral  plates. 

Appendages. — Associated  with  the  remains  are  certain  plates  that  have  the 
same  peculiar  texture  as  the  shields,  and  which  undoubtedly  represent  portions 
of  the  armored  appendages.  A  complete  distal  joint  has  been  found.  (Fig.  241.) 


• 


241. — ,  D,  E,  Distal  joint  of  a  cephalic  appendage  of  Tremataspis;  C,  haemal;  D,  median;  E,  neural 
surface;  F,  appendage  of  some  unknown  animal  associated  with  the  remains  of  Trematapsis.  It  consists  of  five 
or  more  joints.  A  part  of  the  large  distal  joint  has  broken  off,  exposing  the  impression  of  the  outer  surface,  that 
was  marked  by  faint  transverse  ridges.  Superficially,  the  shell  covering  the  joints  resembles  that  of  Tremataspis. 
Seen  from  the  inside.  X  9. 

It  is  oblong,  with  a  broad,  partly  membranous,  posterior  surface,  a  scalloped 
anterior  edge,  and  an  articular  process  at  its  proximal  end.  A  convex,  heart- 
shaped  plate  was  also  found  that  represents  the  dorsal  part  of  the  proximal 
joint.  (Fig.  240.)  It  was  attached  by  a  narrow  roughened  collar  probably  to 
the  larger  anterior  marginal  incision.  In  each  of  the  three  known  specimens  of 
this  plate  the  polished  outer  surface  is  cut  by  a  semicircular  groove,  showing 
where  it  struck  against  the  margin  of  the  shield  in  the  forward  and  backward 
movement  of  the  arms.  (Fig.  240.)  There  is  no  place  where  such  plates  could 
be  attached  to  the  body  or  head  except  in  the  larger  anterior  incisions.  They 
fit  in  this  position  fairly  well  and  in  this  position  they  agree  with  the  corre- 
sponding parts  of  the  cephalic  appendages  of  Bothriolepis. 

The  smaller,  more  posterior  incisions  or  openings  may  have  served  for  the 


THE  ATELEASPID/E. 


363 


attachment,  or  for  the  exit  of  other  organs  of  a  similar  nature,  as  for  example 
external  gills. 

Fragments  of  what  appear  to  be  smaller  appendages,  covered  with  a  thin 
calcareous  shell,  have  been  found  in  the  same  deposits.  They  have  a  large, 
flattened,  terminal  joint  and  two  or  three  small  segments,  or  joints,  with 
the  surface  raised  into  prominent  peaks.  (Fig.  241,  F.)  There  is  no  other  clue 
to  their  origin.  They  may  possibly  represent  the  distal  portion  of  the  cephalic 
arms  of  Thyestes  or  of  some  unknown  arthropod. 

Ateleaspidae.— Head  rounded,  with  heavy  thickened  margin;  cornua  trun- 
cated. Entire  body  covered  with  small,  five-  or  six-sided  plates,  ornamented  with 
bands  or  radiating  lines,  or  canals,  and  with  low  rounded  and  polished  tubercles 


.vm,. 


FIG.  242. — Ateleaspis.  A,  Fragment,  showing  surface  ornament,  magnified;  B,  neural  surface  of  head  and 
part  of  the  trunk,  showing  parts  of  the  "  lateral  line  "  ridges,  marked  by  a  narrow  sinuous  groove.  At  v.m.  the  shell 
is  absent,  displaying  a  cast  of  the  broad,  flat  haemal  margin,  ornamented  with  prominent  rounded  tubercles. 
(From  Miguasha,  Bay  Chaleur,  P.  Q.  About  2/3  natural  size.) 

of  varying  size.  (Fig.  200,  \B,  242,  A.)  No  orbits.  Sensory  canals  on  trunk 
and  head,  consisting  of  zig-zag,  or  interrupted  grooves  on  a  low  narrow  ridge. 
Upper  Silurian  of  England;  Devonian  of  Miguasha,  P.  Q.  Canada. 

I  possess  a  fairly  well  preserved  example  of  this  very  rare  family,  that  was 
obtained  from  the  gray  cliffs  of  Miguasha,  and  which  probably  represents  a  new 
species.  The  greater  part  of  the  head  is  preserved  and  shows  no  trace  of  orbits. 
(Fig.  242.)  The  haemal  side  of  the  thickened  rim  was  broad,  sharply  marginate, 
and  studded  with  large  tubercles.  A  lateral  line  groove  extends  well  back  onto  the 
flanks,  and  there  are  traces  of  a  rostral  and  an  anterior  transverse  line.  The 
trunk,  as  far  as  preserved,  was  covered  with  small  five-  or  six-sided  plates. 

This  genus  is  of  special  importance,  since  it  shows  us  an  undoubted  cephal- 


THE    OSTRACODERMS. 

aspid,  probably  without  orbits,  and  in  which  the  exoskeleton  tends  to  break 
up  into  separate  polygonal  plates,  which  correspond  to  the  polygonal  areas  on 
the  continuous  shield  of  Limulus  and  Cephalaspis.  (Fig.  199.) 

II.  THE  ANASPIDA. 

The  anaspida  include  a  small  number  of  obscure  forms.  They  were  com- 
pletely covered  with  small  dentinal  plates,  without  multipolar  lacunae,  which  prob- 
ably represented  the  fragmented  outer  layer  of  the  primitive  exoskeleton.  Orbits 


FIG.  243. — A,  Restored  outlines  of  Lasanius  problematicus,  Iraq ;  B,  Birkenia  elegans,  Traq;  C,  Thelodus 
scoticus  Traq.;  D,  Lanarkia  spinosa  Traq.;  E,  sagittal  section  of  a  primitive  dermal  denticle  (Coelolepis  Schmidti) 
A-D.  after  Traquair;  E  after  Rohon. 

may  be  covered  with  bony  plates.  Small  marginal  openings  indicate  the 
location  of  gill  clefts  or  the  points  of  attachment  of  the  cephalic  appendages. 

Coslolepidae. — Resembling  somewhat  the  cephalaspids  in  form,  and  covered 
with  separate  rounded,  or  conical,  denticles.  Upper  Silurian,  passage  beds. 
Thelodus.  (Fig.  243,  C.)  Lanarkia.  (Fig.  243,  D.) 

Birkeniidae. — Fishlike  contour,  with  oblong  tuberculate  plates.  A  series  of 
branchial  openings,  like  those  in  Tremataspis.  Birkenia.  (Fig.  243,  B.)  La- 
sanius. (Fig.  243,  A.) 

III.  PTERASPIDA. 

Head  sagittate,  or  oval,  consisting  of  a  small  number  of  large  plates. 
Subdivisions  of  head  united,  forming  a  common  cephalic  buckler  without 


THK    PTERASPIDA. 


mm. . 


en 


FIG.  244. — A,  Restored  outline  of  the  cephalic  shield  and  cephalic  appendages  of  Cyathaspis,  seen  from  the 
neural  surface,  based  largely  on  Lindstrom's  specimen  from  the  Silurian  rocks  of  Gothland;  B  and  C,  partly  restored 
outlines  of  two  individuals  of  Palaeaspis  (male  and  female?) from  Perry  Co.,  Pa.  The  shell  has  been  removed,  showing 
a  cast  of  the  inner  neural  surface.  About  natural  sice. 


-do. 


FIG.  245. — Cephalic  buckler  of  Pteraspis,  neural  surface.     (After  Lankester.slightly  modified)  About  natural  size. 


366 


THE    OSTRACODERMS. 


lateral  eye  openings.     Gills  enclosed  in  peribranchial,  or  atrial  chamber.     Mouth 
parts  unknown.     Body  plates  small,  rhomboidal. 

Pteraspidse. — Dermal  armor  ornamented  with  minute  dentinal  ridges  and 
grooves,  parallel  with  the  margins  of  the  various  plates.  Matrix  sharply  laminate, 
with  numerous  unbranched,  parallel  pore  canals,  terminating  in  spindle-shaped 
unipolar  lacunas.  Cancellae  large,  rectangular,  and  in  a  single  layer.  Parietal 
eye  lodged  in  a  hollow  tubercle,  prominent  externally,  and  with  the  cavity  opening 


FIG.  246. — Restoration  of    Drepanaspis  gemtindenensis,  Schl.     (After  Traquair;    slightly  modified.) 

surface;  B,  neural  surface. 


A,  Haemal 


inward.  Cephalic  appendages,  where  known,  large,  armored.  Oblong  marginal 
openings  on  dorsal  surface  of  cornual  plates  leading  into  the  interior  of  the  head. 
Upper  Silurian  and  lower  Devonian.  Six  to  nine  inches  long.  Pteraspis.  Kner. 
(Fig.  245.)  Palaeaspis,  Claypole.  Oldest  member  of  the  ostracoderms  known  to 
occur  in  America.  Onondaga  Group,  Perry  Co.,  Penn.  (Fig.  244,  B  and  C.) 
Cyathaspis,  Lank.  (Fig.  244,  A.) 

Psamostaedae. — Head,  broad,  flattened;  trunk,  short  and  thick.     Ornament 
minutely  tubercular.     Large  plates  of  the  head  separated  by  rows  of  small, 


THE   ANTIARCHA. 


367 


five-  or  six-sided  plates.  Peribranchial  chamber  opening  on  the  posterior  lateral 
side  of  the  head,  between  the  posterior  lateral  and  the  posterior  ventro-lateral 
plates.  Devonian.  Drepanaspis,  Schltiter.  (Fig.  246.) 

IV.  ANTIARCHA. 

Head  oval,  pentagonal  in  cross-section.  Mesocephalon  united  with  branchio- 
cephalon  by  a  transverse,  movable  joint.  Cephalic  armor  consisting  of  large 
plates;  separate,  but  with  little  or  no  movement.  Trunk  slender,  membranous 
or  scaly;  triangular  in  cross-  section,  with  distinct  but  narrow  lateral  folds.  Two 
dorsal  fins,  membranous,  with  or  without  supporting  rays.  Tail  long,  ending 
in  a  narrow  ribbon.  Gancellae  in  one  or  many  layers,  often  small  and  irregular. 
Dermal  armor  ornamented  with  tubercles  or,  with  concentric  tuberculate  ridges. 
Dentine  layer  ill  defined,  thin,  or  absent.  Lacunae  highly  developed,  multipolar, 
and  extending  close  to  outer  surface.  Large  atrial  and  pre-oral  chambers. 


The  best  known  form  is  Bothriolepis  canadensis,  Whiteaves.  The  author 
has  secured  a  large  number  of  these  fossils,  splendidly  preserved.  Large  slabs 
were  obtained  showing  many  entire  individuals  in  their  exact  attitudes  and 
surroundings  at  the  time  of  death.  With  this  material  at  hand,  which  in  abun- 
dance and  in  perfection  of  preservation  has  never  been  equalled,  it  has  been 
possible  to  form  a  very  accurate  idea  of  the  structure  and.  mode  of  life  of  this 
most  interesting  animal. 

The  details  of  its  anatomy  have  been  worked  out  by  means  of  serial  sections 
and  by  other  methods.  They  will  be  reserved  for  a  separate  publication;  we 
have  space  here  for  only  the  points  of  general  interest,  or  those  bearing  on  the 
subject  under  discussion. 

Exoskeleton.  —  The  outer  surface  of  the  dermal  armor  was  ornamented  with 
low  rounded  tubercles,  arranged  in  concentric  rows,  often  parallel  to  the  margins 
of  the  separate  plates,  or  forming  wavy,  crenulate  ridges. 

The  dorsal  mesocephalic  shield  moves  up  and  down,  to  a  limited  extent,  on 
the  hinge-like  joint  connecting  it  with  the  branchiocephalon.  This  movement 
is  made  possible  by  the  tilting  of  the  suspensory  or  suborbital  plate,  whose  thick, 
ventral  edge  rests  on  the  anterior  lateral  margin  of  the  fixed,  ventral  shield  (an- 
terior ventro-laterals).  (Figs.  253,  s.o,  259,  A.)  When  the  mesocephalon  is  de- 
pressed, the  dorsal  edge  of  this  plate  swings  inward,  without  dislocating  the  plate, 
through  an  angle  of  almost  forty-five  degrees.  In  partly  crushed  heads  it  is 
forced  into  an  abnormal,  horizontal  position,  and  lies  inside  the  head  with  its 
lateral  surface  turned  dorsally. 

As  the  ventral  edge  of  the  suspensory  plate  fits  into  a  shallow  groove,  or 
ridge,  its  posterior  end  cannot  normally  swing  bodily  outward,  after  the  fashion  of 
an  operculum,  as  one  might  at  first  sight  suppose.  In  fact,  there  is  no  passage- 


368 


THE    OSTRACODERMS. 


way  into  the  head  at  this  place  that  could  be  opened  and  closed  by  the  movements 
of  this  plate,  and  there  is  no  reason  for  regarding  it,  either  functionally,  or  morpho- 
logically, as  an  operculum. 


FIG.  247. — -Restoration  of   Bothriolepis  canadensis  (Whiteaves;  from  Miguasha,  P.  Q.,  Canada.)      Neural  surface; 

about  1/2  natural  size. 

The  anterior  end  of  the  suspensory  plate  is  narrow  and  slightly  curved, 
and  adjoins  a  small  quadrangular,  movable  plate  that  probably  represents  the 
maxillary  plate  of  the  arthrodira.  In  depressed  heads,  it  is  bent  over  onto  the 


THE   ANTIARCHA. 


oral  surface,  and  in  side  views  it  then  appears  to  be  absent.  A  branch  of  the 
suborbital  line  extends  over  this  plate,  apparently  connecting  with  the  sensory 
groove  of  the  premaxilla.  (Figs.  254,  262,  moc.) 


FIG.  248. — Restoration  of  Bothriolepis  canadensis.      (Whiteaves.)     Hie mal  surface.      About  1/2  natural  size. 

The  remaining  plates  of  the  pro-  and  meso-cephalic  shield,  except  those 
in  the  orbital  opening,  are  practically  immovable,  and  are  frequently  found  as  one 
piece,  even  when  all  the  other  plates  have  separated  through  maceration. 

The  branchiocephalon  enclosed  the  gills,  atrial  chamber,  viscera,  and  repro- 

24 


370 


THE    OSTRACODERMS. 


FIG.  249. — Photograph  of  a  small  group  of  Bothriolepis.  The  individual,  a,  after  death,  came  to  lie  haemal 
side  up,  on  what  was  the  bottom  at  that  time;  its  dorsal  fin  and  tail  lie  in  a  horizontal  plane.  Individuals,  b  and  c. 
died  in  their  natural  position;  that  is,  buried  in  the  mud,  haemal  side  down.  Their  dorsal  fins  and  tail  lobes  are 
in  a  nearly  vertical  position. 


FIG.  250. — Photographs  of  Bothriolepis,  seen  from  the  haemal  surface,  showing  the  conspicuous  membranous  frills 
surrounding  the  exhalent  opening  of  the  atrial  chamber. 


THE   ANTIARCHA. 


371 


ductive  organs.  The  anterior  part  of  the  trunk 
and  the  branchial  portion  of  the  head  was  sus- 
pended from  the  roof  of  the  branchiocephalic 
shield  by  stout  fibrous  tissue  attached  to  two 
deep  median  processes,  or  ridges  of  cancellous 
bone,  that  projected  downward  and  forward 
(Fig.  251,  pr.)  The  sides  and  ventral  surface, 
were  unattached  and  were  surrounded  by  a 
spacious  peribranchial,  or  atrial  chamber,  which 
opened  outward  between  the  sides  of  the  body 
and  the  ventrolateral  walls  of  the  branchiocep- 
halic buckler. 

Atrial  Frill. — The  lips  of  the  atrial  opening 
were  guarded  by  a  membranous  frill  (Figs.  247, 
248) .  Its  ventral  portion  was  broad,  often  folded 
or  pleated,  and  sometimes  marked  by  faint,  longi- 
tudinal lines.  It  appears  to  project  from  the 
inside  of  the  chamber  and  to  be  attached  to  its 
ventral  wall  by  a  faint  transverse  ridge.  The 
dorsal  portion  was  much  narrower,  and  appeared 
to  be  a  membranous  extension  of  the  posterior 
and  lateral  margin  of  the  shield.  The  contents 
of  the  branchiocephalon  are  never  squeezed  out 
of  the  posterior  end  of  the  buckler.  Sections 
of  many  different  specimens  always  show  the  gills 
and  stomach  in  their  proper  position,  even  when 
the  shields  are  crushed  almost  flat.  If  the  animal 
lies  ventral  side  down,  the  viscera  always  lie 
on  the  ventral  wall.  If  it  died  ventral  side  up, 
they  lie  on  the  dorsal  shield,  and  in  all  cases  well 
in  front  of  the  atrial  opening. 

Gills. — Seven  pairs  of  broad  lamellate  gills 
were  present  on  the  sides  of  the  head,  in  about 
the  center  of  the  branchiocephalon.  They 
appear  as  seven  pairs  of  thin,  parallel,  black 
lines,  somewhat  wavy,  with  minute  black  spots. 
(Fig.  256.)  A  more  conspicuous  dark  band,  or 
a  clear  space,  separates  the  gills  of  one  side  from 
those  of  the  other.  Altogether  they  form  an  ob- 
long nodule  about  an  inch  and  a  half  to  two 
inches  long,  one  inch  broad,  and  from  an  eighth 
to  a  quarter  of  an  inch  thick.  The  nodule 
always  consists  of  a  peculiar  fine-grained,  soft 


372 


THE    OSTRACODERMS. 


matrix  not  visibly  affected  by  acids.  The  part  of  the  atrial  chamber  not 
occupied  by  the  remnants  of  the  viscera  is  always  filled  with  a  coarse  grained  sand 
that  evidently  worked  its  way  in  through  the  atrial  opening  after  the  animal  died. 
When  treated  with  dilute  acids,  this  sandy  matrix  becomes  friable  and  can  often 
be  easily  worked  away,  exposing  the  contour  of  the  gills  and  viscera. 


FIG.  252. — Head  of  Bothriolepis.  A,  The  bony  cranial  plates  have  been  etched  away,  leaving  a  natural  cast 
of  the  inner,  neural  surface  of  the  head;  B,  bony  plates  covering  the  neural  surface  of  the  head,  seen  from  the 
inside.  The  matrix  has  been  chiseled  away.  Photographs;  slightly  restored. 


Km.    IT,'' 


FIG.   253.- — Transverse  sections  of  the  head  of  Bothriolepis.     The  dermal  bones  black;  the  coarser,  more  sandy 
matrix,  dotted;  the  softer  matrix    of  a  chalky  consistency,  shaded.      Slightly  diagrammatic. 

Viscera.— Above,  or  to  one  side  of  the  gills  was  a  large  oblong  stomach,  the 
contour  of  which  was  conspicuous,  owing  to  its  carbonized  contents.  The  cloaca 
was  guarded  by  a  thin  rounded,  cloacal  scale  that  is  faintly  tuberculate  on  its  free 
or  ventral  surface  and  with  concentric  lines  on  its  dorsal  surface.  The  cloaca 
opened  into  the  posterior  part  of  the  atrial  chamber  about  half  an  inch  in  front  of 
the  posterior  margin  of  the  ventral  shield.  (Fig  251,  cl.). 


THE   ANTIARCHA. 


373 


The  pre-oral  chamber  lies  on  the  anterior  ventral  surface;  its  rim  is  formed 
by  the  anterior  margins  of  the  dorsal  and  ventral  shields,  which  were  apparently 
fringed  with  short  fleshy  papillae.  Across  the  roof  of  the  opening  there  is  a  tough 
circum-oral  membrane  in  which  are  imbedded  the  large  premaxillary  plates, 
the  mandibles,  and  the  two  narrow  bony  bands  that  covered  the  hyoids.  The 
membrane  extends  backward,  underneath  the  anterior  ventro-laterals,  as  far  as 
the  transverse  ridge  on  their  inner  surface,  to  which  it  appears  to  be  attached 
(Fig.  251,  254,  om.) 


p.s 


FIG.  254. — E,  The  oral  region  of  Bothriolepis.  On  the  right,  the  anterior  ventro-lateral  plates  have  been  re- 
moved exposing  the  dermal  armor  of  the  double  hyoid  arch,  imbedded  in  the  circumoral  membrane.  The  latter  is 
continuous  with  the  ridge,  b,  on  the  inner  surface  of  the  anterior  ventro-laterals.  A ,  The  mandibles  of  Bothriolepis, 
with  the  smooth,  toothless  margin  rotated  outwards;  B,  same,  rotated  inwards;  C,  longitudinal,  vertical  section, 
a  little  to  one  side  of  the  median  line,  showing  the  pre-maxillary  and  mandibular  plates  in  their  normal  positions; 
D,  cross-section  of  the  premaxillae  in  the  oral  region,  showing  the  thickened,  crushing,  or  cutting  edges  of  the 
premaxillary  plates. 

The  premaxilla  are  thin,  concave  plates  of  dermal  bone  continuous  with  the 
oral  membrane  on  the  sides  and  in  front,  but  with  free  median  and  posterior 
margins.  (Fig.  254.)  The  exposed  surface  presents  the  characteristic  surface 
ornament,  together  with  a  sharply  bent,  sensory  groove.  The  rounded  posterior 
margin  is  smooth  and  bevelled,  ending  in  an  extremely  thin  edge,  nearly  the 
whole  length  of  which  is  broken  into  minute  irregular  tooth-like  serrations.  The 
median  margin  is  very  thick,  with  sharpened  edges  which  in  the  older  individuals 
become  rounded  or  smooth  through  use.  The  anterior  margin  has  a  prominent 
shoulder;  it  is  uniformly  thin  and  rounded,  and  is  continuous  with  the  oral  mem- 
brane. The  lateral  margin  is  narrow  and  slightly  concave;  a  broad  spur  extends 


374 


THE    OSTRACODERMS. 


laterally  and  inward  on  the  visceral  side  of  the  oral  membrane,  serving,  no  doubt, 
for  the  attachment  of  muscles,  s.p. 

The  visceral  aspect  of  the  premaxillae  is  nearly  smooth,  except  for  a  very 
prominent,  transverse,  sharp-edged  ridge,  which  evidently  served  for  the  attach- 
ment of  muscles  that  moved  them  in  a  median  or  lateral  direction.  (Fig.  254,  C.,ir.) 

The  premaxillae  moved  independently  to  and  from  the  median  line,  bringing 
their  stout  crushing  or  biting  edges  together.  They  are  sometimes  found  in  a 
nearly  vertical  position,  or  even  thrown  forward  in  front  of  the  head,  with  their 
ventral  surfaces  facing  dorsally.  Thus  it  is  probable  that,  like  two  great  lids  or 
covers,  they  could  swing  forward  and  backward  on  the  muscles  and  the  mem- 
brane attached  to  their  anterior  visceral  surface;  it  is  improbable  that  normally 
they  ever  passed  beyond  the  vertical  position  during  life.  (Fig.  251.) 


FIG.  255. — The  ocular  and  olfactory  plates  of  Bothriolepis,  enlarged.   A  part  of  the  rostral  and  lateral  plates  have 
been  removed  on  the  left,  in  order  to  expose  the  deeper  lying  sclerotics  and  the  inner  end  of  the  ethmoid. 

The  mandibles  are  peculiar  S-shaped  plates  lying  behind,  or  more  frequently 
underneath,  or  dorsal  to,  the  premaxillae.  (Figs.  248,  251,  254,  md.)  The 
median  end  of  each  mandible  has  a  smooth,  rounded,  anterior,  or  ventral  edge, 
and  a  finely  ornamented,  convex  outer  surface.  Its  visceral  surface  is  deeply 
concave.  The  lateral  arm  of  the  mandible  is  narrow  and  smooth  and  lies  inside 
the  oral  membrane. 

The  mandibles  are  usually  widely  separated  in  the  median  line,  each  being 
quite  independent  of  the  other;  they  are  held  in  place  by  the  tough  skin  in  which 
their  median  ends  are  imbedded.  They  appear  to  have  had  a  free  rotary  move- 
ment, their  ventral  edges  swinging  forward  and  backward;  at  the  same  time, 
their  median  ends  could  be  drawn  together  and  thrown  forward. 

The  difference  in  position  and  structure  between  the  mandibles  and  pre- 
maxillae makes  it  improbable  that  one  pair  acted  directly  against  the  other.  The 
mandibles  probably  pushed  the  food  forward  and  inward,  where  it  could  be 
crushed  or  cut  between  the  stout  margins  of  the  premaxillae,  after  the  manner 
that  prevails  among  the  arthropods. 


THE   ANTIARCHA.  375 

Hyoid  Arches. — Back  of  the  mandibles,  the  circumoral  membrane  is 
strengthened  by  two  bands  of  dermal  armor.  They  are  thin  and  delicately  orna- 
mented ,  and  when  the  head  is  in  a  normal  position  the  posterior  ba  nd  is  entirely,  and 
the  anterior  one  partly,  overlapped  by  the  anterior  ventral  plates.  The  narrow 
anterior  band  consists  of  five  or  six  segments.  The  posterior  one  is  unsegmented. 
Both  bands  are  attached  to  a  large  lateral  plate,  the  lateral  end  of  which  is  bent 
at  right  angles,  and  attached  to  the  lateral  walls  of  the  head.  (Figs.  253,  254,  h.m.) 

The  mouth  was  a  slit-like  opening  situated  between  the  premaxillae  and  in 
front  of  the  membrane  uniting  the  two  mandibles. 

The  oral  membrane  with  the  attached  premaxillae  and  mandibles  could  be 
protruded  a  short  distance,  or  withdrawn  into  the  broad  but  shallow  pre-oral 
chamber. 

The  Eyes  and  Olfactory  Organs. — The  oval  opening  on  the  anterior  part  of 
the  dorsal  shield  contains  the  stalked  lateral  eyes,  the  parietal  eye,  and  the  olfactory 
organ.  These  organs  were  wholly  or  partly  surrounded  by  small  bony  plates 
held  in  place  by  tough  but  flexible  membranes.  (Fig.  255.) 

The  lateral  eyes  were  enclosed  in  short,  rounded  eye  stalks  that  were  attached 
to  the  margin  of  the  parietal  and  olfactory  plates  by  hinge-like  joints  so  that  the 
crescent-shaped  eye  openings  on  the  distal  ends  of  the  stalks  could  be  raised  or 
lowered  into  the  orbits.  The  exoskeleton  of  each  eye  stalk  consisted  of  a  thick 
posterior  dorsal,  d.s.,  a  large  anterior  ventral,  a.s.,  and  two  ventro-lateral  plates, 
l.s.  When  the  posterior  dorsal  plate  was  level  with  the  shield,  the  corneal  opening 
was  concealed  within  the  orbits. 

Parietal  Eyes. — Between  the  lateral  eyes  is  a. thick  quadrangular  plate  that 
is  nearly  perforated  by  a  deep  conical  pit,  opening  inward,  and  covered  externally 
by  a  thin-walled,  lens-like  tubercle.  This  pit  contained  the  anterior  parietal  eye. 
There  are  two  similar  pits,  but  not  so  deep,  on  the  inner  surface  of  the  small  post- 
orbital.  There  is  no  indication  of  their  presence  on  the  external  surface. 

Olfactory  Organs. — The  posterior  end  of  the  rostral  plate  divides  into  an 
inner  and  an  outer  lamella,  enclosing  a  wide  triangular  chamber  between  them. 
(Fig.  251,  r.)  Just  above  the  edge  of  the  inner  lamina  is  a  small,  T-shaped 
ethmoid.  It  stands  nearly  vertically,  with  its  dorsal  transverse  bar  attached  to 
the  anterior  edge  of  the  pineal  plate.  (Fig.  255,  e.)  When  the  anterior  face  of 
the  ethmoid  is  exposed,  it  is  seen  that  its  expanded  arms,  to  which  are  attached 
rod-like  lateral  ethmoids,  I.e.,  and  the  pedicle  form  the  lateral  walls  of  two  rounded 
depressions  in  which  the  paired  olfactory  sacs  were  probably  located.  In  the 
bottom  of  each  pit  is  a  circular  opening,  leading  into  the  interior  of  the  head, 
and  serving  for  the  passage  of  the  olfactory  nerves. 

All  these  plates  are  held  together  and  attached  to  the  sides  of  the  sensory 
opening  by  tough  but  flexible  membranes,  leaving  a  relatively  large  space  for 
the  movements  of  the  organs.  It  is  clear,  from  the  various  positions  of  the  plates 
in  different  specimens,  that  the  parietal  plate  could  move  caudally  for  several 
millimeters,  drawing  the  ethmoid  backward  and  upward,  thus  enlarging  the  open- 


376  THE    OSTRACODERMS. 

ing  to  the  olfactory  chamber.  When  the  outer  end  of  the  ethmoid  was  pushed 
forward  against  the  posterior  wall  of  the  rostral  plate,  two  narrow  passages  would 
still  remain  open,  leading  from  the  olfactory  pits  to  the  exterior.  (Fig.  259,  A.) 

The  olfactory  pits  of  Bothriolepis  correspond  to  the  antorbital  fossae  of  the 
aspidocephali,  indicating  that  the  union  of  the  median  and  lateral  eyes  and  the 
olfactory  organs  into  a  compact  median  dorsal  group  of  sense  organs  is  very 
characteristic  of  the  ostracoderms. 

Sensory  Grooves. — The  cutaneous  sense  organs  were  located  in  distinct  open 
grooves.  (Figs.  259,  A,  262.)  There  is  a  main  suborbital,  r.L,  united  in  front 
in  adults,  but  separate  in  the  young;  a  V-shaped  orbital  line  usually  connecting 
with  the  suborbitals  in  front,  and  at  the  posterior  end,  in  immature  specimens, 
leading  into  the  endolymphatic  ducts;  a  V-shaped  posterior  branchial,  b.L,  and  a 
lateral  line,  /./.,  extending  backward  along  the  sides  of  the  branchiocephalon, 
onto  the  sides  of  the  trunk. 


o.pr. 


pi. 

FIG.  250. — Cross-section  of  Bothriolepis  in  the  branchial  region,  showing  the  seven  pairs  of  plate-like  gills. 

The  cephalic  appendages  are  large,  completely  armored,  and  consist  of  two 
movable  joints.  The  proximal  one  is  covered  with  six  plates;  it  is  triangular  in 
cross-s'ection,  rounded  dorsally,  flat  ventrally,  with  a  sharp  tuberculate  anterior 
margin  and  a  thick,  flat,  posterior  one.  (Figs.  247,  248.)  An  opening  at  the 
proximal  end  of  the  posterior  wall  and  an  adjacent  opening  in  the  side  of  the 
branchiocephalon  served  for  the  passage  of  blood-vessels  and  nerves  into  the 
interior  of  the  appendage.  (Fig.  262.)  The  dorsal  and  ventral  proximal  plates 
formed  a  rounded  articular  head  that  fitted  into  a  socket  in  the  anterior  ventro- 
laterals.  From  the  center  of  the  socket  a  fixed  bony  rod  with  an  expanded 
head  projected  into  the  interior  of  the  appendage,  thus  holding  it  in  place  after 
many  of  the  softer  tissues  had  completely  macerated.  (Fig.  254,  a.)  This  bony 
rod  is  continuous  with  an  axial  plate,  or  bar,  of  fibrous  or  cartilaginous  tissue 
that  extends  into  the  proximal  end  of  the  appendage.  The  distal  joint  of  the 
appendage  was  movable  in  a  horizontal  plane  only.  It  was  hollow,  oval  in  cross- 
section,  with  recurved  marginal  spines,  and  covered  with  numerous  polygonal 
plates. 


THE   ANTIARCHA.  377 

Habitat.    Mode  of  Life. 

Mode  of  Preservation. — Nodules  containing  the  heads,  or  other  fragments  of 
Bothriolepis  may  be  found  on  the  beach,  at  the  foot  of  the  gray  cliffs  in  Miguasha, 
near  Dalhousie,  New  Brunswick;  but  the  best  specimens,  showing  the  whole  body, 
can  only  be  obtained  from  the  unweathered  strata  in  the  cliffs. 

The  greater  part  of  my  material  was  obtained  from  a  small  " table"  about 
twenty  or  thirty  feet  in  diameter  and  sixteen  to  eighteen  inches  thick.  The  edge 
of  the  table  had  been  exposed  by  the  wearing  away  of  the  face  of  the  cliff,  showing 
two  or  three  layers  very  rich  in  fossils.  The  overlying  rocks  were  blasted  out, 
or  worked  out  with  pick  and  bar,  as  far  as,  and  indeed  farther  than  it  was  safe 
to  work  into  the  face  of  the  crumbling  cliff.  We  apparently  reached  the  inner 
limits  of  the  table,  and  succeeded  in  removing  practically  all  the  specimens  con- 
tained in  it. 

Some  of  the  beds  were  crowded  with  fossil  ferns  and  Bothriolepis,  with  here 
and  there  a  specimen  of  Scaumenacia,  Holoptychius,  and  other  fishes.  The 
animals  in  the  richest  beds  were  badly  crushed,  and  the  more  delicate  parts  were 
obscured  by  the  abundant  remains  of  plants  and  carbonized  organic  material. 
In  some  of  the  other  beds,  where  the  rock  was  cleaner  and  more  sandy,  there 
were  fewer  fossils,  but  they  were  not  so  badly  crushed.  In  these  layers  there 
were  many  specimens  of  Bothriolepis  with  the  entire  body  distinctly  preserved 
in  the  attitude  or  position  they  were  in  at  the  moment  they  ceased  to  live. 

The  table  was  no  doubt  at  one  time  the  bottom  of  a  small  seashore  pool,  or 
inlet,  in  which  shallow-water  plants  were  growing.  It  was  either  invaded,  at 
periods  of  exceptionally  high  tides,  by  salt  water  that  carried  with  it  many  fishes 
and  ostracoderms,  or  it  was  from  time  to  time  cut  off  from  the  main  body  of  water 
by  shifting  sand  bars,  or  by  similar  causes.  In  either  case,  the  animals  trapped 
in  the  enclosures  soon  died  from  the  effects  of  the  foul  or  superheated  waters,  and 
were  shortly  afterward  covered  up  and  preserved  by  the  shifting  beach  sands  of 
another  invasion. 

The  great  majority  of  the  specimens  of  Bothriolepis  were  found  in  a  hori- 
zontal position,  ventral  side  down,  and  headed  in  the  same  direction,  i.e.,  a  little 
north  of  east.  These  individuals  evidently  died  in  this  position,  oriented  by  some 
common  external  agent. 

In  nearly  all  such  cases  the  slender  trunk  and  tail  extend  in  a  straight  line 
backward,  while  the  thin  membranous  dorsal  and  caudal  fins,  which  were  usu- 
ally perfectly  flat  and  fully  expanded,  stood  in  a  vertical  direction,  i.e.,  at  right 
angles  to  the  plane  of  stratification.  The  upright  position  of  such  delicate  mem- 
branes shows  beyond  question  that  these  particular  animals  died  quietly  while 
still  in  their  natural  positions.  The  fact  that  these  animals  were  living  partly 
buried  in  mud  or  sand  up  to  the  time  of  their  death,  no  doubt  accounts  for  their 
wonderful  state  of  preservation.  Side  by  side  with  them  were  other  specimens 
lying  either  on  their  back  or  side,  with  the  dorsal  fins  and  tail  expanded  in  a  hori- 


378 


THE    OSTRACODERMS. 


zontal  plane.  They  had  either  fallen  to  the  bottom  in  this  position  when  ex- 
hausted, or  turned  over  in  the  death  struggle,  so  that  the  trunk  and  fins  laid 
flatwise  on  the  upper  surface  of  the  muddy  bottom.  One  specimen  was  found 
standing  on  its  head,  almost  vertically.  It  had  evidently  been  swimming  at  some 
speed,  when  taking  a  sudden  turn,  it  struck  the  soft  bottom,  head  first,  with  suffi- 
cient force  to  stick  there  in  an  upright  position.  This  individual  was  not  in  the 


FIG.  257. — Photographs  of  three  contiguous  slabs,  from  approximately  the  same  level,  and  with  the  same 
orientation.  The  slabs  are  seen  from  their  under  surfaces,  and  show  the  uniformity  in  the  orientation  of  the  numer- 
ous specimens  of  Bothriolepis.  All  are  in  a  natural  position,  and  are  headed  in  the  same  direction,  except  four. 
Of  these,  two  have  the  ocular,  or  neural  surface  exposed  (i.e.,  they  lie,  haemal  side  up,  when  the  slab  is  in  its  original 
position) ;  one  lies  on  its  side;  and  one,  a,  lies  with  its  head  toward  the  reader;  its  tail  is  gracefully  curved  toward 
the  upper  right-hand  corner,  apparently  by  a  gentle  current  of  water,  that  turned  the  tops  of  the  enclosed  water 
plants  in  the  same  direction. 

least  flattened  in  a  dorso-ventral  direction,  and  when  sectioned  showed  the  re- 
mains of  the  viscera  settled  down  in  the  lower  or  anterior  end  of  the  head.  In 
all  other  cases  the  viscera  were  found  lying  either  on  the  dorsal  or  on  the  ventral 
wall  of  the  branchiocephalon,  according  to  the  side  that  happened  to  be  upper- 
most when  the  animal  died. 


THE   ANTIARCHA. 


379 


In  the  large  slabs  containing  many  ferns  or  plant  stems,  it  was  clearly  shown 
that  the  plants  were  laid  down  in  nearly  parallel  lines  with  the  tops  turned  in  the 
same  direction,  as  though  at  the  time  they  were  deposited  they  had  been  bent 
over  by  a  slow  current  of  water. 

One  of  the  larger  slabs,  containing  ferns  and  Bothriolepis  in  great  numbers, 
is  specially  instructive.  (Fig.  257.)  It  shows  most  of  the  fern  tops  turned  in  one 
direction,  with  most  of  the  Bothriolepis  heads  turned  in  nearly  the  opposite 
one,  their  thin,  soft  bodies  extending  in  straight  lines  backward.  But  one  speci- 
men, A,  is  lying  on  its  back  in  a  direction  diagonal  to  the  fern  tops,  thus  show- 
ing that  when  this  individual  died  it  fell  on  the  bottom  oral  side  up;  that  the 
current  then  turned  it  partly  around,  bending  its  tail  in  a  gentle  curve  in  the 
same  direction  as  the  fern  tops. 

In  some  cases  several  individuals  of  Bothriolepis  were  found  close  together, 
and  at  different  levels,  but  nevertheless  all  turned  in  the  same  direction,  showing 
that  they  were  probably  oriented  by  the  same  agents  and  died  at  the  same  time. 
From  these  facts  we  may  infer  that  they  were  moving  along  the  soft  bottom, 
some  completely  covered  with  mud  or  sand,  others  on  the  surface,  just  as  the 
adult  Limuli  do  when  in  great  swarms  they  come  up  the  sandy  beaches  at  high 
tide  to  lay  their  eggs;  or  as  the  young  Limuli,  when  feeding,  plough  about 
through  the  soft  mud,  from  three  to  six  inches  below  the  surface. 

Locomotion. — It  is  evident,  therefore,  that  Bothriolepis  was  a  bottom  feeder, 
moving  about  on  its  flat  oral  side,  either  covered  by  soft  sand  or  mud,  or  leaving  only 
its  projecting  eyes  and  dorsal  surface  exposed.  But  it  is  evident  that  it  was  also 
a  free  swimming  form,  using  both  its  flexible  tail  and  trunk  and  its  large  cephalic 
appendages  for  that  purpose.  They  probably  swam,  with  their  flat  oral  side  up- 
permost, by  powerful  backward  strokes  of  their  cephalic  appendages,  just  as  the 
eurypterids  and  possibly  many  trilobites  did  in  the  palaeozoic  seas,  and  as  many 
phyllopods,  entomostrica,  or  indeed  as  adult  Limuli  continue  to  do  to-day. 

It  will  be  observed  that  the  appendages  are  attached  well  forward  on  the 
margin  of  the  flat  and  narrow  ventral  surface;  that  the  head  is  quite  thick,  and 
the  dorsal  surface  wide  and  strongly  arched.  A  body  shaped  like  this  would 
naturally  move  through  the  water  like  a  boat  right  side  up.  It  is  evident  that 
Bothriolepis  could  not  be  driven  through  the  water,  dorsal  side  up,  without  a 
strong  tendency  to  pitch  downward  head  first,  or  to  roll  over.  As  the  cephalic 
appendages  were  very  narrow  and  had  little  dorso-ventral  movement,  they  could 
hardly  succeed  in  counteracting,  or  preventing,  that  tendency.  Hence  it  is  clear 
that  the  animal  had  to  swim  with  its  dorsal  side  down,  and  with  its  center  of 
gravity  below  the  level  of  the  supporting  appendages.  In  this  position  equilib- 
rium could  easily  be  maintained  either  by  the  arms  or  by  the  tail,  and  the  curved 
anterior  surface  of  the  head,  according  to  the  rate  of  its  forward  movement,  would 
greatly  aid  in  uplifting  the  cumbersome  head  in  the  water. 

Food. — Well  preserved  specimens  always  contain,  in  addition  to  the  rem- 
nants of  other  soft  parts,  a  thick  oval  disc  of  carbonaceous  material,  that  undoubt- 


380  THE    OSTRACODERMS. 

edly  represents  the  contents  of  the  stomach.  A  microscopic  examination  failed 
to  reveal  any  definite  structures  in  it,  such  as  diatomes  or  fragments  of  bones  or 
shells.  It  has  the  same  appearance  as  the  remnants  of  plants  seen  outside  the 
body,  and  like  them  readily  burns  when  heated,  leaving  a  whitish  ash.  We  may 
therefore  infer  that  Bothriolepis  fed  on  the  ferns  or  other  water  plants  that  were 
so  abundant  in  the  places  where  they  have  been  found.  Such  a  diet  was  appa- 
rently better  suited  to  the  peculiar  structure  of  their  jaws  than  any  other. 

We  therefore  again  come  to  the  same  conclusion  that  was  reached  in  another 
way,  namely  that  there  is  a  close  resemblance  between  the  ostracoderms  and 
the  amphibia,  for  Bothriolepis,  with  its  big  head  and  small  tail,  its  vegetable  diet, 
the  peculiar  action  of  its  lower  jaws  white  feeding,  and  in  its  general  mode  of  life, 
greatly  resembles  the  tadpole  larva  of  the  common  frog.  The  most  striking 
difference  between  them  is  the  apparent  absence  in  the  tadpole  of  the  large 
cephalic  appendages,  but  these  organs  are  probably  represented  by  the  sucking 
discs,  which  in  turn  are  comparable  with  the  "balancers"  of  Amblystoma  and 
other  urodeles  (Fig.  169),  and  with  the  long  filamentous  cephalic  appendages 
of  Dactylethra  or  Zenopus.  (Fig.  170.) 


CHAPTER  XXI. 
THE  VERTEBRATES. 

We  are  now  in  a  position  to  see  more  clearly  the  relation  that  the  true  verte- 
brates bear  to  the  ostracoderms.  The  full  recognition  of  the  ostracoderms  as 
the  common  ancestors  of  all  vertebrates  will  prove  to  be  a  fruitful  idea  and  will 
go  far  toward  laying  at  rest  the  obscession  of  the  last  thirty  years  or  more,  that  the 
foundations  of  vertebrate  morphology  rest  on  Amphioxus  and  the  elasmobranchs; 
that  in  them  is  the  beginning  and  the  end,  beyond  which  lies  a  fathomless  abyss. 

It  will  be  a  great  step  forward  if  it  can  be  demonstrated  beyond  a  reasonable 
doubt,  as  I  believe  it  can  be  demonstrated,  that  the  heavily  armored  ostracoderms, 
with  cephalic  appendages  and  a  large  atrial  or  peribranchial  chamber,  form  the 
starting  point  for  all  animals  entitled  to  be  called  vertebrates;  not  the  dermal- 
denticled  shark,  nor  the  naked-skinned  cyclostomes,  nor  the  impotent  remnants  of 
animals  that  constitute  the  acraniates. 

If  we  attempt  precisely  to  define  the  natural  limits  of  the  ostracoderms 
and  their  immediate  descendants,  we  at  once  meet  with  great  difficulties  and 
after  all  we  must  for  the  present  resort  to  arbitrary  definitions.  It  is  doubtful, 
for  example,  whether  we  should,  or  should  not,  exclude  the  ccelolepidae  and  the 
anaspidae  from  the  ostracoderms,  or  whether  we  should,  or  should  not,  include 
with  them  the  arthrodira,  or  even  the  antiarcha.  The  dipnoi  might  be  excluded 
from  the  "fishes"  on  the  ground  that  they  are  really  primitive  amphibians.  If 
so,  we  should  then  have  remaining,  as  representative  fishes,  the  elasmobranchii, 
holocephali,  teleostomi  and  cyclostomes,  forms  very  unlike  in  structure  and  origin. 
However,  while  recognizing  the  inadequacy  of  the  current  terminology,  we  shall 
nevertheless  adopt  it,  as  indicated  in  the  tabular  scheme  of  relationships,  without 
attempting  to  build  up  a  new  one  that  might  express  more  clearly  the  views  herein 
set  forth. 

The  ostracoderms  may  be  briefly  characterized  as  follows:  They  consist 
of  a  comparatively  small  number  of  metameres,  and  are  provided  with  a  highly 
developed  dermal  armor,  cephalic  locomotor  appendages,  paired  jaws,  a  large 
atrial  or  peribranchial,  chamber,  and  with  eyes  and  olfactory  organs  located 
near  the  middle  of  the  aboral  surface  of  the  forehead.  The  form  and  general 
appearance  suggest  that  of  a  trilobite  or  merostome,  or  an  amphibian  tadpole, 
rather  than  that  of  a  true  fish.  Their  negative  characters,  compared  with  those 
of  the  vertebrates,  consist  in  the  presence  of  a  diminutive  notochord,  without 
definite  constrictions  or  thickenings  of  the  sheath  to  form  centra,  and  without 
recognizable  neural  or  haemal  arches,  pectoral  or  pelvic  appendages,  or  teeth. 

It  is  a  remarkable  fact  that  the  descendants  of  the  ostracoderms,  in  their 

381 


382 


THE    VERTEBRATES. 


several  respective  phyla,  have  independently  acquired  the  same  kind  of  organs, 
or  they  pass  through  similar  phases  of  development.  In  other  words,  there  appear 
to  be  present  in  the  ostracoderms  certain  latent  conditions  that  produce,  sooner 
or  later,  the  same  results  in  their  various  descendants,  long  after  the  stock  is 


Arachnids. 


FIG.  258. — Diagram  to  illustrate  the  probable  phylogeny  of  the  arachnid,  ostracoderm,  vertebrate  stock. 

broken  up  into  separate  phyla.  For  example,  in  all  descendants  of  the  ostraco- 
derms the  dermal  skeleton  tends  to  break  up  into  smaller  plates  which  ultimately 
disappear,  leaving  the  skin  naked.  In  the  cyclostomes  and  in  Amphioxus,  the 
dermal  skeleton  probably  disappeared  at  a  very  early  period  in  their  history.  The 
same  thing  took  place,  but  at  a  later  period,  in  the  holocephali;  still  later  in  the 
elasmobranchs,  teleostomi,  and  amphibia.  Teeth,  pectoral  and  pelvic  fins,  neural 


THE    CYCLOSTOMATA.  383 

and  haemal  arches,  and  vertebral  centra  are  not  recognizable  in  the  ostracoderms, 
yet  they  make  their  appearance  in  the  elasmobranchs,  holocephali  and  teleostomi, 
in  each  case  apparently  arising  independently  of  the  other.  There  is  also  a  well- 
marked  tendency  in  all  these  three  groups,  or  in  their  more  remote  descendants, 
for  the  lateral  eyes,  olfactory,  and  auditory  organs,  to  become  greatly  enlarged; 
for  the  eyes  and  olfactory  organs  to  take  up  a  more  lateral  position ;  for  the  paired 
jaws  to  unite;  the  trunk  and  caudal  segments  to  increase  in  number;  the  viscera 
and  anus  to  take  up  a  more  caudal  position,  and  for  the  dental  plates  and  branchial 
chamber  to  disappear. 

At  some  time,  probably  not  later  than  the  Silurian  period,  at  least  three  or  four 
well  denned  phyla  grew  out  of  the  ostracoderms,  as  indicated  in  the  accompanying 
table.  (Fig.  258.)  The  main  line  of  ascent  probably  leads  from  the  typical 
ostracoderms,  through  the  antiarcha  and  arthrodires,  to  the  crossopterygians, 
dipnoi  and  amphibia.  Evolution  along  this  line  is  steady,  comparatively  rapid, 
and  in  every  respect  leads  consistently  upward  to  the  first  air-breathing  land 
vertebrates,  the  culminating  metamorphosis  depending  on  remote  antecedent 
changes  in  the  dermal  skeleton,  appendages,  air  bladder,  and  heart. 

The  remaining  phyla  stand  quite  apart  from  this  main  stem.  They  are 
characterized  by  the  breaking  up  of  the  dermal  armor  into  minute  plates  (elasmo- 
branchs) or  by  their  absence  altogether  (the  later  holocephali);  by  the  absence 
of  an  air  bladder,  branchial  chamber,  and  leg-like  fins.  At  no  time  in  their 
history,  so  far  as  it  is  known,  do  they  show  any  indications  whatever  of  develop- 
ing into  air-breathing  vertebrates.  They  do  not  possess  the  necessary  anatomical 
structures,  and  their  evolution  takes  them  into  quite  other  directions.  The 
cyclostomes  end  in  lampreys;  the  holocephali  in  chimaeras,  and  the  elasmobranchs 
in  sharks  and  rays.  We  may  characterize  the  several  phyla  arising  from  the 
ostracoderms  as  follows: 

I.  CYCLOSTOMATA. 

The  cyclostomes  may  be  regarded  as  one  of  the  earliest  off-shoots  of  the  ostra- 
coderms. We  may  consider  their  chief  characteristics  under  three  heads,  namely 
those  derived  from  the  ostracoderms,  those  gained,  and  those  lost  since  their 
separation  from  them. 

i.  The  cyclostomes  retain  the  following  organs  derived  from  the  ostraco- 
derms: A  median,  practically  unpaired,  olfactory  organ,  merged  with  a  persistent 
hypophysis  that  opens  on  the  dorsal  surface  of  the  head.  An  uncommonly  large 
and  well  developed  parietal  eye.  The  lateral  eyes,  on  the  contrary,  are  relatively 
small,  and  are  very  late  in  acquiring  a  functional  union  with  the  superficial  ecto- 
derm. Hence,  except  for  the  insignificant  visual  power  possessed  by  the  parietal 
eye,  there  is  a  long  post-embryonic  blind  period  corresponding  to  the  permanent 
blind  period  in  some  of  the  earlier  ostracoderms.  It  is  explained  on  the  assump- 
tion that  the  lateral  eyes,  during  the  ostracoderm  stage  in  the  phylogeny  of  the 


384  THE    VERTEBRATES. 

vertebrates,  had  for  the  first  time  been  forced  into  the  brain  chamber  by  the  in- 
folding of  the  medullary  plate,  and  had  not  completely  regained  their  functional 
relations  with  the  outside  world. 

There  are,  in  the  adults  of  some  genera,  three  pairs  of  oral  arches  provided 
with  rudimentary  appendages.  The  arches  are  comparable  with  the  three 
pairs  in  amphibian  embryos,  i.e.,  premaxillae,  maxillae,  and  mandibles,  and  with 
the  three  dental-plate  arches  of  adult  ostracoderms  and  arthrodires.  (Fig.  175.) 
The  true  mouth  is  not  always  circular,  but  may  be  a  narrow,  longitudinal  slit. 
The  oral  region  may  be  surrounded  by  a  wide  papillate  fold  of  ectoderm  that 
forms  a  shallow,  circumoral  antechamber,  or  pre-oral  hood,  comparable  with 
the  membranous  folds  on  the  free  anterior  edges  of  the  dorsal  and  ventral  shields 
of  Bothriolepis. 

The  thyroid  gland,  which  represents  a  liver-like  diverticulum  comparable 
with  that  on  the  haemal  surface  of  the  thoracic  gut  in  arachnids  (Figs.  43,  44, 
181,  182.  308),  is  very  large,  reaching  here  its  maximum  development  in  primi- 
tive vertebrates.  The  peribranchial  chamber  may  be  in  part  retained. 

2.  The  cyclostomes  have  lost,  probably  at  a  very  early  period,  their  an- 
cestral dermal   armor,   including  the  jaw  plates;   also   the  cephalic   swimming 
appendages  and  lateral  folds;  and  the  tadpole-like  form  does  not  appear  in  any 
phase  of  their  development. 

3.  The  cyclostomes  have  failed  to  develop  many  important  organs  that  have 
appeared  in  the  other  descendants  of  the  ostracoderms.     There  are  no  paired 
pectoral  or  pelvic  appendages;  the  notochord  persists  in  a  practically  unmodified 
condition;  no  traces  of  ring-like  calcifications  of  its  sheath  appear,  and  only  the 
most    diminutive    neural  and  haemal  spines  are  developed.     The  air  bladder 
and  teeth  of  the  vertebrate  type  are  absent. 

The  actual  progress  made  by  the  cyclostomes  since  their  separation  from 
the  ostracoderms  is  therefore  insignificant,  the  only  noteworthy  gain  being  in  the 
increased  number  of  body  segments,  giving  additional  freedom  and  facility  of 
locomotion,  and  an  imperfect  adaptation  to  a  parasitic  mode  of  life.  The  cyclos- 
tomes may  therefore  be  regarded  as  very  ancient  animals,  deriving  their  underlying 
primitive  characters  from  the  ostracoderms,  and  owing  the  present  simplicity  of 
their  organization  to  a  precocious  senility  that  was  never  preceded  by  a  vigorous, 
creative  youth. 

II.  THE  ELASMOBRANCHII  AND  HOLOCEPHALI. 

The  Elasmobranchii.— The  advent  of  the  elasmobranchs  is  clearly  fore- 
shadowed in  the  Silurian  period  by  the  appearance  of  the  ccelolepidae  with  their 
fish-like  form  and  shagreen-like  armor.  The  successive  steps  in  the  fragmentation 
and  final  disappearance  of  the  dermal  armor  are  well  shown  in  this  branch  of  the 
ostracoderms.  The  process  begins  in  the  cephalaspidae  with  the  formation  of  well- 
marked,  but  immovable  polygonal  areas,  followed  by  the  appearance  of  the 
small  free  plates  of  Ateleaspis  and  Lasanius,  and  by  the  isolated  dermal  denticles 
of  Thelodus  and  the  elasmobranchs,  which  finally  disappear  altogether  in  their 


THE    ELASMOBRANCHII  AND   HOLOCEPHALI.  385 

naked-skinned,  modern  representatives.  The  primitive  elasmobranchs  quickly 
acquired  a  fish-like  form,  losing  the  extensive  ancestral  peribranchial  chamber, 
and  at  no  phase  of  their  development  showing,  so  far  as  known,  any  trace  of 
a  tadpole  stage. 

The  notochord,  at  an  early  period,  is  invested  and  largely  replaced  by  well- 
developed  cartilaginous  centra,  and  an  elaborate  system  of  more  or  less  calcined 
cartilaginous  gill  bars,  and  neural  and  haemal  arches  is  developed. 

The  lateral  fold  is  generally  retained  for  a  longer  or  shorter  period,  giving 
rise  by  local  enlargement  to  well  defined  pectoral  and  pelvic  fins.  The  parietal 
eye  is  not  conspicuously  developed;  neither  are  the  three  pairs  of  embryonic 
oral  arches,  nor  the  corresponding  appendages.  True  teeth  appear  on  the  upper 
and  lower  margins  of  the  mouth,  but  they  are  not  preceded  by  any  recognizable 
dental  plates.  An  air  bladder  is  absent. 

The  ova  are  very  large,  fertilized  within  the  body,  and  the  males  are  provided 
with  highly  specialized  intromittent  organs  or  claspers. 

The  elasmobranchs,  owing  largely  to  their  well  developed  sensory  and  loco- 
motor  organs,  and  to  their  formidable  jaws  and  teeth,  developed  rapidly  in  efficiency 
during  the  late  palaeozoic  period,  but  they  pass  the  climax  of  their  evolution 
without  producing  a  noticeably  higher  type  of  organization.  While  the  internal 
skeleton  may  be  highly  developed  and  more  or  less  calcified,  it  never  develops 
into  true  bone. 

The  elasmobranchs  are  pelagic  or  deep  water  fishes  rather  than  frequenters 
of  the  shallow  brackish  waters  of  the  shore. 

The  absence  of  an  air-bladder  excluded  the  possibility  of  their  becoming 
air  breathers,  and  the  pectoral  and  pelvic  fins  show  no  signs  of  developing  into 
elongated,  digitate  appendages  suitable  for  locomotion  on  land. 

The  Holocephali  probably  arose  from  the  ccelolepid  branch  of  the  ostraco- 
derms,  developing  along  somewhat  similar  lines  as  the  elasmobranchs,  but 
retaining  certain  features  of  the  parent  stock  not  seen  in  the  latter.  The  noto- 
chord is  persistent,  but  enveloped  by  ring-like  calcifications  of  its  sheath  more 
numerous  than  the  cartilagenous  neural  and  haemal  arches.  They  retain  the 
large  head  and  small  trunk,  or  tadpole  form,  of  the  ostracoderms;  a  peribranchial 
chamber,  and  a  short  body  cavity,  with  the  viscera  and  anus  placed  well  forward. 
Three  pairs  of  dental  plates  are  present,  consisting  of  vascular  dentine  and  grow- 
ing from  persistent  pulps.  They  probably  represent  the  premaxillary,  maxillary, 
and  mandibular  plates  of  the  ostracoderms  and  arthrodires,  and  belong  to  the 
three  pairs  of  primitive  oral  arches  seen  in  amphibian  embryos.  The  general 
shape  of  the  mouth  and  the  form  of  the  jaws  resemble  those  of  the  ostracoderms 
and  amphibian  tadpoles  rather  than  those  of  an  elasmobranch.  No  true  verte- 
brate teeth  are  developed. 

The  primitive  characters  above  mentioned  are  those  of  the  ostracoderms 
and  are  sufficient  to  distinguish  the  chimaeras  from  all  other  adult  vertebrates. 
But  the  cartilaginous  internal  skeleton,  the  large-sized  ova,  the  internal  fertiliza- 
25 


386 


THE    VERTEBRATES 


tion,  the  anal  claspers  of  the  males,  and  the  absence  of  an  air  bladder,  indicate 
an  affinity,  although  probably  a  remote  one,  with  the  elasmobranchs. 

III.  THE  ARTHRODIRA,  TELEOSTOMII,  DIPNOI,  AND  AMPHIBIA. 

The  antiarcha,  at  some  time  probably  not  later  than  the  early  Devonian,  gave 
rise  to  the  arthrodira.     From  the  latter  sprang  the  teleostomii  and  dipnoi,  and 


l.-pl. 


FIG.   259.  FIG.    260.  FIG.   261. 

FIGS.  259,  260,  261. — Figures  illustrating  three  important  stages  in  the  evolution  of  the  head  and  oral  arches  of 
vertebrates,  as  shown  by  an  ostracoderm  (Bothriolepis),  an  arthrodire  (Coccosteus)  and  a  primitive  dipnoan 
(Scaumenacia) .  The  important  steps  are:  (i)  the  union  of  the  three  pairs  of  oral  arches  to  form  an  unpaired  upper 
and  lower  jaw,  with  denticulate  jaw  plates;  (2)  the  separation  of  the  olfactory  organs  and  lateral  eyes,  and  their 
migration  to  their  typical  position  in  vertebrates;  (3)  the  breaking  up,  the  reduction,  and  the  more  intimate  union 
with  the  head,  of  the  dermal  armor  to  the  visceral  and  respiratory  organs,  and  their  transformation  into  the  oper- 
cular  plates  that  cover  only  the  respiratory  organs. 

C  and  D  are  semi-diagrammatic  restorations,  based  on  the  descriptions  of  Dean,  Traquair,  Hussakof,  and 
Jaeckel;  E  and  F  are  restorations  of  Scaumenacia  curta  (Whiteaves),  made  from  a  large  number  of  well  preserved 
specimens  in'  the  author's  collection.  In  F,  the  mandibular  plate,  d,  is  removed  on  the  left,  exposing  the  under 
surface  of  the  large  dental  plate  of  the  lower  jaw,  and  the  pre- maxillary  and  maxillary  plates  of  the  upper  jaw. 

from  them,  near  the  beginning  of  the  carboniferous,  the  first  air-breathing  verte- 
brates, or  amphibians  from  which,  at  some  subsequent  period,  all  the  higher 
vertebrates  had  their  origin,  directly  or  indirectly.  (Fig.  309.) 

These  animals,  the  most  vigorous  and  varied  offspring  of  the  ostracoderms, 


THE   ARTHRODIRA,    TELEOSTOMII,    DIPNOI,   AND   AMPHIBIA.  387 

form  a  homogeneous  stock  that  stands  distinctly  apart  from  all  other  primitive 
vertebrates.  In  practically  all  of  them  the  ancestral  dermal  armor  has  a  pro- 
longed and  flourishing  existence,  and  is  rajely,  or  never,  entirely  suppressed.  In 
the  more  primitive  groups  it  survives  in  the  form  of  large  superficial  plates 
ornamented  with  low  rounded  tubercles,  or  with  sinuous,  beaded  ridges.  On 
the  trunk  and  tail  they  are  usually  smaller,  forming  irregular,  polygonal,  or 
rounded  scales  which  tend  to  sink  deeper  into  the  skin  and  eventually  to  dis- 
appear, leaving  even  in  such  primitive  forms  as  Bothriolepis  and  some  coccosteans  a 
practically  naked  skin  behind.  But  in  the  adults  of  all  branches  of  the  phylum 
a  considerable  number  of  the  ancient,  large-sized  cranial  plates  are  retained  in 
the  head  region,  forming  a  characteristic  covering  for  the  roof  and  sides  of  the 
head,  jaws,  gill  chamber,  and  pectoral  arches.  With  this  prolonged  survival 
of  the  primitive  dermal  armor,  there  is  an  early  and  vigorous  development  of  an 
endoskeleton,  consisting  of  true  bone,  which  here  makes  its  appearance  for  the 
first  time  in  the  history  of  the  animal  kingdom. 

A  bony  floor  and  sides  to  the  endocranium  are  formed,  as  well  as  complete 
bony  vertebrae  consisting  of  centra  intimately  united  with  neural  and  haemal 
arches  and  transverse  processes. 

A  large  peribranchial  chamber  is  always  present,  but  the  rigid,  armored  walls 
of  this  chamber,  so  characteristic  of  the  ostracoderms,  are  greatly  shortened  in  the 
arthrodires,  and  in  the  teleostomes,  dipnoi,  and  amphibia,  give  place  to  membran- 
ous folds  that  may  or  may  not  be  strengthened  by  movable  opercular  plates. 

Three  pairs  of  oral  arches  are  usually  conspicuous  in  the  embryonic  stages, 
and  the  branchial  arches  may  retain  remnants  of  arachnid  appendages,  in  the  form 
of  external  gills,  hyoidian  "balancers,"  oral  arch  papillae,  tentacles,  or  adhesive 
discs. 

The  parietal  eye  is  generally  well-developed,  and  so  far  as  known,  a  large, 
lung-like  air  bladder  occurs  in  all  the  main  subdivisions  of  the  phylum.  The 
primitive  pectoral  and  pelvic  appendages  may  be  narrow  and  elongated,  with 
a  bony  internal  skeleton  that  in  the  earliest  fish-like  descendants  of  the  arthrodires 
shows,  for  the  first  time  in  the  evolution  of  the  vertebrates,  distinct  traces  of  the 
radiate  terminal  digits,  and  the  jointed  axis  characteristic  of  all  primitive  land 
vertebrates  (Eusthanopteron.  Fig.  265.) 

The  ova  are  of  moderate  size,  frequently  covered  with  an  adhesive,  gelatin- 
ous substance,  and  are  generally  fertilized  externally.  The  antiarcha,  coccoste- 
ans, dipnoi,  and  primitive  teleostomes  were  preeminently  shallow  water,  shore- 
loving  forms,  as  are  their  survivors  to-day.  Their  highly  developed  air  bladder, 
leg-like  fins,  specialized  breeding  habits,  and  the  general  structure  and  mode  of 
life  characteristic  of  the  higher  members  of  this  phyla,  afford  an  easy  anatomical 
and  physiological  transition  to  the  amphibia,  and  hence  to  the  higher  air-breathing 
land  vertebrates. 

On  the  other  hand,  the  young  of  many  amphibia,  dipnoi  and,  teleostomes  pass 
through  a  larval,  or  tadpole,  stage  generally  characterized  by  a  large  head,  by  the 


388  THE    VERTEBRATES. 

presence  of  rudimentary  appendages  on  the  oral  and  branchial  arches,  by  a  small 
mouth  with  a  feeble  lower  jaw,  short  body  cavity  and  slender  tail,  and  by  the 
absence  of  postbranchial  paired  appendages.  This  tadpole  larva  is  clearly  the 
recurrence  of  the  ostracoderm  stage  in  their  phylogeny.  (Fig.  167.) 

The  Arthrodira. — The  arthrodires  closely  resemble  the  ostracoderms  in  the 
structure  of  their  jaws  and  in  the  arrangement  of  their  cranial  plates,  and  without 
doubt  are  directly  descended  from  them.  While  it  is  not  possible  to  identify 
in  detail  all  the  various  structures  involved  in  the  general  resemblance  that  runs 
through  them  all,  we  can  trace  a  progressive  series  of  structures  and  events  that 
lead  steadily  upward  from  the  ostracoderms,  through  the  arthrodires,  to  the 
vertebrates. 

Without  entering  into  a  detailed  discussion  of  the  arthrodires,  an  examination 
of  Coccosteus,  a  fairly  well-known  and  typical  representative,  will  show  us  the 
more  important  respects  in  which  they  approach  the  vertebrates. 

In  Coccosteus  (Figs.  260,  263),  the  central  aggregate  of  procephalic  sense 
organs  seen  in  the  ostracoderms  has  separated.  The  lateral  eyes  have  increased 
greatly  in  size  and  have  taken  up  an  antero-lateral  position,  losing  apparently 
some  of  their  dermal  armor  and  their  power  to  rise  and  fall  in  the  orbits;  the 
parietal  eye  is  lodged  in  a  small  median  plate,  located  in  about  the  same  posi- 
tion as  before,  while  the  olfactory  organs  have  moved  forward  and  laterally, 
occupying  a  more  nearly  terminal  position. 

Jaws. — Three  pairs  of  jaws  are  present.  The  premaxillae  are  relatively 
smaller  than  in  the  ostracoderms,  and  although  still  distinctly  paired,  are  less  freely 
movable  in  a  transverse  direction.  The  rudiments  of  toothed  maxillae  are  present, 
for  the  first  time,  as  small  free  plates  that  probably  represent  the  small  ventro- 
lateral  plate  of  Bothriolepis.  The  mandibles  have  increased  greatly  in  size  and 
may  be  provided  with  prominent  tooth-like  spikes  on  their  anterior  and  median 
borders,  being  in  this  respect  more  like  arthropod  jaws  than  those  of  Bothriolepis, 
the  only  ostracoderm  whose  mandibles  are  known.  Like  the  ostracoderm  man- 
dibles, they  were  capable  of  very  complex  movements.  Both  ends  were  free; 
that  is,  they  were  not  firmly  articulated  to  any  cartilage  or  bone,  and  could  be 
either  rotated,  or  moved  in  a  transverse  and  longitudinal  direction.  Their  ex- 
posed median  ends  were  probably  held  in  place  by  the  integument,  while  their 
lateral  ends  were  buried  in  the  tissues  of  the  head,  and  served  for  the  attachment 
of  the  sinews  and  powerful  muscles  that  controlled  their  movements. 

A  single  hyoid  arch  (Fig.  260,  h),  covered  with  dermal  bone,  extended 
across  the  throat  behind  the  mandibles,  in  place  of  the  two  arches  of  similar 
structure  seen  in  Bothriolepis. 

The  cranial  bones  are  more  numerous  than  in  the  antiarcha,  owing  probably 
to  the  breaking  up  of  the  large  orbital  plate  by  the  lateral  migration  of  the  eyes. 
The  same  movement  has  probably  opened  the  way  for  the  formation,  for  the 
first  time,  of  a  supra-orbital  line  of  cutaneous  sense  organs. 

The  branchial  shield  retains  nearly  the  same  number  and  arrangement  of 


THE   ARTHRODIRA.  389 

plates  as  in  the  ostracoderms,  but  it  is  greatly  reduced  in  size,  and  deeply  incised 
on  the  flanks,  thus  opening  up  the  peribranchial  chamber.  It  thereby  takes  on 
more  distinctly  the  character  of  a  true  operculum,  and  apparently  serves  solely 
for  the  protection  of  the  gills  and  heart,  the  digestive  and  urogenital  organs 


de 


p.o. 


FIG.  262,  263,  264. — Side  views  of  the  heads  of :  A,  Bothriolepis;  B,  Coccosteus;  and  C,  Scaumenacia  curta, 
Whiteaves.  In  the  latter,  the  outer  part  of  the  left  mandible  has  been  omitted,  exposing  the  large  mandibular 
dental  plate  and  the  row  of  minute  teeth  on  the  anterior  margin  of  the  mandibles. 


taking  up  a  position  farther  back,  wholly  posterior  to  the  respiratory  region,  as 
indicated  by  the  large  size  of  the  postbranchial  section  of  the  trunk,  and  by  the 
probable  location  of  the  cloacal  opening. 

In  probably  all  arthrodires,  the  notochord  persisted  throughout  life  with  little 
or  no  change.     Although  there  are  no  indications  of  vertebral  centra,  we  see  for 


39° 


THE    VERTEBRATES. 


the  first  time  a  well-developed  series  of  neural  and  haemal  arches,  the  forerunners 
of  a  true  vertebral  column. 

The  cephalic  appendages,  a,  that  were  so  characteristic  of  the  ostracoderms, 
are  here  rudimentary  and  probably  functionless.  With  the  relative  decrease  in 
the  size  of  the  head  and  the  increase  in  the  size  of  the  trunk,  the  whole  body  is 
better  balanced  and  more  suitable  for  an  active,  free  swimming  existence.  Loco- 
motion was  probably  effected  largely  by  the  flexible  trunk  and  tail,  although  im- 
mediately behind  the  branchial  region  there  are  traces  of  supports  for  small 
pectoral  fins,  the  first  appearance  in  this  phylum  of  paired  appendages  of  the 
vertebrate  type.  Pelvic  fins  were  apparently  absent. 

The  arthrodires  clearly  represent  a  higher  type  of  animals  than  the  ostraco- 
derms. They  have  successfully  emerged  from  the  precarious  period  of  profound 
metamorphosis  in  which  the  ostracoderms  were  engaged.  While  it  lasted,  an 
active  life  was  inhibited  by  the  changes  going  on  in  the  old  organs,  and  by  the 
imperfect  adjustment  of  the  new. 

With  the  arthrodires  that  period  is  past.  They  have  increased  notably  in 
size;  the  eyes  are  fully  adjusted  to  their  new  location  within  the  neural  tube;  the 
mouth  has  become  capacious,  and  the  jaws  large  and  powerful,  with  formidable 
cutting,  or  toothed  margins,  well  suited  for  capturing  and  devouring  animal  food; 
the  respiratory  region  is  set  apart  from  the  digestive  and  urogenital  regions,  and 
the  body  is  better  balanced  and  better  adapted  for  an  active,  free  swimming  life. 
The  arthrodires,  therefore,  quickly  developed  from  the  sluggish,  plant-eating 
stage  of  the  ostracoderms,  into  the  most  active,  rapacious,  and  formidable  animals 
of  their  time.  But  as  a  class  they  were  short-lived,  for  the  structural  conditions 
within  had  as  yet  attained  only  a  temporary  equilibrium,  and  the  new  mode  of 
life  was  rapidly  producing  new  creative  forces.  Out  of  these  conditions  arose 
the  first  true  vertebrates  of  this  phylum,  the  dipnoi  and  the  teleostomes. 

The  anatomical  changes  involved  in  the  creation  of  the  new  types  of  animals 
were  comparatively  insignificant.  The  head  became  relatively  smaller  and  more 
compact,  the  trunk  larger,  and  the  whole  body  assumed  a  more  fish-like  appear- 
ance. (Figs.  261-264.)  The  lateral  eyes  grew  still  larger  and  took  up  a  position 
on  the  sides  of  the  head,  well  behind  the  olfactory  pits,  which  have  also  greatly 
increased  in  size.  The  premaxillae  fused,  forming  the  fronto-nasal  process,  and 
together  with  the  maxillae  became  permanently  fixed  to  the  floor  of  the  cranium. 
The  distal  ends  of  the  mandibles  united  in  the  median  line,  and  their  proximal 
ends  articulated  with  a  cranial  cartilage,  to  which  the  hyoid  arch  was  also  at- 
tached; they  then  lose  their  rotary  and  transverse  movements,  and  swing  forward 
and  backward  against  the  maxillary  arch  in  typical  vertebrate  fashion. 

With  the  fusion  in  the  median  line  of  the  three  pairs  of  oral  arches,  their 
dermal  armor  becomes  the  three  pairs  of  fixed  dental  plates  characteristic  of  the 
dipnoi  (Fig.  261,  p.mx,moc.d.p.),  and  from  which  the  isolated  socketed  teeth  of 
the  higher  vertebrates  arose. 

The  plates  in  the  dorso-lateral  walls  of  the  primitive  branchial  shield  now 


THE  ARTHRODIRA. 


391 


form  movable  opercula,  and  those  in  the  ventral  wall  become  attached  to  the 
mandibular  and  hyoid  arches  to  form  the  gular  plates. 

The  dermal  plates  on  the  dorsal  surface  of  the  head  (mesocephalon)  increase 
in  number,  and  approach  the  typical  arrangement  seen  in  the  primitive  air-breath- 
ing vertebrates.  The  base  of  the  endocranium  becomes  ossified;  bony  centra 
appear  in  the  sheath  of  the  notochord  and,  uniting  with  the  neural  and  haemal 
arches,  form  true  vertebrae. 


A  B 

FIG.  265. — Photograph  of  the  left  pectoral  appendage  of  Eusthanopteron  fordi  (Whiteaves).  The  skeleton  of 
the  basal  portion  of  the  appendage  was  exposed;  that  of  the  terminal  portion  was  apparently  covered  by  skin 
which  has  shrunken  sufficiently  to  show  the  arrangement  of  the  internal  skeleton.  The  skeleton  of  this  appendage 
resembles  that  of  the  land  vertebrates,  and  indicates  the  way,  as  shown  in  B,  in  which  the  typical  skeleton  of 
the  pectoral  appendage  of  the  tetrapoda  has  been  derived  from  the  biserial  pectoral  fin  of  fishes.  (From  specimen 
in  the  authors  collection  from  Scaumenac  Bay.  P.  Q.,  Canada.) 

The  pectoral  fins  enlarge  and  the  girdle  extends  dorsally,  uniting  with  the 
occipital  portion  of  the  cranium. 

Within  the  pectoral  fins,  for  the  first  time  in  the  phylogeny  of  the  vertebrates, 
appears  an  axial  skeleton  that  approaches,  in  the  arrangement  of  its  elements, 
the  characteristic  structure  of  the  appendages  of  the  land  vertebrates,  i.e.,  Eus- 
thenopteron.  (Fig.  265.)  Pelvic  fins  appear  in  the  anal  region. 

********* 

Thus  the  broad  foundations  for  the  evolution  of  the  first  air-breathing  land 
vertebrates  is  laid  in  the  dipnoi  and  ganoids,  the  armored  fishes  of  the  upper 


392  THE    VERTEBRATES. 

Devonian;  reaching  down  through  them  to  the  arthrodires  and  to  the  ostracoderms 
of  the  Silurian,  and  then  again  beyond  them  to  the  merostomes,  trilobites,  and 
primitive  phyllopods  of  the  Ordovician,  Cambrian,  and  Proterozoic. 

In  this  vigorous  phylum,  evolution  follows  a  logical  and  consistent  course, 
each  important  event  being  the  direct,  or  indirect  result  of  the  preceding  ones, 
and  they  themselves  creating  the  conditions  that  bring  about  those  that  follow. 
The  various  independent  sets  of  organs,  such  as  the  brain,  sense  organs,  appen- 
dages, jaws,  internal  and  external  skeleton,  in  their  own  peculiar  ways  move  steadily 
onward  toward  the  same  end,  in  a  manner  that  could  hardly  be  possible  except 
in  a  real,  not  an  imaginary,  line  of  evolution.  The  perfection  with  which  these 
immensely  varied  and  complicated  facts  and  details  fit  together  to  form  a  definite, 
intelligible  picture,  carries  with  it  the  overwhelming  conviction  that  that  picture 
is  an  image  of  the  truth.  The  precision  with  which  each  event  creates  again  and 
yet  again,  new  conditions,  new  organs,  and  new  readjustments,  shows  that  the 
primary  forces  that  sustain  and  direct  the  main  lines  of  evolution  are  self-creative, 
and  lie  within,  not  without,  the  organism. 


PART  II.  THE  ACRANIATA. 


CHAPTER  XXIL 
THE  CRANIATES  AND  THE  ACRANIATES. 

The  Statement  of  the  Problem.— One  of  the  chief  difficulties  in  the  study 
of  the  origin  of  vertebrates  is  to  correctly  estimate  the  value  of  contradictory 
evidence  and  to  assign  due  weight  to  opinions  based  on  a  particular  point  of 
view,  or  on  a  particular  source  of  information.  Thus  Amphioxus,  Balanoglossus, 
Cephalodiscus,  the  tunicates,  echinoderms,  and  annelids  have  been  variously 
exploited  as  the  ancestral  stock  from  which  the  vertebrates  arose,  and  each  view 
has  had  its  followers  that  from  time  to  time  have  openly  confessed  their  faith  and 
duly  readjusted  their  estimates  of  morphological  values. 

From  the  very  outset  the  analysis  of  the  vertebrate  head  and  trunk  by  means 
of  comparative  anatomy  and  embryology  seemed  to  demonstrate  that  the  ances- 
tral vertebrate  was  an  elongated  animal  composed  of  many  like  metameres,  each 
one  consisting  of  sharply  defined  members  of  the  more  important  system  of  organs, 
i.e.,  nephridia,  neuromeres,  myo tomes,  sense  organs,  gut  pouches,  and  gill  clefts, 
thus  suggesting  the  condition  so  clearly  presented  by  many  annelids.  For  many 
years,  therefore,  Amphioxus  was  regarded  as  the  most  primitive  existing  verte- 
brate, because  its  simple  structure  and  the  sharply  defined  segmentation  of  its 
mesoderm,  gill  clefts,  and  other  organs,  appeared  to  represent  the  actual  em- 
bodiment of  the  ideal  vertebrate.  But  the  organs  that  are  the  first  to  show  a 
metameric  arrangement  in  the  invertebrates,  and  the  ones  to  maintain  it  most 
persistently,  such  as  the  appendages,  sense  organs,  and  nerve  cords,  we~e  in 
Amphioxus  either  absent  or  without  any  indication  of  segmentation. 

The  fact  that  in  typically  segmented  invertebrates,  such  as  the  arthropods 
and  annelids,  the  nerve  cord  nearly  always  consists  of  sharply  defined  and  widely 
separated  ganglia,  or  neuromeres,  while  in  Amphioxus  little  or  no  indication 
of  such  a  condition  is  visible,  occasioned  little  comment,  while  much  was 
made  of  the  segmental  arrangement  of  the  myotomes,  gill  clefts,  and  later  of  the 
nephridia. 

When  it  was  shown  that  the  development  of  the  tunicates  was  very  similar 
to  that  of  Amphioxus,  and  that  the  tunicate  larva  had  a  well  defined  notochord, 
which  later  disappeared  during  a  process  of  degenerative  metamorphosis,  the 
problem  was  greatly  complicated,  for  the  tunicates  clearly  belonged  to  a  lower 
type  structurally  than  Amphioxus,  yet  they  were  farther  removed  from  the  hypo- 
thetical, ideally  segmented  ancestor  of  the  vertebrates  than  either  Amphioxus  or 
any  of  the  true  fishes. 

393 


394  THE    CRANIATES   AND    THE   ACRANIATES. 

The  clue  was  apparently  leading  in  the  wrong  direction,  into  the  darkness 
rather  than  into  the  light,  and  the  problem  was  by  no  means  simplified  when  it 
appeared,  more  and  more  clearly,  that  Balanoglossus  resembled  Amphioxus  and 
the  tunicates  in  certain  important  particulars,  especially  in  the  structure  and 
development  of  the  coelom  and  gill  clefts,  while  its  larva  resembled  that  of  the 
echinoderms. 

Again  the  problem  was  still  further  complicated  by  the  discovery  of  Cephalo- 
discus  and  Rhabdopleura,  at  first  supposed  to  be  related  to  the  polyzoa,  and  con- 
sequently suspiciously  close  to  the  brachiopods,  but  later  very  generally  recog- 
nized as  also  related  to  Balanoglossus,  and  hence  in  some  way  involved  with  the 
tunicates  and  amphioxus,  which  outwardly  they  did  not  in  the  least  resemble. 
An  amphioxus-balanoglossus-like  animal  with  six  pairs  of  legs,  such  as  those 
of  Cephalodiscus,  was  perforce  accepted  without  a  grimace,  although  it  was  not 
very  readily  assimilated. 

The  trail  was  leading  well  down  toward  the  roots  of  the  animal  kingdom, 
but  certainly  not  toward  anything  like  a  worm-like  ancestral  form  composed  of 
many  well  defined,  similar  metameres.  The  evidence  that  was  accumulating, 
while  in  some  cases  more  concrete  than  that  produced  in  the  earlier  history  of  the 
problem,  became  less  convincing.  It  led  in  too  many  directions,  and  was  forcing 
morphologists  at  large  to  accept  conclusions  against  which  their  better  judgment 
rebelled,  but  from  which  there  was  apparently  no  escape.  While  many  preferred 
to  doubt  the  evidence  rather  than  accept  the  conclusions,  or  even  began  to  lose 
faith  in  the  efficacy  of  comparative  embryology  as  a  means  of  solving  large  problems 
in  phylogeny,  others,  with  commendable  loyalty,  adhered  to  the  particular  faith 
in  which  they  had  been  educated,  and  advocated  it  with  sufficient  ardor,  at  least 
until  the  next  hypothesis  appeared.  But  as  the  number  of  attractive  theories 
increased,  the  older  morphologists  apparently  concluded  that  it  was  wiser  to 
accept  neither  one  nor  the  other,  and  to  beware  of  them  all.  It  was  perhaps 
realized  that  one  might  live  very  happily  wedded  to  one  view,  if  it  was  not  for 
the  others;  for  it  was  increasingly  evident  that  embracing  any  one  theory  created 
more  difficulties  than  were  overcome,  since  each  rejected  one  was  then  sure  to 
look  more  formidable  than  ever. 

However,  our  new  hypothesis  is  not  open  to  these  objections,  for  in  accepting 
the  arachnid  theory  we  shall  have  the  privilege  of  adopting  into  our  household, 
as  her  children,  many  of  the  other  attractive  theories  that  have  from  time  to  time 
won  our  affection.  The  arachnid  theory  opens  the  way  to  a  reconciliation  of  the 
conflicting  views  above  indicated.  It  offers  a  solution  that  is  logical,  consistent 
with  the  facts,  so  far  as  we  know  them,  and  in  harmony  with  the  basic  principles 
of  morphology. 

In  brief,  we  recognize  two  great  groups  of  animals  that  have  independently 
acquired  some  of  those  characters  commonly  associated  with  the  chordata. 
Both  groups  are  descended  exclusively  from  primitive  arthropod  stock,  that  is,  from 
small  bodied  animals  of  a  small  number  of  ill  defined  metameres  (resembling  a 


THE    STATEMENT    OF    THE    PROBLEM.  395 

nauplius  or  an  ostracode)  and  which  in  turn  were  derived  from  rotifer-like  trocho- 
zoans.  Neither  the  annelids  nor  any  other  worm-like  forms  were  included  in  this 
stock.  (Fig.  309.)  The  first  group,  the  syncephalata,  includes  together  with 
other  arthropods,  the  phyllopod-arachnid-ostracoderm-vertebrate  phylum,  or 
the  craniata;  the  second  group,  or  the  acraniata,  includes  the  cirripeds,  tunicates, 
Amphioxus,  echinoderms,  enteropneusta,  pterobranchia,  phoronida,  polyzoa, 
chaetognatha,  and  brachiopods. 

Our  problem  is  in  a  measure  clarified,  and  at  once  assumes  an  entirely  new 
aspect,  as  soon  as  we  recognize  that  the  chordata  consist  of  several  phyla  derived 
from  the  arthropods  via  as  many  separate  lines,  and  that  some  of  the  striking 
features  in  which  they  resemble  one  another  were  independently  acquired.  We 
associate,  for  example,  Amphioxus,  Balanoglossus,  and  the  tunicates  with  the 
vertebrates  because  of  their  notochord,  perforated  gill  slits,  haemostoma,  and 
atrial  chamber.  But,  as  we  have  already  seen,  these  basic  characters,  in  one 
form  or  another,  are  actually,  or  potentially  present  in  all  primitive  arthropods, 
and  presumably,  they  may  be  expressed  in  all  their  descendants,  although,  per- 
haps at  different  times,  and  in  varying  ways,  in  different  phyla. 

The  assumption  that  several  independent  caudate  phyla  have  arisen  from 
the  arthropods  is  of  two-fold  value,  for  it  enables  us  to  explain  why  the  tunicate- 
balanoglossus  group  resembles  at  the  same  time  both  the  arthropods  and  the 
vertebrates,  without  being  in  the  direct  line  of  vertebrate  descent;  and  it  enables 
us  to  attach  these  heretofore  isolated  and  obscure  phyla  to  the  great  trunk  line 
of  organic  evolution,  and  to  thereby  obtain  a  new  basis  for  the  interpretation  of 
their  morphology. 

Let  us  first  consider  in  a  summary  way  the  more  important  features  of  these 
two  great  groups. 

The  Craniates. 

We  have  shown  in  the  preceding  chapters  that  the  trunk  line  of  vertebrate 
descent  runs  through  the  dipnoi-arthrodire-ostracoderm-arachnid-phyllopod 
stock,  which  almost  from  the  very  outset  consisted  of  highly  specialized  segmented 
animals.  Primarily  they  were  neither  sessile  nor  parasitic,  but  large,  vigorous 
free  swimming  forms  with  highly  developed  nervous  system,  sense  organs,  cephalic 
appendages,  and  paired  jaws.  They  were  often,  at  times  predominantly,  fre- 
quenters of  the  warm  littoral,  of  fresh  or  brackish  waters,  and  of  the  land.  At 
every  stage  of  evolution,  with  a  few  notable  exceptions,  the  animals  that  consti- 
tuted the  advancing  crest  of  this  stock  have  been  on  the  whole  the  largest,  and 
the  most  active,  the  greatest  consumers  and  spenders  of  energy,  the  most  highly 
organized,  the  most  progressive  and  innovating,  and  the  most  widely  distributed 
animals  of  their  day  and  generation. 

They  had  a  chitenous,  dentine-like  exoskeleton,  a  cartilaginous  endocranium, 
gill  cartilages,  middle  chord,  gill  sacs,  voluminous  enteric  diverticula,  a  trioc- 


396  THE  CRANIATES  AND  THE  ACRANIATES. 

cellate  parietal  eye,  frontal,  or  olfactory  sense  organs,  and  compacted  cephalic 
neuromeres.  The  vast  majority  of  them  were  free  moving  forms  during  their 
early  post  embryonic,  and  adult  stages.  Their  highly  specialized  appendages, 
well  developed  sense  organs,  and  neuro-muscular  systems  enable  them  to  execute 
varied  movements  with  great  precision  in  response  to  exceedingly  complex  sur- 
roundings; they  were  the  first  animals  to  acquire  an  effective  response  to  stimuli 
of  distant  origin,  to  perceive  the  intangible,  to  pursue,  and  to  capture.  Their 
eggs  contained  a  large  quantity  of  yolk,  and  the  embryos  did  not  leave  the  egg 
till  they  had  attained  an  advanced  stage  of  development. 

In  this  phylum  the  theme  was  metamerism.  Progress  was  first  attained  by 
perfecting  metamerism,  later  by  its  suppression  or  elimination.  The  larger 
possibilities  of  this  type  of  structure  were  practically  exhausted  in  the  arthropods, 
and  it  had  already  entered  another  phase  before  the  critical  period  arrived  that 
was  to  give  rise  to  the  vertebrates.  During  the  early  history  of  the  phylum,  pro- 
gressive evolution  was  effected  by  a  gradual  increase  in  the  number  of  metameres, 
and  by  gradually  increasing  the  perfection,  or  the  fullness  and  precision  with 
which  metamerism  was  expressed.  Metamerism  then  began  to  decline,  owing 
to  the  local  exaggeration,  suppression,  and  fusion  of  organs.  This  process  was 
most  strongly  marked  at  the  anterior,  or  older  end  of  the  lengthening  series  of 
metameres,  thus  leading  to  the  formation  of  an  extensive  and  extremely  complex 
head  region,  to  a  new  linear  arrangement  of  unlike  organs  and  functions,  and  to 
the  production  of  a  higher  and  more  unified  type  of  organization  than  has  been 
attained  in  any  other  phylum  of  the  animal  kingdom. 

The  formation  of  new  metameres  at  the  caudal  end,  and  the  specialization 
of  the  older  ones  at  the  cephalic  end,  on  the  whole  proceeded  simultaneously, 
so  that  at  no  stage  in  the  evolution  of  the  phylum  did  the  body  consist  of  a 
long  series  of  like  metameres.  Only  a  few  metameres,  if  any,  ever  approached 
a  condition  of  ideal  perfection,  that  is,  one  containing  all  the  so-called  segmental 
organs.  Serial  homology  in  the  craniate  phylum  is,  therefore,  necessarily  im- 
perfect, and  the  organs  at  one  end  of  a  series  are  never  fully  comparable  with 
those  at  the  other. 

II.  THE  ACRANIATA. 

The  second  great  group  of  animals  with  arthropod  affinities  constitutes 
the  acraniata.  It  may  be  called  the  cirriped  division  of  the  arthropod  stock, 
for  the  cirripeds  appear  to  form  its  central  figure,  and  because  many  of  the  more 
striking  features  of  the  various  sub-phyla  are  most  clearly  expressed  in  the  cirri- 
peds. We  include  in  the  acraniata,  the  cirripeds,  tunicates,  Amphioxus,  echino- 
derms,  enteropneusta,  chaetognotha,  pterobranchia,  phoronida,  polyzoa,  and 
brachiopoda,  all  of  which,  with  the  exception  of  the  polyzoa,  are  exclusively  marine. 
They  are  all  derived  from  cirriped-like  forms,  or  with  them,  from  ostracoda,  or 
small  nauplius-like  arthropods  that  consisted  of  a  small  number  of  imperfectly 
developed  metameres.  They  form  more  or  less  independent  subphyla,  in  no 


THE   ACRANIATES. 


397 


way  directly  united  with  the  vertebrate  stock,  and  in  no  sense  to  be  regarded  as 
the  ancestors  of  the  vertebrates. 

From  every  point  of  view,  and  from  every  point  of  departure,  the  evidence 
leads  more  and  more  decisively  to  this  conclusion.     The  vertebrates,  however 


JMotocKord. 


_  naemosto  rrva. 


Tarasittc. 

EndocraYvtxxim,. 

., ,  Jore  -"beam  abs 

or  ciea'. 


_Telocoel 
.  Mantle  >•  Vest  ik 
Free . 
Tuted. 
..CepkalstaXK.; 


FIG.  266. — Diagram  to  illustrate  the  probable  interrelations  of  the  acraniata,  and  their  origin  from  a  small 
nauplius-like  arthropod  ancestor.  The  characteristic  shape  of  each  division  and  its  normal  orientation  to  its 
point  of  attachment  is  indicated.  The  neural  surface  may  be  identified  by  the  brain  and  nerve  cord,  in  black.  The 
principal  characters  of  each  division  are  indicated  by  the  presence  of  a  +  or  o  sign  opposite  the  descriptive  term 
on  the  right.  When  a  character  may  be  either  present  or  absent,  or  doubtful,  the  plus  sign  is  enclosed  in  a  circle. 
The  more  specialized  characters  stand,  as  far  as  practical  in  such  a  diagram,  at  the  top  of  the  list. 

far  back  we  may  go,  do  not  lead  to  the  acraniates,  and  the  acraniates  do  not 
take  us  back  to  the  same  stock  as  that  from  which  the  vertebrates  arose.  Indeed 
all  the  acraniates  are  chiefly  notable  for  the  absence  of  those  very  structures 
and  modes  of  growth  so  characteristic  of  the  craniates.  In  the  acraniates  a  differ- 
ent set  of  organs  are  emphasized,  and  the  general  direction  of  evolution  is  different. 


398  THE    CRANIATES    AND    THE    ACRANIATES. 

In  every  subphylum  metamerism  is  suppressed,  the-  structure  simplified,  and 
evolution  has  of  necessity  led  toward  a  less  active,  less  complicated  mode  of  life; 
in  fact,  farther  and  farther  away  from  the  characteristic  condition  in  the  craniate 
stock,  rather  than  nearer  to  it. 


The  chief  characteristics  common  to  the  exceedingly  heterogeneous  group 
of  sub-phyla  constituting  the  acraniates  may  be  defined  as  follows: 

Metamerism. — The  metameres  are  few  in  number,  except  in  Amphioxus, 
and  are  rarely  if  ever  sharply,  or  fully  defined.  Even  when  the  metamerism  is 
fairly  well  expressed  in  the  younger  stages,  it  degenerates  or  becomes  greatly 
obscured,  or  it  may  disappear  altogether  in  the  adult  (cirripeds,  tunicates). 
Whether  this  is  a  true  degeneration,  or  merely  a  special  form  of  development, 
is  a  matter  of  definition.  It  is  certain,  however,  that  the  possibilities  of  the  meta- 
meric  type  of  structure,  so  fully  realized  in  the  arachnid  division,  are  never  real- 
ized here. 

There  is  no  organic  union  of  specialized  metameres  to  form  a  compound 
head,  although  in  the  cirripeds  we  may  recognize  one  or  two  pairs  of  temporary 
antennae,  three  pairs  of  jaws,  and  five  or  six  pairs  of  abdominal  appendages, 
indicating  the  division  of  the  body  into  tagmas,  corresponding  approximately 
to  the  procephalon,  mesocephalon  or  thorax,  and  metacephalon  or  branchial 
region  of  the  arachnids.  (Fig.  275.)  In  the  cirripeds,  the  primitive  head  and 
thorax  are  the  first  to  lose  their  metameric  structure,  the  only  indication  of  it 
left  in  the  adult  being  the  jaws,  and  even  these  may  disappear. 

The  main  divisions  of  the  body,  but  with  little  or  no  indication  of  their  further 
subdivision  into  metameres,  are  recognizable  in  the  other  sub-phyla  as  the  pro- 
boscis, the  collar,  and  branchial  regions  (Amphioxus,  enteropneusta,  ptero- 
branchia) ;  or  they  are  probably  represented  merely  by  the  principal  subdivisions 
of  the  ccelom  seen  in  the  ectoprocta,  phoronida,  chaetognatha,  and  echinodermata. 

The  appendages  are  always  simple  in  structure,  stub-like,  tentacular,  or 
altogether  absent.  Their  serial  identity  is  only  vaguely  indicated,  but  we  may 
recognize  the  rudiments  of  procephalic  appendages,  probably  corresponding  to 
the  antennae  of  cirripeds,  copepods,  and  other  Crustacea,  in  the  adhesive  papillae 
of  the  tunicate  and  echinoderm  larvae,  and  possibly  in  the  tentaculate  arms  of 
ectoproctous  polyzoa,  brachiopods,  rhabdopleura,  and  phoronis. 

The  five  primordial  tentacles  of  the  echinoderms  probably  represent  a  group 
of  thoracic,  or  abdominal  appendages  (Figs.  291-295);  those  of  Cephalodiscus 
and  the  entoproctous  polyzoa  represent  the  thoracic,  or  both  circumoral  and 
abdominal  groups.  (Figs.  299-301.)  The  appendages  may  be  absent  from  the 
outset,  or  if  present  they  may  disappear  completely  in  the  adult.  After  the  larval 
period,  they  are  never  used  as  locomotor  organs  in  any  member  of  the  group. 
In  the  chaetognaths,  the  cephalic  appendages  have  the  appearance  of  typical 
arthropod  oral  appendages. 


THE    NERVOUS    SYSTEM.       DEGENERATION.      ATTACHMENT.  399 

The  nervous  system  is  always  small,  exceedingly  simple  and  primitive  in 
structure.  Sense  organs,  such  as  visual  and  auditory  organs,  are  absent,  or  very 
rudimentary.  Even  in  the  cirripeds,  the  lateral  and  parietal  eyes  of  the  larva 
quickly  disappear,  or  become  functionless.  Only  in  a  few  tunicates  does  the 
parietal  eye  function  to  any  extent  in  the  adult.  A  neuro-muscular  apparatus, 
capable  of  elaborate  or  varying  responses  to  external  stimuli,  is  never  present. 

The  mode  of  life  is  rigidly  prescribed  by  these  conditions.  The  absence  of 
armored  grasping  appendages,  of  well  developed  sense  organs,  and  of  a  complex 
neuro-muscular  apparatus,  excludes  the  possibility  of  elaborate  reflexes,  of  percep- 
tion at  a  distance,  of  pursuit  and  capture.  The  inevitable  result  has  been  the 
practically  universal  adoption  of  either  a  sessile,  a  subterranean,  or  a  parasitic 
mode  of  life,  depending  for  food  on  micro-organisms,  or  on  other  finely  divided 
matter  sifted  from'  water  or  soil,  or  on  fluids  absorbed  from  other  animals. 

Degeneration. — There  is  a  strong  tendency  in  the  entire  group  toward  a 
retrograde  or  degenerative  development,  that  appears  to  be  due  to  some  prevalent 
lack  of  adequate  internal  conditions  or  of  materials.  It  makes  its  appearance 
during,  or  shortly  after  the  larval  stages,  cutting  down  the  first  promises  of  a  clear 
cut,  vigorous  organogeny  to  one  that  is  feeble,  blurred,  or  defective  in  definition; 
or  one  in  which  important  parts  are  absent.  It  may  manifest  itself  in  the  absence 
of  structural  detail,  in  diminished  local  outgrowths,  or  in  the  absence  of  appen- 
dages (Amphioxus,  enteropneusta,  tunicates,  chaetognaths) .  It  is  seen  in  the 
degenerative  metamorphosis  of  tunicate  larvae;  in  the  reduction  in  size,  or  absence 
of  organs,  so  common  in  male  cirripeds;  and  in  the  progressive  disappearance 
in  many  parasitic  cirripeds  of  both  sexes,  of  mouth,  anus,  appendages,  nervous 
system,  and  alimentary  canal;  in  fact,  of  practically  everything  except  the  integu- 
ment and  reproductive  organs. 

In  some  cirripeds  (rhizocephala) ,  this  process  is  carried  so  far  that  if  it  were 
not  for  the  presence  of  the  characteristic  appendages  in  the  larvae,  their  identity 
would  be  exceedingly  difficult,  perhaps  impossible,  to  determine.  We  have 
merely  to  assume  that  the  suppression  of  appendages  has  been  carried  a  step 
further  back  in  the  ontogeny,  to  account  for  their  total  absence  in  Amphioxus  and 
the  enteropneusta. 

In  the  rhizocephala,  the  parts  of  the  body  left  after  the  extensive  degeneration 
of  organs,  and  the  casting  off  of  the  abdomen,  acquire  a  new,  almost  unlimited 
power  of  growth,  forming  extensive,  root-like  processes  that  penetrate  in  every 
direction  the  tissues  of  its  host.  In  the  ectoprocta,  there  is  also  an  extensive 
degeneration  of  organs,  similar  to  that  in  parasitic  cirripeds.  That  is,  the  nervous 
system,  appendages,  and  alimentary  canal  disappear,  or  fail  to  develop,  and  from 
the  apparently  formless  remnants,  strangely  enough,  buds  are  formed,  destined  to 
give  rise  to  new  and  more  perfect  zoids.  It  may  be  that  there  is  some  relation 
between  the  degenerative,  or  retrograde  development  of  the  tunicates,  ptero- 
branchia,  and  polyzoa,  and  this  retention  and  renewal  of  the  power  of  budding. 

Attachment. — We  have  seen  that  many  phyllopods  are  temporarily  attached 


400  THE  CRANIATES  AND  THE  ACRANIATES. 

to  foreign  objects  by  means  of  a  sort  of  sucking  or  adhesive  disc  on  the  haemal  side 
of  the  head.  Many  parasitic  copepods  and  cirripeds  are  permanently  attached 
in  this  manner,  aided  by  a  pair  of  modified  appendages  that  have  moved  round 
onto  the  haemal  side  of  the  head.  In  cirripeds  the  larva  attaches  itself,  head 
first,  neural  side  down;  it  then  turns  a  forward  handspring  on  its  rudimentary 
adhesive  antennae,  bringing  the  neural  side  up;  meantime  an  enormous  out- 
growth develops  from  the  haemal  surface  of  the  head,  forming  the  peduncle  by 
which  the  animal  is  permanently  attached.  (Fig.  274.)  This  extraordinary 
mode  of  attachment,  accompanied  by  the  same  peculiar  rotation  and  cephalic 
outgrowth,  occurs  with  but  slight  variation  in  the  tunicates,  echinoderms,  ptero- 
branchia,  phoronida,  polyzoa,  and  brachiopods,  in  fact  in  every  subdivision  of 
the  acraniates  except  Amphioxus,  the  enteropneusta  and  chaetognatha. 

Mantle. — Before  the  young  cirriped  becomes  attached,  the  valves  of  the 
thoracic  shield  make  their  appearance  as  a  longitudinal  circular  fold.  The  free 
edge  of  the  fold  gradually  extends  toward  the  neural  surface,  enclosing  the  body 
and  appendages  in  a  large  vestibular,  atrial,  or  branchial  chamber.  (Figs.  289, 
281.)  A  similar  mantle  forms  a  familiar  and  conspicuous  feature  in  the  larval 
and  adult  stages  of  the  tunicates  (Figs.  284-286),  echinoderms  (Figs.  291, 
295),  polyzoa  (Fig.  301),  phoronida  (Fig.  305),  and  brachiopods  (Fig.  304).  In 
the  polyzoa  and  echinoderms,  the  vestibule  may  develop  very  early  as  a  closed 
chamber;  but  it  is  soon  ruptured  by  the  growing  appendages  within,  which  then 
protrude  through  the  opening  in  the  same  manner  as  those  of  a  cirriped.  In 
the  enteropneusta  the  mantle  consists  of  two  longitudinal  pleural  folds  that  form 
an  imperfect  branchial  chamber.  (Fig.  298.) 

The  rudimentary  mantle  fold  is  a  conspicuous  feature  of  the  larvae  of  cirripeds, 
echinoderms,  enteropneusta,  polyzoa,  phoronida,  and  brachiopods.  Its  free 
margin  may  be  drawn  out  into  characteristic  projections  or  lobes,  that,  heavily 
ciliated,  form  the  primary  longitudinal  ciliated  band  characteristic  of  the  naupula. 
It  should  not  be  confused  with  the  transverse  ciliated  band  characteristic  of  the 
trochophore  larva.  (Figs.  267-296.) 

Skeleton. — An  endocranium  occurs  only  in  the  enteropneusta  and  chaeto- 
gnatha. In  the  enteropneusta  it  consists  of  a  low  grade  nbro-cartilage,  probably 
of  mesodermic  origin,  and  comparable  with  the  primitive  endocranium  of  the 
phyllopods.  See  page  312.  No  neural  or  branchial  cartilages  appear,  but 
in  Amphioxus  and  the  enteropneusta,  a  complicated  framework  of  chitenoid  gill 
bars  supports  the  margins  of  the  gill  clefts  and  the  tongue  bars. 

The  exoskeleton  may  consist  of  a  voluminous  chitenoid,  celluloid,  or  gelatin- 
oid  secretion  of  the  ectoderm;  it  is  not  shed  at  regular  intervals,  as  in  the  arthropods, 
but  is  retained  throughout  life.  The  only  exception  appears  to  be  the  appendicu- 
laria,  which  often  shed  their  enormous  "  gelatinous  house"  soon  after  its  formation. 
It  may  be  heavily  calcified,  forming  characteristic  polygonal  plates,  and  greatly 
complicated  by  epidermal  folds  and  channels  containing  vascular  or  other  tissues 
(cirripeds,  tunicates,  brachiopods). 


CIRCULATION.      SEXUAL   ORGANS.      DEVELOPMENT.  4OI 

The  Heart  and  Circulation. — In  the  acraniates  the  heart  and  the  vascular 
channels  are  feebly  developed  and  may  be  altogether  absent  or  unrecognizable. 
There  is  no  definite  vascular  system  in  the  polyzoa  and  chaetognatha;  and  a  distinct 
heart  is  absent  in  cirripeds,  Amphioxus,  the  phoronida,  and  the  echinoderms; 
it  exists,  if  at  all,  in  a  highly  modified  condition  in  the  enteropneusta  and  ptero- 
branchia.  In  the  tunicates  and  brachiopods  a  small,  oval  heart  is  present.  In 
the  ascidians  it  is  a  small  fusiform  tube,  unsegmented  and  valveless,  and  composed 
of  epithelio-muscular  cells.  Its  cavity  is  derived  from  the  so-called  blastoccele 
and  it  is  enclosed  in  a  pericardium  derived  from  the  ccelom.  In  these  respects, 
and  in  respect  to  its  location,  the  distribution  of  the  principal  blood  channels, 
and  in  its  mode  of  development,  it  resembles  the  "heart"  of  phyllopods.  The 
tunicate  heart  is  notable  for  its  "reversing  circulation,"  that  has  been  regarded  as 
something  unique  in  the  animal  kingdom;  but  a  similar  phenomenon  has  been  ob- 
served by  Scott,  in  the  parasitic  copepod  Lepeophtheirus.  See  page  418. 

The  Sexual  Organs. — Both  ovaries  and  testis  may  be  present  in  the  same 
individual.  In  the  cirripeds,  the  testis  usually  occupies  the  posterior  part  of  the 
trunk,  opening  to  the  exterior  at  the  apex  of  the  caudal  lobe.  The  ovaries  are 
lodged  in  the  cephalic  region,  extending  also  into  the  peduncle  and  mantle  folds, 
and  even  into  the  recesses  of  the  exoskeleton.  The  oviducts  open  outward  near 
the  middle  of  the  body,  at  the  base  of  the  first  pair  of  abdominal  cirri.  (Fig.  275.) 
In  the  tunicates  a  similar  condition  may  prevail,  e.g.,  in  Polycarpa,  where  "there 
are  many  complete  sets  of  both  male  and  female  systems  attached  to  the  inner 
surface  of  the  mantle,  on  both  sides  of  the  body."  Moreover  embryonic  "kalym- 
mocytes,"  or  egg  follicle  cells,  frequently  pass  through  the  ectoderm  into  the  cellu- 
lose test,  suggesting  a  former  connection  with  the  mantle,  like  that  in  cirripeds. 

In  the  brachiopoda  there  are  two  pairs  of  genital  glands,  both  pairs  located  in 
the  mantle,  one  in  the  anterior  fold,  the  other  in  the  posterior.  The  genital  cells 
are  usually  discharged  by  a  pair  of  nephridia-like  ducts  that  open  on  the  neural 
surface  near  the  middle  of  the  body,  or  just  behind  what  appears  to  represent  the 
thoracic  region;  chaetognatha,  pterobranchia,  polyzoa,  brachiopods,  phoronida. 

The  germ  cells  of  arthropods  may  make  their  appearance  as  the  so-called 
pole  cells  at  a  very  early  period,  before  any  germ  layers  are  recognizable.  In 
parasitic  copepods,  they  arise  from  the  undifferentiated  blastoderm,  and  later 
form  a  small  but  conspicuous  cluster  of  cells  on  the  neural  surface,  between 
the  abdominal  and  thoracic  neuromeres.  (Fig.  242.)  In  the  chaetognatha  and 
polyzoa,  the  germ  cells  are  conspicuous  at  an  early  period  in  a  corresponding 
position.  (Figs.  301,  306.) 

Development. 

It  will  be  necessary  to  abandon,  or  greatly  modify  some  deep-rooted  con- 
ceptions as  to  the  significance  of  the  germ  layers  and  early  embryonic  processes 
in  segmented  animals,  for  they  are  based  either  on  errors  of  observation,  or  upon 

26 


402 


THE  CRANIATES  AND  THE  ACRANIATES. 


a  too  literal  interpretation  of  the  phenomenon  of  embryonic  growth  in  terms  of 
adult  coelenterates.  There  is  little  or  no  foundation  for  the  prevalent  assumption 
that  the  blastopore  elongates,  closes  in  the  middle,  leaving  a  mouth  at  one  end 
and  an  anus  at  the  other.  On  the  contrary,  segmented  animals  elongate  primarily 
by  a  localized  apical  growth,  never  by  stretching  a  gastrula  lengthwise.  Neither 
the  mesoderm  nor  the  notochord  ever  helped  form  the  walls  of  a  primitive,  func- 
tional, alimentary  canal,  with  the  mesoccele  opening  into  the  enteroccele. 

The  principal  source  of  confusion  in  the  interpretation  of  these  fundamental 
processes  has  been  the  failure  to  recognize  the  difference  between  a  true  gastrula 
and  a  mesentocoele,  between  a  trochosphere  and  a  naupula,  neurostoma  and 
haemostoma,  or  the  haemal  and  neural  surfaces;  and  in  assigning  a  fictitious  and 
artificial  significance  to  the  so-called  " archenteron"  and  "ccelomic  pouches." 
We  base  our  conclusions  on  the  following  assumptions,  that  we  regard  as 
axiomatic. 


FIG.  267. — A,  B,  C,  Diagrams  of  an  annelid  larva  in  the  trochosphere,  or  coelentreate  stages,  showing  the  rela- 
tions of  the  gastrula,  blastopore,  mouth,  and  anus  to  each  other;  also  the  origin  of  the  trunk,  as  an  outgrowth 
from  the  primitive  head;  D,  E,  F,  same  of  a  molluscan  larva. 

i.  The  fixed  point  in  all  morphological  problems  is  the  central  nervous 
system.  When  it  is  located,  and  its  direction  of  prolongation'determined,  we  may 
identify  the  six  sides  of  any  bilaterally  symmetrical,  acrogenous  animal,  and  approx- 
imately locate  the  characteristic  organs  of  each  side.  2.  In  all  bilaterally  sym- 
metrical animals,  the  primitive  mouth,  or  neurostoma,  and  the  neuron,  or  axial 
cords  of  the  central  nervous  system,  are  always  laid  down  on  the  same  side  of  the 
body,  or  egg,  i.e.,  the  neural  surface.  3.  The  neural  surface  increases  in  length 
primarily  by  apical  growth  at  the  anal  or  posterior  end  of  the  principal  axis.  4. 
The  right  and  left  sides  of  the  body  are  formed  as  lateral  outgrowths  from  the 
principal  axis,  the  growth  and  differentiation  being  in  a  neuro-haemal  direction. 
5.  The  primitive  mouth,  or  neurostoma,  always  lies  between  the  anterior  ends  of 
the  lateral  cords.  6.  The  anal  or  caudal  end  of  the  body  is  always  the  youngest, 


MOLLUSCS   AND   ANNELIDS.  403 

the  cephalic  or  oral  end  the  oldest.  7.  The  neural  surface  of  the  embryo  is  laid 
down  and  differentiated  earlier  than  the  haemal  surface,  the  difference  between 
the  time  of  formation  and  the  amount  of  specialization  in  the  two  surfaces  being 
governed  largely  by  the  volume  and  distribution  of  the  yolk  mass.  See  Chapter 
XIII,  p.  219. 

There  is  a  well  marked  difference  between  the  embryonic  processes  in  the 
craniates  and  acraniates  on  the  one  side,  and  the  molluscs  and  the  annelids  on  the 
other,  due  to  a  prevailing  difference  in  the  volume  of  yolk  in  the  four  groups,  the 
period  at  which  the  embryo  is  liberated,  and  in  the  unequal  emphasis  placed  on 
growth  at  the  cephalic  and  caudal  ends  of  the  body. 

The  principal  features  of  these  groups  may  be  summarized  as  follows: 

A.  Molluscs  and  Annelids. — In  these  animals  we  have  a  true  gastrulation 
in  its  original  meaning,  for  the  blastular  infolding  gives  rise  to  endoderm  only, 
and  the  blastopore,  without  noticeable  elongation  persists  as  the  mouth.     The 
mesoderm  arises  from  cells  lodged  in  or  near  the  posterior  lip  of  the  blastopore. 
The  young  usually  escape  from  the  egg  as  so-called  trochospheres,  a  larval  form 
representing    the    ccelenterate    phase    of    their   development.     Its  characteristic 
feature  is  a  transverse,  or  equatorial  ciliated  band  encircling  the  principal  axes 
between  the  mouth  and  the  apical  plate.     (Fig.  267.) 

In  the  annelids  the  trochosphere  forms  only  the  head,  the  body  always 
arising  as  a  new  local  outgrowth,  not  by  the  elongation  of  the  trochosphere 
as  a  whole.  When  there  is  a  considerable  amount  of  yolk  present,  the  trocho- 
sphere stage  may  be  passed  within  the  egg  membranes;  in  these  cases  the  trunk  is 
formed  by  the  rapid  proliferation  of  a  special  group  of  large  terminal  cells,  or  telo- 
blasts,  of  which  there  may  be  several  kinds,  each  giving  rise  to  a  linear  series  of 
some  particular  kind  of  organ,  as  nerve  cords,  nephridia,  entoderm,  etc. 

In  the  molluscs  the  trochosphere  is  transformed  into  the  adult  with  little  or  no 
axial  elongation. 

The  molluscs  and  annelids,  therefore,  are  characterized  by  small  or  medium 
sized  eggs  that  pass  through  a  true  gastrulation;  the  blastopore  persists  as  the 
mouth,  and  the  larva  is  a  ccelenterate-like  trochosphere.  The  germ  layers  of  the 
trunk  arise  simultaneously  with  the  progress  of  apical  growth,  no  one  layer  arises 
from  another,  the  mesoderm  never  forms  a  part  of  the  functional  enteron,  and 
the  mesocoele  never  opens  into  the  enteroccele. 

B.  Craniates. — In  the  arachnid-vertebrate  stock,  the  embryo  grows  film- 
like  over  the  surface  of  a  large  body  of  yolk.     It  passes  rapidly  through  the  gas- 
trula  and  trochosphere  stages,  and  is  retained  within  the  egg  membrane  till  the 
trunk  is  well  developed,  rarely  being  liberated  with  less  than  fifteen  or  twenty 
highly  specialized  metameres.     (Figs.  25  to  32.) 

The  only  recognizable  remnants  of  the  gastrula  are  found  in  the  primitive 
cumulus,  or  circular  germ  disc,  at  the  point  where  the  primitive  mouth  and  pro- 
cephalic  lobes  are  formed.  (Figs.  25, 269.)  The  body,  or  trunk,  is  a  new  formation 
that  has  no  real  homologue  in  a  ccelenterate,  and  it  is  formed  solely  by  the  multi- 


404 


THE  CRANIATES  AND  THE  ACRANIATES. 


plication  of  cells  that  lie  beyond  the  limits  of  the  true  blastopore,  that  is  on  the 
posterior  margin  of  what  represents  the  body  of  the  gastrula.  The  bands  of  new 
tissues,  or  organs,  formed  by  the  teloblasts,  such  as  the  nerve  cords,  mesoderm, 
and  entoderm,  are  usually  quite  distinct  and  preserve  their  normal  position  and 
relation  to  one  another  from  the  outset.  But  as  a  result  of  diverse  conditions 
created  by  growth,  the  entire  mass  of  proliferating  cells  may  be  bodily  invaginated 
(insects,  Crustacea,  amphibia)  forming  extensive  axial,  or  terminal,  infoldings 
from  which  the  products  of  apical  growth  are  gradually  separated.  (Fig.  269,  t.p.) 
These  infoldings,  so  often  confused  with  gastrulation,  are  of  a  purely  secondary 


FIG.  268. — Diagrams  of  a  molluscan  trochosphere ;  A  ,B,  in  sagittal  section ;  C,  D,  seen  from  the  neuralor  oral  surface. 
The  diagrams  indicate  the  relation  between  the  gastrula,  blastopore,  mouth,  and  anus;  and  the  site  of  apical  growth. 

nature  and  of  no  special  phylogenetic  significance.  The  process  has  nothing  in 
common  with  gastrulation,  and  the  invaginated  cells  do  not  represent  a  primitive 
enteron.  The  cavity  of  the  infolded  teloblasts  may  be  called  a  teloccele,  and  its 
external  opening  the  telopore. 

The  concrescence  that  occurs  in  the  craniates  is  the  concrescence  of  the 
peripheral  margins  of  an  expanding  embryonic  area,  not  that  of  an  elongated 
blastopore.  (Fig.  157.) 


FIG.  269. — Diagrams  of  an  arthropod  embryo.  A.  B,  Sagittal  sections;  C,  D,  the  embryo  seen  from  the  neural 
surface.  The  figures  indicate  the  relations  between  the  gastrula,  cephalic  navel,  neurostoma,  telopore,  and  telo- 
blasts; and  the  axial  structures  formed  from  the  latter.  Here  the  gastrula  gives  rise  only  to  those  structures  be- 
longing to  the  primitive  head,  or  that  part  of  the  embryo  derived  from  a  coelenterate  ancestor.  The  teloblasts, 
with  or  without  the  formation  of  a  terminal  infolding,  or  telopore,  give  rise  to  the  axial  cords,  out  of  which,  like 
an  appendage  to  the  old  radially  symmetrical  head,  the  new,  bilaterally  symmetrical,  segmented  trunk  is  formed. 

In  the  craniates,  therefore,  the  gastrula  and  trochosphere  stages  and  the 
blastopore  are  omitted,  or  are  but  faintly  repeated  at  the  head  end  of  the  medul- 
lary plate,  while  a  conspicuous  false  gastrulation  is  produced  by  the  infolding  of 
the  teloblastic  areas  at  the  end  of  a  rapidly  growing  trunk.  The  organs  of  the 
trunk,  produced  by  apical  growth  become  recognizable,  or  separate  from  each 
other  from  before  backward,  at  varying  periods  in  different  members  of  the  group, 
and  there  is  a  general  tendency  to  carry  the  development  within  the  egg  up  to 
later  and  later  stages,  so  that  the  young,  when  liberated,  usually  develop  into  the 
adult  without  a  marked  metamorphosis. 


DEVELOPMENT. 


405 


C.  The  Acraniates. — In  the  acraniates  the  embryonic  development  takes 
place  under  a  new  set  of  conditions  and  is  expressed  in  a  new  set  of  forms.  The 
eggs  as  a  rule  are  very  small,  practically  devoid  of  yolk,  and  develop  rapidly,  with 
continuous  epithelial  layers  and  folds.  The  gastrula  stages  are  passed  within 
the  egg  membranes,  and  the  embryo  escapes  at  an  early  period  as  a  small  unseg- 


SE. 


Tut.' 


FIG.  270. — Semi- diagrammatic  sagittal  sections  through  the  embryo  and  young  larva  of  Balanoglossus,  to  illustrate 
the  relation  of  the  neostoma  and  haemostoma  to  the  gastrula,  telocoele  and  telopore. 

mented  larva,  or  naupula,  representing  under  various  disguises  the  nauplius  stage 
of  their  crustacean  ancestors.  For  such  a  large  and  diversified  group  of  animals, 
the  early  embryonic  and  larval  stages  are  remarkably  uniform.  The  small,  more 
or  less  transparent  eggs  undergo  a  total  and  nearly  equal  cleavage,  forming  a  small, 
hollow  blastula  (cirripeds  excepted).  No  true  gastrulation,  such  as  that  in  the 


FIG.  271. — Diagrams  of  the  development  of  a  vertebrate  and  acraniate  seen  as  a  semi-transparent  object  from 
the  neural  surface.  The  primitive  gastrula  is  indicated  only  by  the  remnants  of  the  neurostoma,  now  a  shallow  pit 
lying  on  the  floor  of  the  procephalic  lobes,  and  later,  after  the  closure  of  the  medullary  plate,  giving  rise  to  the 
infundibulum  and  the  saccus  vasculosus.  The  teloblasts  have  increased  greatly  in  importance,  and,  like  those  in 
the  arthropods,  give  rise  to  a  large  terminal  infolding,  or  false  gastrula,  from  the  walls  of  which  the  axial  cords,  such 
as  the  notochord,  mesoderm  and  ended erm  are  formed.  These  structures  separate  at  various  periods,  and  in 
various  manners,  as  shown  by  the  cross- sections  E-H,  but  in  all  cases  the  end-result  is  the  same,  and  the  real 
sources  of  the  axial  cords  are  special  groups  of  proliferating  cells  lying  at  the  caudal  apex  of  the  trunk. 

annelids,  occurs  in  the  group,  but  there  is  a  large  infolding,  or  mesentoccele,  at 
the  posterior  end  of  the  blastula,  in  which  the  teloblasts  and  their  earlier  products 
are  involved.  (Fig.  271,  A.)  It  opens  outward  by  a  telopore  that  marks  the 
caudal  end  of  the  body,  and  closes  near  the  point  where  the  anus  is  formed.  It 
never  remains  open  as  the  primitive  mouth,  and  is  never  formed  in  the  oral  or 
cephalic  region. 

The  component  parts  of  the  infolded  layer  begin  to  separate,  or  first  become 


406  THE  CRANIATES  AND  THE  ACRANIATES. 

recognizable,  at  the  head  end,  the  process  extending  backward  with  the  growth 
of  the  body.  When  viewed  in  cross-section,  the  mesoderm  and  notochord  ap- 
pear, at  first  sight,  to  be  in  the  act  of  arising  from  the  endoderm.  (Fig.  271,  G.) 
As  a  matter  of  fact,  what  is  really  taking  place  is  the  belated  separation  of  the 
mesoderm  bands,  notochord,  and  endoderm  from  one  another,  while  all  of  them 
owe  their  origin  to  terminal  groups  of  proliferating  cells,  just  as  in  the  typical 
craniate  embryos. 

An  enteron,  or  primitive  gut,  does  not  exist  till  after  the  products  of  telo- 
blastic  growth  have  separated,  and  the  lateral  bands  of  the  endoderm  have  united 
to  form  a  closed  tube.  The  mesoderm  may  separate  from  the  lateral  walls  of  the 
mesentoccele  as  hollow  vesicles,  either  during  (Amphioxus),  or  before  its  division 
into  somites,  or  ccelomic  chambers  (echinoderms).  In  either  case  the  process  is 
not  a  primitive,  but  a  secondary  one.  It  is  merely  another  way  of  attaining  the 
same  conditions  seen  in  the  annelids  and  arachnids,  and  is  probably  the  result  of 
a  rapid  development  of  yolkless  eggs.  Owing  to  the  almost  universal  absence 
of  a  considerable  volume  of  yolk  in  the  acraniates,  there  is  no  terminal  concres- 
cence like  that  in  the  craniates. 

Mouth.— A.  functional  neurostoma  and  primitive  stomodaeum  are  formed 
between  the  anterior  ends  of  the  nerve  cords  in  the  cirripeds,  chaetognatha,  echino- 
derms, brachiopods,  and  polyzoa.  In  all  the  other  sub-phyla,  a  vestigial,  or  trans- 
itory neurostoma  is  formed  as  a  median  depression  in  the  anterior  end  of  the 
medullary  plate.  It  opens  outward,  in  those  forms  in  which  the  anterior  end  of 
the  medullary  plate  is  not  infolded  to  form  a  closed  forebrain  vesicle  (enterop- 
neusta,  pterobranchia,  phoronida,  polyzoa,  chaetognatha.  Where  the  medullary 
plate  is  infolded  and  closed,  the  neurostoma  lies  in  the  floor  of  the  forebrain 
vesicle  (tunicates,  Amphioxus).  The  infolding  for  the  primitive  mouth  may  be 
of  considerable  depth,  forming  a  true  primitive  stomodaeum,  opening  perma- 
nently into  the  mesenteron  as  in  the  case  of  the  dorsal  tubercle  and  subneural 
gland  of  the  tunicates;  or  a  blind  pocket  may  arise  from  the  midgut,  that  grows 
toward  the  primitive  mouth,  representing  either  the  cut  off  remnant  of  the  primi- 
tive stomodaeum,  or  that  part  of  the  midgut  that  formerly  communicated  with  it. 
(enteropneusta,  pterobranchia,  (and  phoronida  ?)). 

The  haemostoma  is  a  new  formation  arising  independently  of  the  old  mouth, 
from  the  anterior  haemal  surface  of  the  body. 

The  Naupula. — The  larva  of  the  acraniates,  or  the  naupula,  is  a  small, 
usually  free  swimming,  pelagic  form  resembling  a  cirriped  nauplius.  It  may 
undergo  a  part  of  its  development  in  the  brood  pouches,  or  recesses,  of  the  atrial 
chamber  (cirripeds,  tunicates,  echinoderms,  brachiopods,  and  polyzoa).  The 
naupula  differs  from  a  trochosphere  in  that  it  represents  a  distinctly  older  phylo- 
genetic  stage,  and  undergoes  a  special  kind  of  metamorphosis.  It  possesses  a 
longitudinal,  circular  fold,  representing  a  larval  mantle  fold  or  carapace,  that 
usually  has  a  ciliated  margin.  The  larva  comes  to  rest,  neural  side  down,  and 
becomes  permanently  attached,  usually  by  an  adhesive  disc,  aided  by  rudimentary 


THE    CCELOM.  407 

cephalic  appendages.  Subsequently  the  larva  rotates,  bringing  its  neural  side 
up  (cirripeds,  tunicates,  echinoderms,  polyzoa,  brachiopods  (?)),  and  the 
haemal  surface  of  the  head  rapidly  grows  into  the  characteristic  voluminous  stalk, 
or  peduncle,  by  which  the  animal  is  permanently  attached. 

The  Ccelom. — The  primitive  body  cavity  is  the  space  enclosed  between  the 
somatic  and  splanchnic  mesoderm.  It  was  probably  primarily  segmented,  form- 
ing two  completely  closed  chambers  for  each  metamere.  On  the  sides  and  haemal 
surface,  the  walls  separating  adjacent  chambers  tend  to  break  down,  forming 
extensive  sinuses  containing  loosely  united  or  isolated  cells  (blood  cells).  On  the 
neural  surface  the  original  segmentation  is  usually  more  strongly  marked  and 
more  permanent,  and  certain  portions  may  be  retained,  or  set  apart,  as  thin 
walled  chambers,  lined  with  epithelium,  and  devoid  of  free  amoeboid  cells  or 
blood  corpuscles. 

They  may  consist  of  a  small  part  of  a  single  mesoblast  segment,  the  part 
that  is  directly  connected  with  the  excretory  or  nephridial  duct,  or  that  developed 
mainly  as  a  tubular  outgrowth  from  it  (Figs.  279,  294);  or  several  such  portions 
may  unite  to  form  more  extensive  chambers.  They  are  lined  in  part  by  flat, 
indifferent  endothelium,  and  in  part  by  more  specialized  excretory  cells,  and 
they  may  open  to  the  exterior  by  glandular  ducts,  or  nephridia,  of  which  only 
the  terminal  portion  is  of  ectodermic  origin.  These  so-called  ccelomic  chambers 
are  often  referred  to  as  the  true  ccelom,  and  have  been  regarded  as  the  primitive 
body  cavity,  but  they  are  in  reality  either  special  portions  of  more  primitive  and 
more  extensive  spaces,  or  the  parts  that  remain  hollow,  after  the  other  portions 
have  been  shut  off  as  vascular  spaces  or  canals,  or  have  been  completely  filled  by 
the  growth  of  fibrous,  muscular,  or  other  tissues. 


CHAPTER  XXIII. 
THE  CIRRIPEDS,  TUNICATES  AND  ECHINODERMS. 

I.  THE  CIRRIPEDS. 

The  cirripeds  are  the  only  members  of  the  acraniates  in  which  the  more 
typical  arthropod  characters  are  retained.  They  present  an  extraordinary  diver- 
sity of  form  and  structure,  but  many  of  their  peculiarities,  such  as  the  enormous 
cephalic  stalk,  the  mantle,  pigmyism,  the  absence  or  disappearance  of  appendages, 
of  sensory  and  alimentary  organs,  and  of  external  segmentation,  we  shall  see  ex- 
pressed in  a  more  stable  and  permanent  form  in  other  members  of  the  group. 

The  Nauplius  and  the  Naupula. — The  young  leave  the  egg  like  many 
other  primitive  arthropods,  in  the  nauplius  stage,  as  a  small,  free  swimming  larva 


B. 

FIG.  272. — Diagrams  of  a  nauplius,  based  in  part  on  Pedeschenko's  figures  of  Lernaea;  A,  neural  surface;  B,  haemal. 

with  three  pairs  of  appendages.  (Fig.  289.)  Its  minute  structure  is  doubtless 
very  similar  to  that  of  a  parasitic  copepod.  The  larva  of  the  latter  being  better 
known,  it  may  be  taken  to  illustrate  the  structure  of  the  nauplius,  the  basic  larval 
form  of  the  entire  group  of  acraniates. 

In  Lernaea  branchiata,  which  has  been  carefully  studied  by  Pedaschenko, 
the  nauplius  (Fig.  272),  is  provided  with  a  well  developed  brain,  br.,  frontal  or- 
gans, ol.,  rudimentary  lateral  eyes,  I.e.,  a  trioculate  median  eye,  p.e.,  and  stomo- 
daeal  ganglia,  st.g.,  very  similar  to  those  we  have  already  seen  in  Branchipus,  in 
other  phyllopods,  and  in  Limulus.  There  is  also  a  large  cephalic  navel,  do. 
(dorsal  organ)  that  undergoes  a  characteristic  degeneration  and  absorption  by 
the  yolk  cells. 

Behind  the  mandibular  ganglion  the  nerve  cords  are  widely  separated,  but 
they  unite  again  at  the  caudal  end,  forming  the  rudiments  of  the  abdominal  neuro- 
meres,  or  ventral  cord.  The  latter  increases  in  length  by  the  multiplication  of 
prominent  telo-neuroblasts.  A  middle  cord,  m.ch.,  is  clearly  indicated. 

408 


THE    CIRRIPEDS. 


409 


The  germ  cells  appear  in  the  undifferentiated  blastoderm  at  a  very  early 
period,  and  later  form  a  conspicuous  cluster  of  cells  in  the  middle  of  the  neural 
surface  of  the  nauplius,  between  the  separated  nerve  cords,  g.c.,  in  the  same  posi- 
tion, therefore,  that  they  have  in  the  polyzoa  and  pterobranchia. 

In  the  cirripeds  two  prominent  longitudinal  pleural  folds  are  formed  that 


FIG.  273. — Limnadia  lenticularis.      (After  Nowikoff.)   x  7  1/2. 

represent  the  beginning  of  a  two-lobed  thoracic  shield  comparable  with  that  of 
the  ostracodes  and  other  primitive  Crustacea,  and  from  which  the  mantle  will 
develop  later.  (Fig.  289,  mt.) 

In  the  later  stages  of  the  nauplius  there  is  usually  developed  an  enormous 
labrum  (Figs.  7,  ro.  289,  /.),  that  overhangs  and  conceals  the  mouth,  and  a  promi- 


ci 


Fig.  274. — Diagrams  of  a  cirriped  larva,  illustrating  its  mode  of  attachment,  revolution,  and  metamorphosis. 

nent  caudal  lobe,  a.l.,  with  the  anus  on  its  haemal  side.  An  adhesive  disc  forms 
near  the  apex,  or  the  haemal  surface  of  the  head,  by  which  the  larva,  for  a  longer 
or  shorter  period,  is  attached  to  foreign  objects. 

A  nauplius-like  larva,  with  most  of  the  characters  indicated  above,  occurs 
under  various  disguises  and  modifications  in  all  the  subphyla  of  the  acraniates. 
In  its  modified  form  we  shall  refer  to  it  as  the  naupula. 


4io 


THE    CIRRIPEDS,    TUNICATES    AND    ECH1NODERMS. 


The  Metamorphosis.— In  the  cirripeds  the  larval  cephalic  shield  is  usually 
very  large  and  the  body  lies  on  its  concave  neural  surface.  Its  margin  is  sensitized 
and  drawn  out  into  very  prominent  horns  or  lobes,  of  which  there  are  usually  two 
especially  long  ones  in  front,  two  behind,  and  minor  ones  between.  (Fig.  289.) 
After  a  time  the  caudal  lobe  elongates,  the  antennae  migrate  forward  and  haemally, 
and  the  thoracic  appendages  and  the  rudimentary  lateral  eyes  make  their  ap- 
pearance. 

The  larva,  having  taken  on  the  shape  and  general  appearance  of  an  ostracode 
(cypress  stage)  which  it  will  be  recalled  is  one  of  the  first  forms  to  make  its  appear- 


oc 


ov. 


FIG.  275. — Semi-diagrammatic  sagittal  section  of  a  cirriped.      Lepas, 


FIG.  276. — Mouth  parts  of  Lepas. 


ance  in  the  geological  record,  attaches  itself  to  some  foreign  object,  haemal  side 
down,  by  glandular,  disc-like  expansions  of  its  first  pair  of  appendages.  (Fig. 
274.)  Cement  glands  appear  and  the  greatly  enlarged  cephalic  outgrowth  be- 
comes firmly  attached.  The  body  next  turns  completely  over;  the  stalk  elongates, 
carying  with  it  the  remnants  of  the  first  pair  of  appendages,  and  the  animal  grad- 
ually takes  on  the  adult  form.  (Figs.  274,  275.) 

Appendages.— Eleven  pairs  of  appendages  may  be  represented;  two  pairs 
of  antennae,  three  pairs  of  jaws,  including  the  mandibles,  and  the  first  and  second 
pairs  of  maxillae,  and  six  pairs  of  abdominal  appendages.  With  the  metamor- 
phosis there  is  a  strongly  marked  tendency  toward  the  reduction,  or  atrophy  of 
many  important  larval  organs.  All  of  the  eleven  pairs  of  appendages  present  in 
the  larvae  are  never  fully  developed  in  the  later  stages.  Even  in  the  least  modified 
forms,  the  two  pairs  of  antennae  are  either  greatly  reduced  or  absent;  the  three  pairs 
of  jaws  or  oral  appendages  are  very  small,  and  two  or  three  pairs  of  the  six  pairs  of 
abdominal  appendages  may  be  absent.  Only  nine  pairs  are  present  in  the  alcip- 


THE    CIRRIPEDS.      ALIMENTARY    CANAL.       CCELOM. 


411 


pidae  and  eight  in  the  cryptophialidae.  In  apodous  forms  only  the  jaws  and  one 
pair  of  antennae  are  retained,  while  in  the  rhyzocephalidae  all  traces  of  the  appen- 
dages disappear. 

Alimentary  Canal. — The  stomodaeum  is  generally  small,  and  leads  into  a 
large  pear-shaped  enteron  provided  with  prominent  gastro-hepatic  glands.  (Fig. 
275.)  The  larger  ones  form  a  circle  of  racemose  diverticula,  or  pouches,  said  to 
be  provided  with  two  kinds  of  cells,  hepatic  and  pancreatic,  h.d.  and  g.h.  Circular 
bands  of  smaller  pouches,  or  patches  of  cells  having  a  special  structure,  are  ar- 
ranged at  regular  intervals  over  the  remainder  of  the  stomach,  diminishing  in 
distinctness  toward  the  caudal  end,  h.c.  The 
anus,  a,  is  located  on  the  haemal  side  of  the 
elongated  caudal  lobe. 

Coelom. — True  ccelomic  chambers  are  well 
developed  in  the  cirripeds  and  copepods.  In 
Lernaea  ten  pairs,  one  apparently  for  every 
metamere  except  the  first,  have  been  described 
by  Pedaschenko.  (Fig.  279,  A.)  They  form 
large  segmentally  arranged  chambers  enclosed 
in  thin  but  well  defined  epithelial  walls.  They 


oc. 


FIG.  277. — Petrarca  mira  (after  Fowler); 
commensinal  in  the  mesenteric  chamber  of  the 
coral  Bathyactis.  Ramifications  of  testis,  ovary, 
and  liver  in  the  mantle  folds. 


FIG.  278. — Ibla  quadrivalis,  male;  parasitic 
on  the  female,  within  the  palial  chamber.  (From 
Gruvel,  slightly  modified  3-4  mm.  long.) 


are  completely  shut  off  from  the  remaining  parts  of  the  primitive  ccelom,  or 
haemoccele,'  which  consists  of  irregular,  extensive,  unsegmented  spaces  that  are 
surrounded  by  mesoderm  and  contain  free  mesodermic  cells. 

In  the  adult  cirripeds  three  pairs  of  ccelomic  chambers  have  been  recognized, 
although  their  early  history  and  identity  are  not  known.  (Fig.  279,  B.)  There 
is  a  small  pair  of  completely  closed  sacs  in  the  head  region,  lined  with  excretory 
cells  that  probably  represent  the  remnants  of  the  antennary  ccelom  and  the  anten- 
nary  gland,  c1.  The  second  pair  lie  in  the  thoracic  or  circumoral  region,  on 
either  side  of,  and  close  to,  the  stomach,  c2.  They  are  extensive  chambers  that 
communicate  with  each  other  across  the  median  line,  in  front  of  and  behind  the 
mouth  (Gruvel).  They  are  completely  separated  from  the  haemoccele,  but  open 


412 


THE    CIRRIPEDS,    TUNICATES    AND    ECHINODERMS. 


to  the  exterior  through  the  nephridia-like  ducts  of  the  shell  gland,  mx.d.  They 
probably  represent  the  combined  thoracic  cceloms  together  with  the  excretory 
portion  of  the  "shell  gland"  or  "coxal  gland"  of  the  second  pair  of  maxillae. 
The  third  pair,  c3,  lie  external  to  and  somewhat  behind  the  second.  They  are 
completely  closed,  and  probably  represent  the  remnants  of  several  pairs  of  united 
abdominal  ccelomic  chambers. 

Excretory  Organs. — Two  pairs  of  nephridia-like  excretory  organs  are 
conspicuous  in  the  arthropods  and  remarkably  constant  in  their  location.  The 
so-called  green  gland  of  the  second  antennae,  and  the  shell  gland  of  the  second 
maxillae  of  Crustacea  (coxal  gland  of  the  fifth  pair  of  thoracic  appendages  in 
arachnids).  The  characteristic  thin-walled  end  sac  of  these  organs  is  derived 
from  a  portion  of  the  ccelom,  but  a  variable  one.  It  may  represent  a  single 


Pc.C. 


?r.C. 


c. 


FIG.  279. — Diagrams  to  illustrate  the  relations  of  the  coelomic  chambers,  and  their  excretory  ducts,  to  the 
main  subdivisions  of  the  body  (tagmata)  and  to  the  central  nervous  system.  A,  Larval copepod  (Lernaea);  B,  an 
adult  cirriped;  C,  Balanoglossus. 


somite  (Limulus)  or  a  small  part  of  one  (Lernaea) ,  or  the  whole,  or  a  part  of  several 
combined  somites  (cirripeds  and  other  acraniates). 

In  the  cirripeds  the  excretory  organs  of  the  fifth  metamere  have  very  volum- 
inous end  sacs,  c2,  and  the  nephridia-like  tubes  that  lead  off  from  them  open  at 
the  base  of  the  second  pair  of  maxillae.  A  small  pair  of  excretory  sacs  lie  in  the 
head  region  in  front  of  the  mouth  that  probably  represent  the  remnants  of  the 
antennary  glands  of  other  Crustacea. 

The  sexual  organs  are  of  exceptional  volume  in  the  cirripeds,  the  racemose 
testis  ramifying  through  the  whole  trunk  and  opening  at  the  apex  of  the  modified 
tail  lobe.  (Fig.  275.)  The  location  of  the  ovaries  is  noteworthy  in  that  they 
lie  mainly  in  the  cephalic  stalk  and  anterior  portion  of  the  mantle,  the  ovarian 
lobules,  in  some  cases,  extending  into  the  recesses  of  the  exoskeleton.  The 
oviduct  opens  outward  near  the  middle  of  the  body,  at  the  base  of  the  anterior 
pair  of  abdominal  appendages,  o.d. 


THE    CIRRIPEDS.      DEGENERATION. 


413 


In  the  more  degenerate  cirripeds,  the  sexes  are  separate,  and  the  males  are 
reduced  to  minute  forms  parasitic  on  the  females.  (Figs.  278-281.) 

Degeneration. — The  wide  range  of  variation  in  the  cirripeds  is  largely 
due  to  the  varying  amount  of  degeneration  following  the  metamorphosis.  The 
mantle,  integument,  and  sexual  organs  are  often  the  only  parts  that  retain  the 
normal  powers  of  growth.  The  degeneration  may  be  manifest  in  the  dimin- 
ished size  of  the  whole  body,  and  by  the  absence,  or  dwindling  of  appendages, 
muscles,  alimentary  canal,  sense  organs,  and  nervous  system.  The  anal  opening 


FIG.   281. 

FIG.  280. — A,  Alcippe  lampas,  female;  about  8  mm.  long;  B,  male,  parasitic  on  disc  of  female;  about  r  mm. 
long.  Probable  position  of  the  remnants  of  the  mouth,  indicated  at  m;  cement  glands  and  alimentary  canal, 
absent;  excretory  organs  closed  (?).  (After  Berndt,  slightly  modified.") 

FIG.  281. — C,  Scalpellum  vulgare,  dwarf  male;  surface  view;  D,  in  optical  section.      Fixed  to  the  hermaph- 
roditic individuals;  mouth  and  ailmentary  canal  absent.       (After  Scott,  slightly  modified.) 

may  close  (Petrarca,  Fig.  277,  females  of  Alcippe,  Fig.  280),  or  the  proctodaeum, 
stomodaeum,  and  the  entire  mesenteron  may  disappear,  as  in  the  dwarf  males 
of  Scalpellum  (Fig.  281),  of  Alcippe  (Fig.  280),  and  many  copepods  (Figs.  282 
and  283). 

It  is  doubtful  whether  these  dwarf  males  survive  long  after  the  maturation 
of  the  spermatozoa,  but  there  is  a  certain  vegetative  vigor  in  the  surviving  organs 
of  the  larger  individuals,  i.e.,  females  and  hermaphrodites,  that  is  not  so  seri- 
ously affected  by  degeneration. 

The  Old  Mouth  and  the  New. — A  highly  significant  aspect  of  degeneration 
in  cirripeds  is  the  closing  of  the  mouth  (neurostoma),  and  the  dwindling  or  dis- 


414 


THE    CIRRIPEDS,    TUNICATES    AND    ECHTNODERMS. 


appearance  of  the  stomodaeum.  We  have  already  witnessed  a  tendency  in  this 
direction,  in  the  arachnid-vertebrate  stock,  and  the  origin  of  a  new  mouth, 
or  h^mostoma,  from  the  dorsal  organ,  or  cephalic  navel,  p.  253.  Similar  condi- 
tions are  latent  in  the  cirripeds.  A  typical  embryonic  dorsal  organ  is  a  conspic- 
uous feature  in  the  young  nauplius  of  Lernaea  and  of  many  other  primitive  crus- 
tacea.  Its  invagination,  or  ingrowth  into  the  yolk,  followed  by  disintegration 
and  absorption,  probably  played  an  important  part  in  establishing  a  permanent 
opening  into  the  enteron  at  this  point,  from  which,  in  the  other  sub-phyla  of  the 
acraniates,  the  new  mouth  arose.  The  exact  way  in  which  the  new  opening 


c.r 


cr. 
LerncBa. 


FIG.  282. — Lernaea  branchialis.     A,  Young  fertilized  female;  B,  penella  stage;  fertilized  female  in  gill  of  Whiting;  C, 
adult  female,  attached.      (After  Scott,  slightly  modified.) 

was  established,  and  its  relation  to  the  adjacent  gut  pouches,  to  the  adhesive 
glands,  and  to  the  cephalic  stalk  are  not  clear  because  very  little  is  known  about 
the  details  of  these  important  structures.  But  it  is  certainly  significant  that  in  the 
rhizocephala,  where  the  old  mouth  closes,  the  animals  manage  to  survive  by  the 
absorption  of  nutrition  through  the  root-like  outgrowth  of  the  cephalic  stalk  that 
is  formed  at  the  place  where  the  dorsal  organ  closes.  The  condition  in  Tubicinella 
is  likewise  suggestive,  for  Gruvel  states,  p.  279,  that  according  to  Marloth,  the 
tubicinellae  secrete  a  peptonizing  ferment  that  diffuses  through  the  membranous 
base,  transforming  into  peptones  the  albuminoid  substances  of  the  skin  of  the  whale, 
to  which  these  forms  are  attached.  Without  doubt  we  have  here  a  glimpse  of  the 
way  in  which  the  old  mouth  disappeared,  and  the  way  in  which  the  new  one  was 


THE    TUNICATES. 


415 


formed  that  became  the  haemostoma  of  the  tunicates,  enteropneusta,  and  other 
acraniates. 

It  is  often  assumed  that  a  sessile  or  parasitic  mode  of  life  is  the  initial  cause 
of  degeneration.  The  various  anatomical  peculiarities  common  to  the  copepods, 
cirripeds,  and  acraniates  do  not  bear  out  this  conclusion.  The  fact  that  in  these 
diverse  sub-phyla  we  see  the  same  shifting  of  cephalic  appendages  to  the  haemal 
side,  the  same  cephalic  outgrowths,  and  the  same  degeneration  of  the  neuro- 
muscular  organs,  indicates  that  there  are  certain  initial  defects,  or  peculiarities 
of  germinal  material,  common  to  the  whole  group,  that  is  the  underlying  cause  of 
a  defective  organization,  and  the  defective  organization  is  in  every  case  of  such 
a  nature  that  a  sessile,  or  parasitic,  or  vegetative  mode  of  life  is  the  only  one 
possible. 


D 


LepeopKtKevcus 


Calicjus. 


FIG.  283.  —  Lepeoptheirus.     D,  Haemal,  E,  neural  surface.      F,  G,  Caligus.      (From  Scott,  slightly  modified.) 

II.  THE  TUNICATES. 

It  was  from  a  stock  similar  to  that  of  the  copepods  and  cirripeds,  and  one 
dominated  by  the  same  fundamental  defects  in  germinal  material,  that  the 
tunicates  arose.  They  are  built  on  the  same  general  plan,  pass  through  a  similar 
larval  existence,  and  undergo  a  similar  retrograde  metamorphosis.  But  all  this 
is  at  first  sight  effectively  disguised  by  the  permanent  closing  of  the  old  mouth, 
and  the  opening  of  a  new  one  on  the  haemal  surface;  by  the  absence  of  both  the 
chitenous  and  calcareous  exoskeleton  and  appendages;  and  by  the  conspicuous 
development  of  perforated  gill  sacs,  and  of  the  middle  cord,  or  notochord.  Like 
the  cirripeds  they  begin  life  with  the  same  brave  display  of  animal  vigor,  of  well 
developed  brain,  eyes,  and  locomotor  organs  that  bespeak  an  efficient  past  and  a 
hopeful  future,  only  to  have  them  dwindle  almost  to  extinction  in  a  peaceful, 
sedentary  existence. 

The  early  stages  of  the  tunicate  embryo  are  essentially  like  those  of  a  primi- 
tive crustacean  (Moina,  Cetochiles,  Balanus,  or  Palaemon).  In  both  types  there 
is  the  same  kind  of  cleavage;  the  same  terminal  proliferation  or  infolding  to  form 
a  teloccele;  the  same  slipper-shaped  medullary  plate;  the  same  middle  cord  or 


4i6 


THE    CIRRIPEDS,    TUNICATES    AND    ECHINODERMS. 


notochord  arising  from  the  ectoderm  and  projecting  forward  from  the  anterior 
lip  of  the  teloccel;  the  same  lateral  bands  of  mesoderm;  the  same  kind  of  trioculate 
median  eye;  the  same  infolding  of  the  medullary  plate  on  the  neural  side  of  the 
head  to  form  the  primitive  stomodaeum,  or  subneural  gland,  and  a  similar  organ 
on  the  opposite  side  of  the  head  representing  an  acutal  or  a  potential  haemostoma; 
and  a  similar  larval  metamorphosis. 

When  the  tunicate  larva  escapes  from  the  egg,  three  glandular  tubercles 
appear  on  the  anterior  haemal  surface  of  the  head,  representing  the  remnants 
of  arthropod  cephalic  appendages.  After  a  short,  free  swimming  existence,  it 
attaches  itself  by  these  appendages  to  some  foreign  object,  head  down,  and  in  a 
nearly  vertical  position,  and  then  begins  its  metamorphosis.  (Fig.  284,  A.)  It 
undergoes  a  partial  rotation,  turning  neural  side  up;  the  cephalic  tubercles  are 
gradually  merged  in  the  larger  cephalic  stalk;  the  body  contracts,  taking  on  a  pro- 


FIG.  284. — Diagrams  illustrating  the  mode  of  fixation  and  the  metamorphosis  of  an  ascidian. 

nounced  curvature  that  draws  the  head  end  upward  toward  the  root  of  the  tail; 
the  latter  atrophies,  and  with  the  growth  of  the  mantle,  the  remnants  of  the  body 
are  completely  enclosed  in  the  large  atrial  chamber. 

In  Figs.  285  and  286  I  have  attempted  to  show,  in  a  purely  diagrammatic  way, 
the  manner  in  which  a  cirriped-like  animal  could  be  metamorphosed  into  a 
tunicate.  These  figures  should  be  compared  with  what  actually  takes  place  in  a 
tunicate  (Fig.  284),  and  with  the  conditions  that  actually  occur  in  the  metamorpho- 
sis of  a  typical  cirriped,  such  as  Lepas  (Fig.  274),  or  with  that  which  prevails  in 
the  adult  condition  of  more  degenerate  cirripeds,  such  as  Ibla  and  Alcippe, 
Scalpellum,  Petrarca,  and  Sacculina.  (Figs.  278  and  280.) 

With  the  atrophy  of  the  body,  the  notochord  disappears  and  the  elongated 
nerve  cord  is  reduced  to  a  small,  compact  cerebral  ganglion  which,  like  that  in 
many  parasitic  cirripeds,  still  surrounds  the  proximal  end  of  the  old  stomodaeum, 
i.e.,  the  subneural  gland,  s.g. 

In  the  cirripeds  (Lepas) ,  the  anterior  end  of  the  enteron  forms  an  immense 
chamber  with  numerous  enteric  pouches  arranged  in  transverse  bands.  (Fig. 


THE  TUNICATES.   THE  HEART  AND  VASCULAR  SYSTEM. 


417 


275.)  In  the  tunicates  similarly  arranged  enteric  pouches  are  formed,  which 
unite  with  the  lateral  walls  of  the  body,  or  with  infolded  rudimentary  appen- 
dages, forming  gill  clefts  that  lead  from  the  enteron  into  the  atrial  or  peribranchial 
chamber,  and  thence  to  the  exterior. 


The  Heart  and  Vascular  System. — In  both  tunicates  and  cirripeds  the 
vascular  system  is  imperfectly  developed;  nevertheless  there  are  some  interesting 
points  for  comparison.  In  the  tunicates  there  is  a  short,  oval  heart  on  the  poster- 
ior haemal  surface  of  the  thoracic  region.  It  is  similar  in  form  and  location  to  that 
in  primitive  Crustacea,  and  the  general  arrangement  of  the  associated  blood  chan- 
nels is  also  similar. 


FIGS.  285,  286. — Diagrams  illustrating  the  manner  in  which  a  sessile,  cirriped-like  arthropod  is  supposed  to  give 
rise  to  a  tunicate.     A-D,  Seen  from  the  side;  E-H,  same,  seen  from  the  neural  surface. 

In  the  cirripeds  the  heart  is  absent.  But  in  many  of  the  primitive,  short- 
bodied  Crustacea,  a  heart  is  present,  and  consists  of  a  short  oval  sac  containing 
but  one  pair  of  openings  (cladocera,  Fig.  9,  h.),  or  a  small  number  of  them. 
It  no  doubt  arises  in  typical  arthropod  fashion  from  the  fusion  of  the  lateral  meso- 
derm  plates  of  the  posterior  thoracic  metameres.  The  circulation  in  the  tunicates 
is  chiefly  remarkable  for  the  periodic  reversal  of  the  direction  in  which  the  blood 
is  made  to  flow,  a  condition  generally  assumed  to  occur  nowhere  else  in  the  animal 
kingdom. 

In  the  copepods  a  slowly  pulsating  heart,  similar  in  appearance  to  that  of 
cladocera,  may  be  present,  but  in  parasitic  forms  it  is  said  to  be  absent,  although 
there  are  certain  channels  through  which  the  blood  flows  in  a  definite  direction. 
In  Lepeophtheirus,  for  example,  according  to  Scott, 1  the  circulation,  while  wholly 

Liverpool  Marine  Biol.  Com.  Memoirs.   VI,   1901. 
27 


4i8 


THE    CIRRIPEDS,    TQNICATES   AND    ECHINODERMS. 


lacunal,  follows  well  marked  channels.  The  remarkable  part  of  it  is  that  the  blood 
currents,  as  he  states,  "  Do  not  continue  to  flow  for  any  length  of  time  in  the  one 

direction.  At  one  period  they  may  be  flowing  as  indicated They  then 

suddenly  slacken  and  reverse  and  stream  for  a  time  in  exactly  the  opposite  course." 
(p.  21.)  In  this  particular,  therefore,  the  circulation  is  astonishingly  like  the 
well  known  reversing  circulation  of  the  tunicates. 

The  Eyes. — In  the  tunicates  the  lateral  eyes  are  absent,  as  they  are  in  the 
adult  stages  of  cirripeds  and  all  other  acraniates.  The  parietal  eye,  however,  may 
be  retained,  and  in  some  forms,  as  Salpa,  it  may  be  present  and  even  well 
developed  in  the  adult  stages,  resembling  in  a  very  striking  manner  the  nauplius 
eye,  or  trioculate  median  ocellus  of  primitive  crustaceans. 

The  details  of  its  early  development  are  obscure,  especially  the  manner  in 
which  the  several  retinas  are  infolded  and  become  lodged  on  the  roof  of  the  neural 


FIG.  287. — The  forebrain,  ventral  cord,  and  ocelli  of  Cyclosalpa,  chain  form,  adult.     A,  Seen  from  the  neural  sur- 
face; B,  in  cross-section;  C,  in  longitudinal  section.      (After  Metcalf,  slightly  modified.) 

tube.  Judging  from  the  structure  of  the  eye,  and  from  what  is  known  of  the  de- 
velopment of  the  medullary  tube,  the  tunicate  parietal  eye  appears  to  develop 
in  essentially  the  same  manner  as  the  median  eye  of  Branchipus.  In  other  words, 
it  is  to  be  regarded  as  a  true  parietal  eye,  consisting  of  two  pairs  of  ocelli,  the 
retinas  of  which  have  become  loosely  united  to  form  the  walls  of  a  common 
unpaired  vesicle,  opening  into  the  cavity  of  the  forebrain,  and  forming  a  part 
of  its  roof.  (Fig.  287.)  The  eye  is  innervated  by  two  principal  nerves,  «,  that 
arise  from  a  dorsal  ganglionic  mass,  probably  representing  the  forebrain  plus  the 
optic  ganglia. 

The  Old  Mouth  and  the  New. — When  the  medullary  plate,  which  represents 
the  entire  brain  and  nerve  cord  of  the  ancestral  arthropod,  was  infolded,  the  parietal 
eye  and  primitive  stomodaeum  were  carried  in  with  it,  and  when  the  medullary 
tube  finally  closed,  the  primitive  mouth  and  the  stomodaeum  were  permanently 
shut  off  from  the  exterior;  but  they  retained  their  original  structure  and  relations 
essentially  unchanged,  for  the  stomodaeum  persisting  as  the  subneural  gland  and  its 
duct,  opens  at  its  outer  end,  through  the  floor  of  the  embryonic  brain  into  the 
rudimentary  cerebral  vesicle;  while  the  inner  end  still  opens  into  the  enteron  as 
the  so-called  dorsal  tubercle.  (Figs.  284-286.) 


THE  TUNICATES.   THE  MANTLE.  419 

In  the  adult  Salpa  the  oval  ventral  portion  of  the  brain  (Fig.  287,  m.ce),  from 
which  arise  numerous  pairs  of  peripheral  nerves,  and  which  formed  the  floor  of 
the  anterior  part  of  the  medullary  plate,  probably  represents  the  condensed  rem- 
nants of  several  thoracic  neuromeres.  The  dorsal  portion,  p.ce.,  represents  the 
primitive  forebrain,  or  supracesophageal  ganglion.  The  cavity,  or  space  between 
them,  represents  the  rudiments  of  a  cerebral  ventricle.  Before  the  neural  tube  is 
completely  closed,  the  subneural  gland,  or  primitive  stomodaeum,  opens  into  this 
space  and  thence  to  the  exterior  (Fig.  287,  C.) 

The  new  mouth,  or  haemostoma,  arises  from  the  opposite  side  of  the  head, 
in  the  region  of  the  cephalic  naval,  or  dorsal  organ. 

The  Mantle.— A  characteristic  feature  of  the  tunicates  is  the  thick,  fibroid, 
translucent  secretion  of  the  ectoderm  that  forms  a  flexible  covering  for  them,  and 
that  takes  the  place  of  the  ectodermal  skeleton  of  arthropods.  In  its  general 
appearance  and  consistency  it  is  not  unlike  the  softer  forms  of  chiten;  but  it 
differs  from  it  chemically,  consisting  of  a  special  substance,  tunecine,  said  to  be 
identical  with  cellulose,  although  it  is  doubtful  whether  a  more  careful  analysis 
will  bear  out  this  conclusion.  It  may  be  regarded,  provisionally  as  some  modifi- 
cation of  chiten,  or  of  a  closely  related  substance. 

At  an  early  stage  of  development,  it  is  invaded  by  numerous  spindle-  or  star- 
shaped  cells,  differing  in  character  and  in  origin;  some  arise  from  the  ectoderm, 
others  from  the  mesoderm,  others  from  the  ovary.  It  is  also  broken  up  by  the 
presence  of  canals  and  spaces  that  permit  the  invasion  of  blood-vessels  and  other 
tissues,  and  there  are  occasional  deposits  in  it  of  calcareous  and  silicious  specules. 
In  one  form,  Chelyosoma,  it  consists  of  " horny  plates"  that  recall  those  of 
cirripeds. 

The  mantle  of  tunicates  may  be  regarded  as  a  special  modification  of  the 
exoskeleton  of  arthropods,  resembling  most  nearly  that  of  the  cirripeds.  The 
cirripeds  have  a  type  of  exoskeleton  not  known  to  occur  elsewhere  in  the  arthropods; 
it,  therefore,  has  a  special  interest  and  significance  for  us.  Unfortunately,  I  have  not 
had  an  opportunity  to  examine  at  first  hand  into  the  details  of  its  minute  structure 
and  development.  It  develops,  according  to  Gruvel,  underneath,  or  inside,  the 
hypodermis  of  the  mantle,  by  the  secretion  of  concentric  or  parallel  layers  of 
chiten  which  then  become  heavily  calcified.  It  is  never  cast  off,  and  continues 
to  increase  in  volume  during  life. 

In  the  early  stages,  the  chitenous  matrix  is  crowded  with  regular  spaces,  each 
containing  a  live  cell,  which  however  dies  with  the  progress  of  mineralization. 
(Gruvel,  p.  362.)  In  Pachylasma  (Fig.  288,  A),  the  shell  consists  of  two  principal 
layers,  an  outer  one  of  small  chambers  with  thick  laminated  walls  secreted  by 
infoldings  of  the  hypodermis,  h,  and  a  basal  layer  derived  from  the  inner  surface 
of  the  mantle,  m.l.  In  Balanus  (Fig.  288,  B),  the  outer  layer  contains  large  spaces 
or  parietal  canals,  en.,  and  the  hypodermis,  h,  extends  inward  in  the  form  of 
spreading  or  branching  plates.  The  peripheral  spaces  or  parietal  canals,  en., 
were  formed  by  ingrowths  from  the  inner  layer  of  the  mantle  and  contain,  for  a 


420 


THE    CIRRIPEDS,    TUNICATES   AND    ECHINODERMS. 


time,  living  tissue.  In  Corunula,  the  shell  contains  large  spaces  which  stand  in 
direct  communication  with  the  central  cavity  of  the  test,  and  in  which  is  imbedded 
that  part  of  the  ovary  that  gives  rise  to  the  eggs.  (Gruvel,  p.  368.) 

The  remarkable  structure  of  the  shell  in  the  cirripeds  recalls  that  of  Limulus 
and  Pteraspis  (Figs.  196-204.)  At  the  same  time  the  invasion  of  its  matrix  by 
isolated  hypodermal  cells,  by  pyramidal  ingrowths  of  the  mantle,  and  by  ovarian 
tubules,  is  comparable  respectively  with  the  presence  of  the  test  cells,  vascular  in- 
growths, and  the  so-called  "kalymmocytes,"  or  egg  follicle  cells,  in  the  mantle  of 


en. 


A  m 

FIG.  288. — Sections  of  the  shell  of  cirripeds.     A,  Pachylasma;  B,   Balanus.      (Afte;  Gruvel.) 

the  tunicates.  The  principal  difference,  therefore,  between  the  exoskeleton  of  the 
tunicates  and  the  complicated  ectodermal  skeleton  of  Limulus,  the  pteraspids,  and 
the  cirripeds  apparently  lies  in  the  different  chemical  composition  of  the  non- 
cellular  matrix. 


Tunicates  and  Cirripeds.    Summary. 

It  is  unnecessary  to  carry  our  comparison  into  further  detail.  The  structure 
and  mode  of  growth  of  the  tunicates  justify  the  conclusion  that  they  are  descended 
from  animals  in  which  the  salient  characteristics  of  primitive  arthropods  were 
fully  established;  in  fact,  from  that  particular  subdivision  of  the  arthropods  to 
which  the  cirripeds  and  copepods  belong.  Here  some  inherent  defects  in  the 
germinal  material  impose  on  both  ancestors  and  descendants  those  peculiarities 
of  structure  and  mode  of  life  that  are  common  to  both. 

The  tunicates  resemble  the  cirripeds:  i.  In  the  structure  of  their  larvae,  in 
their  mode  of  attachment,  and  in  their  subsequent  revolution,  degeneration,  and 
metamorphosis.  2.  In  the  investment  of  the  body  by  a  huge  fold  of  the  skin,  or 
mantle,  that  encloses  an  atrial,  or  peribranchial,  chamber.  3.  In  the  outgrowth 
that  arises  from  the  haemal  surface  of  the  head  to  form  the  stalk  or  pedicle.  4.  In 
the  occurrence  of  a  reversing  circulation.  5.  In  the  presence  of  a  parietal,  tri- 
occellate  eye.  6.  In  the  presence  of  an  exoskeleton,  which  in  the  one,  consists  of  a 
calcified,  chitenous  secretion  of  the  ectoderm;  in  the  other  of  a  substance  resem- 
bling cellulose;  each  has  a  complex  and  unusual  structure,  but  one  that  is  essentially 
the  same  in  both.  7.  In  the  tunicates,  the  enteric  pouches  have  perforated  the 
body  wall;  the  old  mouth  is  permanently  closed,  and  a  new  one  has  opened  on  the 


THE    ECHINODERMS.  421 

anterior  haemal  surface  of  the  head.  On  the  other  hand,  in  the  cirripeds  the  gut 
pouches  may  have  an  arrangement  in  transverse  rows  similar  to  that  in  the  tuni- 
cates;  the  old  stomodaeum  often  closes  up  and  ceases  to  function,  while  the  cephalic 
navel  may  form  a  temporary  opening  into  the  enteron,  in  the  place  where  the  new 
tunicate  mouth  is  formed;  and  in  the  parasitic  cirripeds  the  region  of  the  cephalic 
navel  may  actually  serve  for  the  absorption  of  nutrition. 

Thus  many  important  conditions  essential  to  the  evolution  of  tunicates  are 
present  in  cirripeds,  and  on  the  whole  the  present  structure  of  the  tunicates  is 
more  like  that  of  cirripeds  than  that  of  any  other  known  animals. 

III.  THE  ECHINODERMS. 

The  echinoderms  must  be  assigned  a  position  in  our  scheme  because  they 
appear  to  be  in  some  way  connected  with  the  enteropneusta,  and  hence  with  the 
tunicates  and  with  the  main  phylum  from  which  the  chordates  arose.  The  prob- 
lem is  a  difficult  one.  It  demands  some  explanation  of  the  apparent  resemblance 
between  echinoderms,  enteropneusta,  and  tunicates,  and  if  there  is  a  real  resem- 
blance, indicating  genetic  relationship,  it  calls  on  us  to  harmonize  the  probable 
origin  of  the  echinoderms  with  the  explanation  we  have  given  above  for  the  origin 
of  the  other  chordate  phyla.  The  key  that  unlocks  this  series  of  problems  and 
places  at  our  disposal  a  consistent  and  ever  ready  explanation  of  the  multitude  of 
details  involved,  must  indeed  be  the  master  key. 

The  echinoderms  are  notable  for  their  contrasts  and  contradictions.  Their 
outward  appearance  and  their  pronounced  radial  structure  distinguish  them  from 
all  other  animals,  and  at  first  sight  suggest  a  very  primitive  organization  similar  to 
that  of  the  ccelenterates.  On  the  other  hand,  they  display  a  high  degree  of  histo- 
logical  and  anatomical  specialization  that  is  in  marked  contrast  with  their  low 
grade  of  organic  efficiency.  They  begin  their  early  embryonic  development  with 
a  bilaterally  symmetrical  body  and  with  clear  indications  of  metamerism,  only  to 
change  it  in  the  later  stages  for  one  that  is  radially  symmetrical,  and  in  which  all 
outward  traces  of  metamerism  have  disappeared.  After  a  short  free  swimming 
larval  existence  they  attach  themselves,  neural  side  down,  by  means  of  larval 
appendages  and  a  cephalic  outgrowth;  they  then  turn  neural  side  up,  and  remain 
so  attached  for  life;  or,  in  some  cases,  they  give  up  their  sessile  existence  and  again 
become  free,  moving  slowly  about,  neural  side  down. 

There  are,  therefore,  three  chief  characteristics  of  the  echinoderms  that 
demand  our  first  consideration:  The  early,  bilateral  symmetry  and  metamerism; 
the  sessile  life  and  mode  of  attachment  by  cephalic  outgrowths;  and  the  asymmetry. 
There  appears  to  be  but  one  explanation  for  these  remarkable  conditions,  which 
is  as  follows:  The  early  development  of  bilateral  symmetry  and  metamerism  in 
the  echinoderms,  and  the  presence  of  a  teloccele  and  telopore  in  place  of  the  more 
primitive  gastrula  and  blastopore,  clearly  indicate  that  they  had  their  origin  in 
bilaterally  symmetrical  animals  of  the  acraniate  type,  that  had  already  acquired  a 


422 


THE    CIRRIPEDS,    TUNICATES   AND    ECHINODERMS. 


considerable  degree  of  complexity.  These  ancestral  forms  probably  belonged  to 
the  cirriped  group,  for  before  the  latent  asymmetry  becomes  effective  the  young 
echinoderm  larva  resembles  a  cirriped  in  its  form,  mode  of  attachment,  and  sub- 
sequent metamorphosis,  more  than  any  other  animal. 

The  radiate  structure  of  the  later  stages  was  due  to  a  persistent  local  defect, 
or  to  the  absence  of  a  definite  part  of  the  embryonic  formative  material,  which  in 
turn  created  a  condition  of  unstable  organic  equilibrium,  the  result  of  which  is  that 
the  whole  side,  following  the  path  of  least  resistance,  bends  toward  the  defective 
area,  forming  an  arch  that  increases  in  curvature  till  an  approximate  equilibrium 
is  again  attained  by  the  union  of  its  two  ends  to  form  a  circle.  The  original  half 
metameres  and  segmental  organs  are  then  arranged  in  radiating  lines,  thus  creat- 
ing a  new  radiate  type  and  a  new  set  of  internal  conditions  that  dominate  the 
future  growth  of  the  organism. 

If  we  assume  that  a  strongly  marked  asymmetry,  like  that  which  occurs  so 
frequently  as  an  abnormality  in  Limulus,  or  even  as  a  normal  character  in  the 


FIG.  289.— Cirriped  larvae.     E,  Early  nauplius  stage,  seen  from  the  neural  surface;  F,  from  the  side;  G,  metanauplius 

sage.      Semi-diagrammatic. 

bopeiridae  and  paguridae,  was  a  fixed  feature  of  the  hypothetical  ancestral  cirripeds, 
and  was  capable  of  a  successful  organic  adjustment,  we  shall  have  a  perfectly 
simple  and  natural  explanation  of  the  origin  and  structure  of  the  echinoderms, 
and  of  their  resemblance  to  the  tunicates,  enteropneusta,  and  to  the  other  chordate 
phyla. 


The  Echionderm  Larva.— A  young  echinoderm  larva  may  be  represented 
in  a  generalized  diagrammatic  form  as  shown  in  Figs.  291,  293.  In  form  and 
structure  it  is  much  like  the  familiar  cirriped  nauplius,  but  differs  from  it  in  general 
appearance,  largely  because  it  has  no  chitenous  covering,  and  because  it  begins 
its  free  swimming  existence  (the  absence  of  yolk  demanding  an  early  liberation  of 


THE   ECHINODERM   LARVA. 


423 


the  embryo)  with  the  aid  of  cilia  instead  of  appendages.  But  in  both  larae  there 
is  the  same  enormous  labrum,  /.;the  same  caudal  lobe,  a/.,  with  its  similarly  placed 
anus;  the  same  lateral  thoracic  folds  enclosing  a  central  depressed  area;  a  similarly 
located  adhesive  disc,  and  the  same  simple,  U-shaped,  alimentary  canal. 

The  ciliated  band  is  one  of  the  characteristic  features  of  the  echinoderm  larva. 
It  has  been  compared  with  the  prototroch  of  annelids  and  molluscs,  but  it  is  of  an 
entirely  different  nature.  Its  main  course  is  longitudinal,  and  when  completed  it 
surrounds  the  neural  surface  only  (Fig.  293,  mt.)  on  the  other  hand,  the  proto- 
troch is  always  equatorial,  extending  around  the  long  axis,  across  both  neural  and 
haemal  surfaces.  (Fig.  267,  pt.) 


ab.a. 


FIG.  290.  FIG.   291. 

FIG.  290. — A  hypothetical  cirriped-like  larva,  in  which  the  posterior  part  of  the  trunk  is  taking  on  a  false 
radial  symmetry  due  to  the  absence  of  the  left  half  of  the  middle  gioup  of  metameres.  The  figure  is  designed  to 
illustrate  the  probable  origin  of  radial  symmetry  in  the  echinoderms. 

FIG.  291. — Diagram  of  a  late  stage  in  the  development  of  a  star-fish  larva.  The  asymmetrical  metanauplius 
stage,  before  the  asymmetry  has  produced  the  characteristic  radial  structure  of  the  later  stages. 

The  ciliated  band  of  the  echinoderm  larva  is  merely  an  adaptation,  or  modi- 
fication, of  the  thickened  and  sensory  margins  of  the  thoracic  folds  and  of  the 
preoral  and  caudal  lobes.  In  embryonic  arthropods,  the  margins  of  the  thoracic 
folds,  and  to  a  less  degree  those  of  the  preoral  and  caudal  lobes,  are  studded  with 
rows  of  minute  hairs,  many  of  which  are  sensory.  Long  before  the  folds  are  actu- 
ally formed,  or  any  chitenous  covering  is  secreted,  their  future  location  is  clearly 
indicated  by  a  deeply  stained,  thickened  band  of  ectoderm.  In  Limulus  the  band  is 
first  formed  as  two  lateral  thickenings,  which  extend  forward  and  backward, 
unite,  and  form  a  continuous  girdle  around  the  neural  surface.  (Figs.  140-142.) 

When  the  echinoderm  larva  grows  older,  the  ciliated  band  is  thrown  into  folds, 
or  tentacle-like  lobes  (Fig.  292),  the  larger  ones  corresponding,  approximately, 
to  the  marginal  outgrowths  so  characteristic  of  the  cirripeds.  (Figs.  289  and  290.) 


424 


THE    CIRRIPEDS,    TUNICATES   AND    ECHINODERMS. 


The  apparent  difference  between  them,  therefore,  is  largely  due  to  the  absence 
of  chiten  in  one  case  and  its  presence  in  the  other. 

The  Larval  Cephalic  Appendages. — The  ciliated  lobes  of  the  echinoderm  larva 
must  not  be  confounded  with  primitive  appendages,  with  which  they  have  nothing 
in  common.  The  true  larval  appendages  appear  in  the  same  place  as  the  cephalic 
appendages  of  the  nauplius,  that  is,  on  the  neural  side  of  the  head  in  front  of  the 
mouth,  and  in  the  angle  between  the  lateral  margin  of  the  thorax  and  the  preoral 
lobe.  (Figs.  290,  291,  292,  c.a.p.)  There  appear  to  be  two  pairs;  or  at  least  when 


FIG.  292. — Star-fish  larvae,  seen  as  opaque  objects,  illustrating  the  mode  of  fixation  and  the  metamorphosis. 

Semi-diagrammatic. 

they  have  attained  their  full  development  and  have  taken  up  their  final  position 
on  the  projecting  surface  of  the  head,  there  are  three  appendages,  an 
unpaired  one  lying  beyond  the  adhesive  disc,  and  one  on  each  side  of  it.  (Fig. 
291,  c.a.p.)  Each  appendage  is  thick-walled  and  muscular,  with  a  long  basal  por- 
tion and  a  short  terminal  knob  studded  with  small  adhesive  papillae.  At  this 
time  they  greatly  resemble  the  three  cephalic  appendages  of  a  tunicate  larva 
(Figs.  284,  285),  or  the  minute  adhesive  antennas  of  the  cirripeds  and  parasitic 
Crustacea.  (Figs.  274,  278,  280,  283.) 

Attachment. — The  young  star-fish  larva  is  said  to  attach  itself  voluntarily 
at  first  and  for  a  short  time  only.  Later  it  becomes  permanently  attached,  head 
first  and  neural  side  down,  in  the  same  remarkable  manner  as  a  young  cirriped, 


THE    ECHINODERMS.      ATTACHMENT.      DEVELOPMENT.  425 

both  the  cephalic  appendages  and  adhesive  disc  taking  part  in  the  process.  (Fig. 
292,  F.)  In  the  more  primitive  echinoderms,  such  as  the  crinoids  (Antedon),  the 
metamorphosis  is  even  more  illuminating.  The  larva  attaches  itself  wholly  by 
means  of  the  cephalic  disc,  as  the  adhesive  appendages  appear  to  be  absent.  Its 
first  position  is  with  the  neural,  or  oral  surface  down,  as  in  the  cypress  stage  of  the 
cirriped.  (Figs.  274,  A  and  295,  D.)  The  disc  then  elongates,  forming  a  slender 
cephalic  stalk,  or  peduncle,  and  the  larva  turns  a  somersault,  bringing  its  neural 
side  uppermost.  Meantime  the  vestibule,  or  peribranchial  chamber,  which  at 
first  is  small  and  temporarily  closed,  enlarges,  then  ruptures,  and  the  five  appen- 
dages project  from  the  cup-like  head,  in  typical  cirriped  fashion.  (Fig.  295,  G.) 
The  cirriped  stage  (pentacrinoid)  is,  however,  a  transitory  one,  and  the  young 
Antedon  becomes  a  free  swimming  feather-star,  by  the  breaking  down  of  the 
stalk  and  the  elongation  of  the  appendages. 

There  are  many  other  representatives  of  the  more  modern  echinoderms  in 
which  the  fixed  stage  is  temporary  (asteroidea)  or  appears  to  be  omitted  altogether 
(ectinoidea  and  holothurioidea) ,  and  the  young  echinoderm,  after  its  metamor- 
phosis, again  acquires  a  limited  power  of  locomotion.  But  in  the  most  primitive 
echinoderms,  such  as  the  stalked  crinoids,  blastoids,  and  cystoids,  a  permanent 
attachment  by  an  elongated  cephalic  stalk,  in  typical  cirriped  fashion,  was  the 
almost  invariable  rule,  and  no  doubt  represented  the  primitive  condition  for  the 
whole  class.  When  an  echinoderm  does  become  free,  it  acquires  only  a  very 
limited  power  of  locomotion  and  of  coordinated  movement.  Its  characteristic 
lack  of  efficiency  in  this  respect  is  due,  not  so  much  to  its  simple  or  primitive  struc- 
ture, as  to  the  fact  that  its  freedom  was  gained  at  a  late  period  in  the  phylogeny  of 
a  very  ancient  group,  where  sessile  inaction  was  the  prevailing  condition. 

Early  Embryonic  Development. — Let  us  now  return  to  the  early  embryonic 
stages  to  trace  the  beginning  of  the  metameres,  ccelomic  cavities,  and  thoracic 
appendages.  These  structures,  when  definitely  formed,  have  the  same  or  a  very 
similar  structure,  location,  and  mutual  relation  that  they  have  in  the  arthropods; 
but  they  make  their  appearance  in  a  somewhat  different  manner,  and  at  relatively 
different  times,  owing  largely  to  the  absence  of  yolk,  which  has  here,  as  elsewhere, 
an  important  influence  over  the  method  and  relative  rate  of  development. 

The  Teloccde — After  cleavage,  which  resembles  that  of  the  cirripeds,  a 
blastula  is  formed;  from  its  walls  many  mesenchyme  cells  arise,  comparable  with 
those  which  in  so  many  arthropods  wander  into  the  yolk  from  the  blastoderm. 
They  are  usually  more  numerous  at  the  point  where  later  the  so-called  "gastrula" 
infolding  takes  place.  But  this  infolding  is  formed  at  the  caudal  end,  and  extends 
forward,  like  the  products  of  all  teloblastic  growth;  and  it  finally  closes  in  the  anal 
region,  not  the  oral.  (Fig.  293.)  We  have  already  seen  (p.  219),  that  the  real 
gastrula  is  always  developed  at  the  head  end;  that  the  resulting  endoderm  grows 
from  its  point  of  origin  backward,  never  forward;  that  the  true  gastrula  opening 
always  persists,  if  at  all,  as  the  oral  opening;  it  is  always  associated  with  the  sto- 


426 


THE    CIRRIPEDS,    TUNICATES    AND    ECHINODERMS. 


modaeal  infolding;  it  never  gives  rise  to  coelomic  epithelium;  and  is  never  coinci- 
dent with  the  anus. 

It  is  evident,  therefore,  that  the  terminal  infolding  of  the  blastula  in  the 
echinoderm,  tp.,  is  in  nowise  comparable  with  the  true  gastrula  of  molluscs  and 
annelids.  It  does  represent,  however,  the  apical  infolding  of  arthropods  (teloccele 
and  telopore)  and  gives  rise,  as  it  does  there,  to  the  main  mass  of  the  postoral 
mesoderm  and  endoderm,  but  which  are  here  temporarily  united  to  form  a 
continuous  layer.  Whatever  remnant  of  the  true  gastrula  is  preserved  in  the 
echinoderms  must  be  looked  for  at  the  anterior  end  of  the  neural  surface,  at  the 
point  where  the  primitive  mouth  arises. 


FIG.  293. — Semi-diagrammatic-figures  illustrating  the  development  of  echinoderm  larvse. 

The  Mesoderm  and  the  Ccelom. — After  the  teloccele  is  formed,  the  primitive 
mesoderm  and  endoderm  that  form  its  walls  separate,  the  latter  giving  rise  to  the 
enteron,  the  former  to  the  paired  anlagen  of  the  mesoderm  (hydro-enteroccele). 
The  peculiar  way  in  which  these  two  layers  separate  is  merely  a  specialized  process 
of  differentiating  the  lateral  bands  of  mesoderm  and  endoderm,  and  is  pecu- 
liar to  segmented  animals  with  a  very  small  percentage  of  yolk,  and  which  develop 
with  extreme  rapidity  (Amphioxus,  Balanoglossus,  etc.). 

The  mesodermic  vesicles  thus  formed  then  break  up  on  each  side  into  two  or 
three  main  divisions.  They  are  not  to  be  regarded  as  primitive  mesoblastic  seg- 
ments, but  as  groups  of  imperfectly  divided  ones,  corresponding  probably  to  the 
three  groups,  or  tagmas,  so  commonly  present  in  the  arthropods,  one  belonging 
to  the  head,  one  to  the  thorax,  and  one  to  the  abdomen.  (Figs.  293,  294.) 

The  middle  section  of  one  side  then  divides  imperfectly  into  the  five  chambers 
of  the  hydroccele,  h.c.,  which  represent  five  thoracic  somites.  If  any  segmentation 
occurs  in  the  other  vesicles  of  either  side,  it  is  imperfect,  and  of  short  duration. 


THE    ECHINODERMS.       CCELOM.      APPENDAGES.      EXCRETORY    ORGANS.       427 

The  thoracic  somites,  as  in  the  arthropods,  remain  relatively  small  and  without 
lateral  plates,  and  do  not  expand  laterally  onto  the  haemal  surface. 

The  more  posterior  division  of  the  mesoderm,  ab.cl.,  probably  represents 
several  abdominal  somites  and  lateral  plates  which  have  combined  to  form  the 
general  body  cavity,  or  ccelom.  Like  the  corresponding  structure  in  the  arach- 
nids (Fig.  138),  it  extends  rapidly  in  a  cephalic,  haemal,  and  caudal  direction,  till  it 
meets  the  opposite  ccelom.  (Fig.  294,  E.F.)  The  mesentery  formed  by  this 
union  naturally  lies  to  one  side  of  the  median  haemal  line  owing  to  the  unequal 
growth  of  the  two  chambers. 

Thus  the  principal  difference  between  the  mesoderm  in  a  young  echinoderm, 
as  shown  in  mercator  projection  (Fig.  294,  F.),  and  that  of  an  arachnid  (Fig. 
138),  lies  in  the  asymmetrical  development  of  the  mesoderm  and  in  the  absence 
of  segmentation  in  the  abdominal  ccelom. 

Thoracic  Appendages. — Sometime  after  the  five  chambers  of  the  hydroccele 
appear,  finger-like  outgrowths  of  the  ectoderm  are  formed  over  the  thoracic 
somites.  They  are  the  five  primary  tentacles,  or  tube  feet,  which  represent  five 
modified  thoracic  appendages,  th.ap.  An  outgrowth  of  the  underlying  somite 
grows  into  each  appendage,  in  typical  arthropod  fashion,  but  instead  of  breaking 
up  into  separate  muscles  for  the  appendage  it  remains  permanently  in  the  form 
of  a  membranous  diverticulum  of  the  hydroccel,  and  becomes  the  distal  end  of 
a  radiating  water  vascular  canal.  Only  the  distal  end  of  the  original  appendage 
separates  from  the  body  as  the  primary  tentacle;  the  remainder  of  the  appendage, 
however  long  it  may  eventually  become,  may  be  regarded  as  lying  in  the  surface 
ectoderm,  developing  on  either  side,  as  it  increases  in  length,  paired  cirri  that 
become  the  double  row  of  tube  feet  for  each  arm,  and  into  each  of  which  a  pro- 
longation of  the  water  vascular  canal  extends.  (Fig.  294,  G.) 

Excretory  Organs. — A  portion  of  the  ccelom,  probably  belonging  to  the  cephalic 
division,  undergoes  a  special  modification.  A  narrow  dorso-lateral  outgrowth 
arises  from  it  that  unites  with  an  ectodermic  infolding  on  the  anterior  aboral,  or 
haemal  surface.  From  it  develops  the  stone  canal  and  the  madreporite.  (Fig. 
294,  ec.d.)  The  ectodermic  opening  places  the  hydroccele  in  communication  with 
the  exterior,  so  that  the  organ  has  often  been  compared,  in  whole  or  in  part,  to 
an  annelid  excretory  organ  or  nephridium.  It  is,  however,  more  like  one  of  the 
typical  excretory  organs  of  the  head  region  of  the  arthropods  (shell  gland,  green 
gland,  coxal  gland)  which  consists  of  thin-walled  ccelomic  sacs,  with  a  thick-walled 
tubular  outgrowth  of  varying  length,  united  to  a  short  duct,  infolded  from  the 
ectoderm.  The  haemal  location  of  the  external  opening  to  the  duct  in  the  echino- 
derms  is  no  doubt  due  to  the  same  causes  that  have  carried  the  corresponding, 
or  at  least  adjacent,  cephalic  appendages  to  the  haemal  surface,  not  only  in  the 
echinoderms,  but  in  the  entomostraca,  cirripeds  and  tunicates. 

The  Formation  of  the  Disc. — By  the  time  the  larva  is  attached,  the  asymmetry 
of  the  thoracic  region  has  become  very  pronounced,  due  to  the  fact  that  practically 
all  the  growth  is  now  taking  place  on  the  right  side  of  the  thorax.  In  the  cephalic 


428 


THE    CIRRIPEDS,    TUNICATES    AND    ECHINODERMS. 


and  caudal  regions  the  asymmetry  is  less  apparent.  (Fig.  294.)  As  the  right 
half  of  the  oral  surface  of  the  thorax  takes  on  a  circular  form,  its  haemal  surface 
divides  into  five  thickened,  tubercular,  and  calcareous  plates,  each  plate  corre- 
sponding to  one  of  the  thoracic  appendages  and  representing  the  right  half  of  a  tho- 
racic tergite.  (Figs.  290,  292.)  Each  of  the  five  plates,  or  half  tergites,  becomes 
the  aboral  surface  of  a  starfish  arm.  The  cephalic  and  caudal  ends  of  the  thorax 
finally  unite,  forming  the  five-rayed  body  of  the  new  animal.  (Figs.  294,  292,  G.) 
The  mouth,  m,  is  drawn  into  the  center  of  the  neural  surface,  and  is  surrounded 
by  the  remnants  (right  half  ?)  of  the  nerve  cord,  while  the  five  pedal  nerves  form 
the  radiating  nerves  of  the  arms,  and  the  thoracic  ccelomic  chambers  form  the 
circular  and  radiating  water  vascular  canals.  The  anus  and  madrepore  are 
crowded  toward  the  center  of  the  haemal  surface. 


abcl- 


FIG.  294. — Diagrams  illustrating  the  probable  origin  of  the  echinoderms  from  asymmetrical,  cirriped-like  larvae. 
The  larval  organs  are  seen  from  the  neural  surface,  in  mercator  projection.  E,  Hypothetical  symmetrical  larva;  F, 
hypothectia  asymmetrical  larva;  G,  the  radially  symmetrical  echinoderm. 

When  the  circle  is  completed,  the  head  of  the  larva  is  either  gradually  drawn 
into  the  disc  and  absorbed,  or  according  to  Corens  andDanielson,it  is  amputated, 
and  for  a  short  time  may  lead  a  separate  existence  after  its  separation  from  the 
posterior  part  of  the  body,  thus  recalling  the  amputation  thatoccurs  in  Sacculina 
near  the  close  of  its  metamorphosis,  except  that  here  it  is  the  head  alone  that 
survives;  the  abdomen  dies. 

The  Vestibule,  or  Peribranchial  Chamber. — In  the  starfish,  during  the  forma- 
tion of  the  appendages,  the  neural  surface  of  the  thoracic  region  is  depressed  and 
partly  enclosed  by  the  thoracic  folds,  forming  a  rudimentary  atrial  or  vestibular 
chamber  into  which  the  primary  tentacles  project  like  the  appendages  of  an  ar- 
thropod into  the  peribranchial  chamber.  In  Echinus  and  Antedon,  the  right  peri- 
branchial  chamber  deepens  at  a  very  early  period  and  forms  a  closed  vestibule, 
from  the  floor  of  which  the  five  primary  tentacles  develop  in  the  usual  way. 
(Fig.  295,  D.)  Finally  the  membranous  roof  of  the  vestibule  ruptures,  allowing 
the  appendages  to  protrude  (Fig.  295,  G),  as  they  do  in  a  cirriped,  or  in  a 
polyzoan,  (Fig.  301.) 

Asymmetry— The  morphology  of  the  echinoderms  is  dominated  by  a 
strongly  marked  asymmetry.  While  asymmetry  of  originally  symmetrical 
animals  is  no  doubt  common  enough  as  an  embryonic  abnormality,  it  is  rarely 


THE   ECHINODERMS.      ASYMMETRY. 


429 


retained  to  any  marked  extent  as  a  permanent  feature  of  the  adult.  It  is,  how- 
ever, a  familiar  occurrence  in  the  bopeiridae  and  paguridae,  although  in  the  last 
two  cases  it  affects  only  the  terminal  metameres,  producing  various  degrees  of 
curvature,  but  in  no  wise  disguising  their  morphological  characters. 

In  Limulus,  a  considerable  number  of  half  embryos  are  always  present  in 
material  that  has  been  produced  and  developed  under  apparently  normal  condi- 
tions; such  embroyos  probably  occur  in  other  arthropods  more  frequently  than 
we  have  supposed.  The  half  embryos,  in  their  readjustment  to  the  new  con- 
ditions of  growth,  inevitably  take  on  a  bow-shaped,  spiral,  or  semicircular  form, 
and  may  live  for  several  months,  although  I  have  never  known  them  to  develop 
beyond  the  trilobite  stage. 


FIG.  295. — Diagrams  illustrating  the  development  and  metamorphosis  of  a  crinoid  (Antedon).  C,  Plan  of  an 
early  stage  seen  from  the  neural  surface  in  mercator  projection;  D,  E,  F,  G,  larvae  seen  from  the  side,  as  semi- 
transparent  objects,  illustrating  the  mode  of  fixation,  the  revolution,  formation  of  the  atrial  chamber  and  of  the 
cephalic  stalk. 

We  assume  that  half  embryos  of  this  nature  occurred  frequently  in  the  ar- 
thropod ancestors  of  the  echinoderms;  that  they  were  capable  of  an  organic  re- 
adjustment that  enabled  them  to  survive,  and  that  they  became  the  prevailing 
form.  Whatever  animals  are  assumed  to  be  the  ancestors  of  the  echinoderms, 
it  is  obvious  that  half  embryos  have  been  produced  by  them,  they  have  survived, 
and  they  have  given  rise  to  a  new  class  of  animals. 

It  is  a  significant  fact  for  the  student  of  creative  evolution  that  the  absence 
of  growth  on  one  side  of  an  originally  bilateral  animal,  whatever  may  have  been 
the  cause  of  that,  inevitably  compels  the  remaining  side  to  assume  a  new  form, 
and  thus  creates,  at  almost  a  single  stroke,  a  new  class  of  animals.  In  other 


430  THE    CIRRIPEDS,    TUNICATES    AND    ECHINODERMS. 

words,  a  negative  character  at  one  point  can  create  a  new  character  at  a  dif- 
ferent point. 

Echinoderms  and  Cirripeds.     Summary. 

1.  The  echinoderms  are  descended  from  cirriped-like  arthropods  in  which, 
as   a  result  of  some  unknown  condition,  the  absence,  or  degeneration  of  organs 
on  one  side  of  the  middle  section  of  the  body  had  become  a  frequent  or   fixed 
character. 

The  five  half  metameres  corresponding  to  the  five  defective  ones,  assumed 
in  consequence,  a  new  architectural  arrangement,  forming  a  closed  ring  with  the 
segmental  organs  arranged  in  radiating  lines,  instead  of  a  double  linear  series  of 
parallel  lines.  The  five  half  metameres  successfully  established  a  new  condition 
of  organic  stability,  and  gave  rise  to  a  new  kind  of  body,  and  a  new  class  of  animals; 
the  bilaterally  symmetrical  head  and  tail  ends  of  the  old  body  either  atrophied 
completely,  or  were  reduced  to  structural  insignificance. 

2.  The  relation  of  the  echinoderms  to  the  arthropods  is  shown  by  the  absence, 
or  imperfect  repetition  of  a  true  gastrula  stage;  by  the  conspicuous  development 
of  a  telopore  and  teloccele;  by  the  development  of  a  nauplius-like  larval  form,  or 
naupula;  and  by  the  mode  of  development  of  the  mesoderm,  mesoblastic  somites, 
ccelom,  and  excretory  ducts. 

3.  The  echinoderms  more  particularly  resemble  the  cirripeds  in  the  form 
of  the  larvae,  and  in  their  mode  of  attachment  and  rotation;  also  in  the  mode  of 
growth  of  the  mantle,  atrial  chamber,  appendages,  and  cephalic  stalk;  and  in  the 
peculiar  sessile  mode  of  life  that  is  so  characteristic  of  practically  all  the  more 
primitive  members  of  the  class. 


CHAPTER  XXIII. 

THE   ENTEROPNEUSTA,   PTEROBRANCHIA,  POLYZOA,  BRACHIO- 
PODA,  PHORONIDA  AND  CH^ETOGNATHA. 

IV.  THE  ENTEROPNEUSTA.     (Fics.  296,  297,  298.) 

The  resemblance  between  the  larvae  of  the  enteropneusta  and  echinoderms 
has  been  frequently  emphasized,  and  it  is  generally  assumed  that  it  indicates  a 
common  origin  for  both  classes  in  some  worm-like  ancestors.  We  shall  show  that 
the  resemblance  between  these  larvae  is  largely  superficial;  that  between  them  there 
are  underlying  differences,  so  great  as  to  preclude  a  direct  genetic  relation  of  one 
to  the  other,  and  that  neither  one  nor  the  other  resembles  the  larva  of  annelids. 

The  enteropneusta  have  also  been  compared  with  the  vertebrates,  because  of 
their  aboral  nerve  cord,  perforated  gill  sacs,  " enteric"  ccelom,  and  "notochord." 
The  first  two  points  are  of  real  significance,  and  together  with  other  evidence, 
indicate  an  intimate  relation  with  Amphioxus,  tunicates,  and  other  acraniates,  but 
not  with  vertebrates.  The  last  two  points,  the  "  enteric  ccelom"  and  " notochord" 
have  a  purely  artificial  value  that  has  been  created  by  a  false  interpretation  of  the 
early  processes  of  development,  for  the  "  archenteron "  and  the  "gastrula"  that 
lie  at  the  bottom  of  the  whole  system,  have  no  existence. 

The  Enteropneusta  and  the  Echinoderms. — The  early  development  of  the 
enteropneusta  is  very  significant.  We  have  already  seen  that  when  the  gastrula  is 
retained  in  an  essentially  unmodified  condition,  and  has  the  significance  originally 
attached  to  it,  it  always  gives  rise  to  the  endoderm  only,  its  cavity  remains  as  the 
cavity  of  the  enteron,  and  its  external  opening  remains  as  the  neurostoma;  or  the 
latter  is  formed  at  the  point  where  the  gastrula  opening,  or  blastopore,  closed. 
These  conditions  prevail  in  the  annelids  and  molluscs.  Wherever  the  two  inner 
germ  layers  arise  from  a  caudal  infolding  that  gives  rise  to  both  mesoderm  and 
endoderm,  to  mesoccele  and  endoccele,  and  that  in  closing  gives  rise  to  the  anus,  not 
to  the  mouth,  we  are  dealing  with  a  highly  modified  process  of  development  that  is 
only  remotely  comparable  with  gastrulation,  and  the  presence  of  which  is  prima 
facie  evidence  that  the  animals  in  which  it  occurs  are  descended  from  arthropod 
ancestors. 

It  is  the  latter  type  of  development  that  is  characteristic  of  the  enteropneusta, 
hence  it  is  apparent  at  the  very  outset  that  their  simple  structure  is  not  a  primitive 
condition  but  a  secondary  one,  and  that  the  location  of  the  functional  mouth  on 
the  haemal,  instead  of  the  neural  surface,  the  formation  of  an  ento-mesoccele  in 
place  of  a  gastrula,  and  the  presence  of  a  telopore  in  place  of  a  blastopore,  de- 
finitely excludes  the  enteropneusta  from  both  the  molluscs  and  annelids,  and 
places  them  among  the  descendants  of  the  arthropods. 


432 


THE    ENTEROPNEUSTA. 


With  these  facts  in  mind,  if  we  compare  an  echinoderm  larva  with  that  of 
Balanoglossus,  it  will  be  seen  that  the  resemblances  between  them  in  general  form, 
and  in  the  arrangement  of  the  ciliated  bands,  are  not  of  special  significance;  or  at 
least  they  can  be  explained  on  the  assumption  that  both  echinoderms  and  entero- 
pneusta  are  descended  from  arthropods,  and  that  their  larval  forms  are  similarly 
adapted  to  a  temporary  pelagic  existence. 

The  differences  are  more  fundamental,  for  in  the  echinoderm  the  mouth  lies 
on  the  primitive  neural  surface  and  is  still  surrounded  by  the  nerve  cord,  or  a  rem- 
nant of  it.  Although  the  larval  mouth  of  the  echinoderms  is  said,  in  some  cases, 
to  disappear,  or  to  close  for  a  time,  it  opens  again,  or  a  new  one  is  formed  very 
close  to  where  the  old  one  was  last  seen.  There  are  no  indications  that  the  per- 
manent mouth  of  the  echinoderms  is  a  haemostoma,  and  there  are  no  indications 
that  a  functionless  remnant  of  a  neurostoma  is  present.  In  the  enteropneusta 
the  case  is  different.  The  permanent  mouth  lies  on  the  haemal  surface,  and  out- 
side of  the  medullary  plate;  it  cannot,  therefore,  be  compared  with  the  functional 
mouth  of  the  echinoderms,  annelids,  molluscs,  or  arthropods.  Moreover,  in 
the  enteropneusta  there  are  definite  indications  of  a  primitive  infolding  of  the 
neural  surface  that  perforates  the  medullary  plate  where  the  primitive  mouth  should 
be  located.  This  opening  is  not  comparable  with  the  hydropore,  because  it  lies  on 
the  neural  side,  not  the  haemal.  It  is,  however,  comparable  in  position  and  de- 
velopment with  the  neurostoma  of  annelids  and  arthropods,  and  probably  repre- 
sents the  subneural  gland  of  the  tunicates,  and  the  infundibular  tube  of  true  verte- 
brates. (Compare  Figs.  43,  44,  284-297.) 

The  Enteropneusta  and  the  Arthropods. — The  enteropneusta  are  prob- 
ably descendants  of  primitive  arthropods  in  which  the  essential  features  of  the  class 
were  definitely  developed.  Through  one  of  those  inexplicable  internal  condi- 
tions that  are  exemplified  in  other  sub-phyla  of  the  acraniates,  the  enteropneusta 
have  failed  to  develop  to  their  full  extent  many  of  the  characteristic  structures  of 
segmented  animals.  The  power  of  growth  is  vigorous  enough,  for  the  dwarfed 
forms  of  other  phyla  do  not  occur  here,  but  the  power  of  organic  definition  is 
feeble,  producing  animals  without  conspicuous  appendages,  or  outgrowths  of  any 
kind,  and  without  a  sharp  definition  or  specialization  of  the  organs  ordinarily 
segmentally  arranged,  such  as  neuromeres,  myotomes,  sense  organs  and  ccelomic 
chambers.  We  have  frequently  seen  this  condition  in  the  abnormal  embryos  of 
Limulus,  where  certain  individuals  appear,  as  it  were,  to  have  passed  through  a 
flame  that  softened  or  partly  melted  the  usual  surface  details  (Fig.  184),  or  that 
reduced  a  whole  group  of  metameres  to  a  single  appendage,  or  to  an  ill-defined 
unspecialized  mass  of  cells.  (Fig.  186.)  A  somewhat  similar  condition  is  familiar 
enough  in  the  maggot-like  larvae  of  many  insects  and  arachnids,  and  especially 
in  those  forms  where  these  conditions  become  more  and  more  accentuated  in  the 
later  stages,  as  in  Pentastoma,  and  in  innumerable  parasitic  Crustacea. 

The  initial  factor  in  these  cases  is,  no  doubt,  a  defective  germinal  structure 
that  determines  the  character  of  the  older  stages  and  rigidly  prescribes  the  mode 


STRUCTURE  AND  DEVELOPMENT.  433 

of  life  for  the  adult.  These  germinal  defects  are  manifestly  cumulative  in  their 
results  for,  as  a  rule,  only  the  older  stages  are  so  modified  by  them  that  practically 
all  traces  of  the  initial  organs  are  obliterated.  The  adult  parasitic  crustacean 
with  its  worm-like  body,  devoid  of  metameres  and  appendages,  and  with  sense 
organs,  nerves,  muscles,  heart,  mouth,  and  alimentary  organs  either  imperfectly 
developed,  or  altogether  absent,  could  not  have  been  recognized  as  arthropods, 
with  highly  specialized  ancestors,  if  it  were  not  for  the  absolutely  conclusive  testi- 
mony of  the  embryonic  and  larval  stages. 

In  the  enteropneusta  we  have  a  precisely  similar  condition,  except  that  the 
particular  form  of  reduction  characteristic  of  these  animals  is  not  dependent  for 
its  perpetuation  upon  a  parasitic  mode  of  life;  and  the  loss  of  organic  definition 
extends  farther  back  into  the  ontogeny,  modifying  and  disguising  the  embryonic 
and  larval  stages,  almost  as  effectually  as  it  does  the  later  ones.  Nevertheless, 
certain  characteristic  conditions  have  been  retained  that  demonstrate  with  reason- 
able certainty  that  the  enteropneusta  are  descended  from  arthropod  stock. 

Structure  and  Development. — The  cleavage  is  total  and  the  resulting 
blastomeres  are  of  remarkably  uniform  size.  A  deep  infolding  is  formed  at  the 
caudal  end,  representing  a  typical  teloccele,  or  mesentocoele.  The  telopore  soon 
closes,  but  opens  again  as  the  anus,  or  the  anus  forms  at  the  point  where  the 
telopore  closes.  (Fig.  270,  A.C.) 

The  Gastrula  and  the  Telocxle. — At  a  very  early  stage  a  large  portion  of  the 
inner  tube  is  constricted  off,  and  a  funnel-shaped  outgrowth  extends  from  it 
toward  the  anterior  neural  surface,  where  a  median  ectodermal  infolding  is  formed 
that  unites  with  it,  putting  the  inner  chamber  into  communication  with  the 
exterior. 

The  history  of  the  important  events  that  take  place  at  this  point  is  by  no 
means  clear  or  conclusive.  The  early  opening  in  the  median  neural  surface  of 
the  proboscis  has  been  called  the  proboscis  pore  and  has  been  compared  with  the 
hydropore  of  echinoderms,  but  the  difference  in  their  location  is  apparently 
irreconcilable,  one  being  on  the  haemal  side  and  the  other  on  the  midneural  side. 
Moreover,  the  primitive  proboscis  pore  does  not  appear  to  be  the  same  thing  as 
either  the  single  unsymmetrical  proboscis  pore,  or  the  two  pores  that  may  be 
present  in  the  adult. 

A  more  satisfactory  explanation  may  be  given,  it  seems  to  me,  and  is  as  fol- 
lows: The  anterior  section  of  the  inner  tube  represents  the  remnants  of  the  gas- 
trula,  and  consists  of  the  primitive  cephalic  mesoderm  and  endoderm,  at  first 
united  with  the  walls  of  the  telocoele,  later  separating  from  them  as  the  so-called 
"  proboscis  ccelom."  (Fig.  270,  g.)  The  point  of  union  with  the  procephalic  lobes 
represents  the  remnants  of  the  blastopore,  and  the  coincident  ectodermic  infolding, 
the  neurostoma,  and  primitive  stomodaeum,  n.st.  The  latter  closes  and  loses  its 
connection  with  the  cephalic  endoderm,  but  leaves  for  some  time  a  faint  central 
depression  that  marks  its  original  location.  (Fig.  296,  n.st).  The  definitive 
proboscis  pores  arise  later,  close  to  the  neurostoma,  and  open  into  the  proboscis 
28 


434 


THE    ENTEROPNEUSTA. 


cavity  or  cavities,  which  may  now  be  regarded  as  the  definitive  coelomic  cavities 
of  the  procephalon.  (Figs.  296  and  297.) 

The  Mesoderm  and  Ccelom.—It  is  clear  that  the  primitive  proboscis  ccelom, 
or  cephalic  vesicle,  is  not  comparable  with  the  paired  coelomic  vesicles  of  the 
collar  and  trunk,  for  the  cephalic  vesicle  arises  at  a  very  much  earlier  period;  it 
consists  of  a  special  type  of  rounded  cells  not  seen  elsewhere,  and  from  them  arise 
the  first  mesenchyme.  It  is  unpaired,  located  on  the  neural  side  of  the  head  and 
widely  separated  from  the  coelomic  vesicles  of  the  trunk.  The  latter  are  paired, 
have  a  posterior  lateral  position,  are  thin  walled,  and  arise  either  as  evaginations, 
or  as  solid  outgrowths  of  the  teloccele  wall,  or  as  segments  of  mesodermic  bands 
of  teloblastic  origin. 

The  cells  forming  the  primitive  proboscis  vesicle  (Fig.  270,  B.),  are  comparable 
with  those  that  in  arachnids  (Fig.  141,  ac.)  arise  from  the  inward  proliferation  of 
the  anterior  primitive  cumulus,  or  with  those  formed  in  the  region  of  the  cephalic 


FIG.  296. — Diagrams  to  illustrate  the  larval  development  and  metamorphosis  of  Balanoglossus. 

lobes  in  insects.  In  both  cases,  and  this  condition  prevails  no  doubt  in  many 
other  arthropods,  two  distinct  infoldings  or  solid  ingrowths  are  formed,  the  anterior 
or  cephalic  one  representing  the  gastrula,  the  posterior,  the  teloblasts  or  teloccele. 
(Fig.  269.)  The  anterior  one,  g,  gives  rise  to  a  mass  of  cells  at  the  point  where 
the  stomodaeum  is  forming,  or  where  it  will  appear  later.  From  them  arises  the 
mesoderm  of  the  procephalic  lobes,  and  the  endoderm  which  either  forms  the 
anterior  portion  of  the  enteron,  or  is  scattered  through  the  yolk  and  degenerates. 
The  Larva. — The  young  larva,  at  about  the  time  of  hatching,  is  covered  with 
cilia  and  has  somewhat  the  appearance  of  that  in  Fig.  296,  B.  It  may  now  be 
compared  with  an  echinoderm  larva,  or  with  a  legless,  ciliated  nauplius.  (Fig. 
296,  A.)  The  main  longitudinal,  ciliated  band  represents,  as  it  does  in  the  echino- 
derms,  the  free  margin  of  the  thoracic  fold,  and  the  margin  of  the  caudal  and  pre- 
oral  lobes.  The  typical  tornaria  arises  as  a  result  of  the  contraction  and  subse- 
quent infolding  of  the  anterior  haemal  surface  of  the  head.  We  have  studied  this 
process  and  the  part  it  play  sin  the  formation  of  the  cephalic  navel  of  arachnids,  and 
in  the  formation  of  the  mouth  and  the  closing  up  of  the  haemal  surface  of  the  head 
vertebrates.  Chapter  XIV,  p.  253.  Here  we  may  again  recognize  the  same  process. 
The  cephalic  navel  first  appears  as  a  thickened  depression  of  the  haemal  surface,  B. 


METAMORPHOSIS. 


435 


An  outgrowth  of  the  enteron  extends  toward,  and  then  unites  with  it,  to  form  the 
haemostoma ;  meantime  the  surrounding  haemal  surface  is  greatly  shortened  by  the 
drawing  together  of  the  anterior  and  posterior  ends,  the  progress  of  the  contraction, 
which  may  be  compared  with  the  reversed  curvature  commonly  seen  in  arthropod 
embryos  during  the  formation  of  the  "dorsal  organ,"  being  indicated  not  only  by 
the  general  shape  of  the  larva,  but  in  a  specially  striking  manner  by  the  changes 
in  the  form  of  the  ciliated  band,  B,  C  and  D. 

Metamorphosis. — During  the  metamorphosis  the  ciliated  bands  disappear, 
and  the  larva  rapidly  takes  on  the  form  of  the  adult.  (Fig.  296,  E.)  The  preoral 
region  elongates  and  forms  the  proboscis,  or  procephalon,  the  middle  section 
forms  the  collar,  or  thorax,  and  the  posterior  one,  the  abdomen,  or  tail. 

Cephalic  Ccecum. — Meantime  a  median  outgrowth,  or  cephalic  caecum, 
arises  from  the  anterior  end  of  the  enteron,  that  marks  the  beginning  of  the  so- 
called  "notochord"  (Fig.  296,  E.)  But  neither  in  structure,  nor  origin,  nor  in  its 
anatomical  relations,  has  it  any  resemblance  to  a  notochord,  for  we  have  seen  in 


FIG.  297. — Diagrams  illustrating  the  structure  of  Balanoglossus.  A,  Sagittal  section  through  the  procephalon 
(proboscis)  and  mesocephalon  (collar,  thorax);  B,  transverse  section  of  the  mesocephalon ;  C,  transverse  section  of 
the  abdominal,  or  branchial  region. 

Chapter  XIX,  that  the  notochord  is  a  modification  of  the  middle  cord,  that  it  arises 
from  the  ectoderm,  and  that  it  is  never  connected  with  or  forms  a  part  of  a  func- 
tional alimentary  canal.  It  is  therefore  obvious  that  an  organ  cannot  be  re- 
garded as  a  notochord  merely  because  it  is  a  diverticulum  of  the  gut.  In  fact,  such 
an  origin  or  connection  may  be  accepted  as  conclusive  evidence  that  the  organ  in 
question  is  not  a  notochord. 

The  cephalic  caecum  of  Balanoglossus,  in  its  minute  structure  and  location 
is  not  essentially  different  from  the  caecal  outgrowths  of  the  foregut  which 
are  so  common  in  arthropods,  as  for  example  in  cirripeds  (Figs.  275  and  277), 
entomostraca  (Fig.  282,  A),  phyllopods  (Fig.  273),  cladocera  (Fig.  10),  and 
arachnids  (Fig.  43). 

The  cephalic  caecum  of  the  enteropneusta  probably  represents  the  remnants 
of  that  part  of  the  foregut  that  originally  opened  into  the  primitive  stomodaecum 
and  neurostoma. 

The  Late  Larval  and  the  Adult  Stages. — In  the  later  stages  the  principal  events 
are:  The  further  differentiation  of  the  main  subdivisions  of  the  body,  that  is,  of  the 
procephalic,  thoracic,  branchial,  and  caudal  regions;  and  the  development  of 
the  gill  slits,  lateral  folds,  endocranium,  -excretory  ducts,  and  nervous  system. 


436 


THE    ENTEROPNEUSTA. 


The  relations  of  these  various  parts  to  one  another  are  shown  in  a  diagram- 
matic way  in  Fig.  297,  which  represents  a  sagittal  section  of  the  head  and  thorax, 
and  in  Fig.  298,  which  represents  the  entire  animal  viewed  from  the  neural 
surface.  At  this  stage  Balanoglossus  may  be  compared  to  a  naked  phyltopod- 
like  arthropod,  with  the  basal  portion  of  its  abdominal  appendages  infolded  in 

the  typical  arachnid  method  to  form  respiratory  sacs, 
the  distal  portion  persisting  as  the  tongue  bar.  The 
gill  sacs  eventually  unite  with,  and  then  open  into, 
corresponding  caecal  outgrowths  from  the  gut,  giving 
rise  to  the  new  type  of  respiratory  organs  characteristic 
of  the  vertebrates  and  of  several  other  phyla  of  the 
acraniates. 

New  respiratory  appendages  and  branchial  clefts 
arise  in  varying  numbers  behind  those  already  formed, 
in  the  usual  manner  for  segmented  animals.  The 
wing-like  lateral  folds  (Figs.  297  and  298,  /./.),  that 
are  so  characteristic  of  the  adults,  may  be  compared 
to  the  pleural  folds  on  the  abdominal  metameres  of 
a  trilobite, arachnid,  or  crustacean  (Figs.  2,  n),  or  with 
the  lateral  folds  of  the  trunk  in  a  primitive  vertebrate. 
(Fig.  232.)  In  all  these  cases  the  pleural  folds  repre- 
sent the  extended  lateral  margins  of  the  neural  sur- 
face, they  lie  lateral  to  the  appendages  and  are  turned 
in  a  neural  direction.  (Fig.  297.) 

The  Endocranium. — We  have  seen  that  the  endocra- 
nium  forms  a  most  characteristic  structure  in  the 
arachnids,  and  in  primitive  Crustacea  related  to  the 
phyllopods.  In  its  simplest  condition,  as  in  Apus 
and  Branchipus,  it  consists  of  a  transverse  bar,  or 
plate  of  cartilage,  lying  in  the  anterior  thoracic  region, 
just  back  of  the  neurostoma,  and  on  the  haemal  side 
of  the  nerve  cord.  The  main  body  of  the  endocra- 
nium  is  of  mesodermic  orgin,  and  often  contains  two 
kinds  of  cartilage-like  tissues  that  differ  in  structure 
and  in  their  reaction  to  stains;  one  is  a  dense  fibroid 
tissue  with  small  flattened  nuclei  irregularly  distri- 
buted; the  other  has  more  the  appearance  of  hyaline 
cartilage,  and  contains  large  rounded  cells,  in  the 

peculiar  grouping  characteristic  of  certain  kinds  of  primitive  cartilage.  In  a 
few  cases,  notably  Apus,  paired  ectodermic  ingrowths  are  formed,  lined  internally 
with  chiten,  which  unite  with  and  form  an  integral  part  of  the  endocranium. 

These  facts  have  an  important  bearing  on  the  origin  of  the  enteropneusta, 
where  the  endocranium  has  a  similar  form  and  location.   Here  it  is  a  flattened  plate, 


FIG.  298. — Diagram  of  Balano- 
glossus, seen  from  the  neural 
surface. 


MUSCLES.      CCELOM.      NERVOUS    SYSTEM.  437 

lying  on  the  haemal  side  of  the  cephalic  caecum,  with  two  slender  arms  extending 
backward  on  either  side  of  the  pharynx.  (Fig.  297,  en.c.)  The  body  of  the  en- 
docranium  consists  of  a  sharply  defined  mass,  consisting  of  clear,  concentric 
"chitenoid"  lamellae,  said  to  arise  from  the  basement  membrane  of  the  cephalic 
caecum  and  the  pharynx,  and  in  some  cases  containing  cells  lodged  between  the 
lamellae.  The  lateral  portions  are  less  clearly  defined  and  consist  of  a  cartilage- 
like  matrix,  containing  small  clusters,  or  nests  of  cartilage  cells,  said  to  be  derived 
from  the  epidermis. 

Whatever  may  be  the  origin  of  the  endocranium  in  the  enteropneusta,  it 
certainly  bears  a  strong  resemblance  in  its  location,  form,  and  histological  struc- 
ture to  the  endocranium  of  the  arachnids  and  phyllopods. 

Muscles. — The  arrangement  of  the  muscles  in  the  collar  and  trunk  supports 
the  interpretation  indicated  above.  We  have  shown  (p.  230)  that  in  the  arachnids 
practically  all  the  somatic,  or  intersegmental,  muscles  are  absent  in  the  thorax, 
this  condition  being  either  the  cause  or  the  result  of  the  fusion  of  all  the  thoracic 
and  cephalic  tergites  into  one  cephalothoracic  shield.  The  muscles  that  remain 
belong  to  the  appendages,  or  to  the  forward  extension  of  longitudinal  abdom- 
inal muscles  that  are  attached  to  the  posterior  portion  of  the  endocranium.  There 
is,  therefore,  a  marked  difference  in  the  arachnids  as  well  as  in  many  other  arthro- 
pods between  the  musculature  of  the  thorax  and  that  of  the  abdomen.  According 
to  Ritter  there  is  in  Balanoglossus  a  similar  difference  between  the  musculature 
of  the  collar  (thorax)  and  trunk.  He  states  that  circular  somatic  muscles  are 
wholly  wanting  in  the  collar.  Here  the  principal  muscles  are  radio-longitudinal, 
attached  to  the  posterior  wall  of  the  collar  at  one  end,  and  at  the  other  mainly  to 
the  "notochord"  and  nuchal  skeleton,  but  also  to  the  walls  of  the  oesophagus. 
The  muscles  of  the  branchial  and  caudal  regions,  both  longitudinal  and  circular, 
are  always  strictly  somatic. 

The  Codom  consists  of  three  main  divisions.  (Figs.  279,  298,  c.1'3.)  That 
in  the  proboscis,  c1,  may  be  regarded  as  the  remnants  of  the  procephalic  ccelom, 
and  is  drained  by  one  or  two  excretory  ducts,  comparable  with  the  antennary 
ducts  of  Crustacea,  or  the  cheliceral  ducts  of  the  arachnids  (Galeodes).  The 
collar  ccelom,  c2,  represents  that  of  the  thorax,  and  its  excretory  duct  represents 
one  of  the  posterior  thoracic  ducts,  i.e.,  the  so-called  shell  gland  of  the  phyllopods 
or  the  coxal  gland  of  the  arachnids.  The  trunk  ccelom  represents  that  of  the 
abdominal  and  post  abdominal  segments  united  to  form  a  continuous  chamber, 
and  is  devoid  of  excretory  ducts  as  it  is  in  the  arthropods  generally.  (Com- 
pare with  Fig.  279.) 

The  Nervous  System.  The  Neural  and  Hamal  Cords. — The  nervous  system 
of  arthropods  forms :  i .  A  "ventral "  medullary  plate  consisting  of. parallel,  gang- 
lionated,  segmented  cords,  perforated  by  the  primitive  stomodaeum.  It  coincides 
with  the  primitive  axis  of  growth  and  differentiation,  ontogenetically  and  phy- 
logenetically.  It  is  the  primary  nervous  system  because  it  is  the  oldest,  and 
because  it  is  always  associated  with  the  oldest  sense  organs,  muscles  and  append- 


438  THE    ENTEROPNEUSTA. 

ages,  which  develop  side  by  side  with  it  on  the  neural  surface.  It  rarely  keeps 
pace  with  the  growth  of  the  body,  usually  concentrating  at  the  anterior  end, 
around,  or  close  to,  the  stomodaeum. 

2.  The  haemal  nerve  cord  is  unpaired,  never  distinctly  segmented,  and  never 
associated  with  important  sense  organs  or  appendages.  It  is  the  last  part  of  the 
nervous  system  to  develop,  and  is  associated  most  intimately  with  the  heart  and  the 
haemal  and  somatic  musculature,  which  are  the  last  parts  of  the  embryonic  body 
to  develop.  The  haemal  cord  extended  primarily  from  end  to  end  of  the  trunk 
and  was  united  with  the  neural  cord  by  segmental,  circular  nerves.  It  likewise 
fails  to  keep  pace  with  the  growth  of  the  body;  its  point  of  concentration,  however, 
is  never  in  the  anterior  cephalic  region,  but  in  the  cardiac  and  respiratory  region, 
which  tends  to  shift  farther  and  farther  back,  beyond  the  oral  and  thoracic,  and 
finally  into  the  abdominal  region.  (See  Chapter  XII.)  Its  principal  connection 
with  the  neural  system  is  with  the  stomodaeal  nerve  centers  at  the  head  end,  and 
with  the  respiratory  neuromeres  of  the  abdominal  region.  But  even  the  former 
connection  is  lost,  or  greatly  reduced  in  the  highest  types,  leaving  only  the  seg- 
mental cardiacs  of  the  vagus  and  respiratory  neuromeres  as  a  means  of  connect- 
ing the  main  haemal  nerve  with  the  nerve  cords  on  the  opposite  side  of  the  body. 
(Limulus). 

The  principal  parts  of  the  nervous  system  of  the  enteropneusta  correspond 
with  those  of  the  arachnids  as  outlined  above,  the  most  important  point  being  the 
identity  of  the  primary  neural  surface  of  the  arthropod  with  the  principal  neural 
surface  of  the  enteropneusta.  In  the  arthropods,  it  is  true,  the  medullary  plate 
is  on  the  oral  side,  while  in  the  enteropneusta  it  is  on  the  aboral  side,  but  it  is  the 
mouth  that  has  changed,  not  the  neural  surface  with  all  its  fundamental  relations. 
The  haemal  mouth  is  a  new  formation,  while  the  old  mouth  may  still  be  recog- 
nized in  its  proper  place  in  the  median  depression  of  the  cephalic  lobes.  (Fig. 
297,  n.st.)  The  medullary  plate  of  the  proboscis  represents  the  remnants  of 
the  supra-cesophageal  ganglion,  and  is  still  connected  with  the  small  eye  spot,  ac.j 
which  probably  represents  the  remnants  of  a  parietal  eye.  (Figs.  296-298.) 

The  thoracic  neuromeres  form  the  principal  part  of  the  nervous  system, 
and  at  an  early  period  are  bodily  infolded  to  form  the  floor  of  a  canal  or  tube. 
The  latter  usually  remains  open  at  either  end,  and  a  variable  number  of  vertical 
canals,  or  strands  of  tissue,  persist  over  the  median  line  and  appear  to  mark  the 
point  where  the  medullary  folds  are  imperfectly  united  (Figs.  296,  297,  iv.), 
recalling  the  vertical  strands  of  ectoderm  between  the  compacted  thoracic  neu- 
romeres in  the  insects  and  arachnids.  (Figs.  221,  229  and  231.) 

In  cross  sections  the  thoracic  or  collar  neuromeres  form  a  thick  band  of  small 
nerve  cells  with  an  underlying  layer  of  "punct  substanz."  The  general  appear- 
ance of  the  cord  is  similar  to  that  of  a  young  scorpion,  the  more  so  since  the  nerve 
cells,  for  a  time,  may  be  arranged  in  radiating  lines  around  minute  cavities  (Har- 
rimania,  Ritter)  like  those  so  characteristic  of  the  embryonic  cords  of  the  arach 
nids.  (Figs.  15,  16  and  227.) 


THE    PTEROBRANCHIA. 


439 


The  posterior  end  of  the  thoracic  nerve  cord  is  united  by  nerves  of  consider- 
able size  with  the  median  haemal  nerve  (Fig  297,  h.nc.),  the  latter  representing  the 
median  cardiac  nerve  of  the  arachnids.  (Compare  Figs.  78,  115  and  117.) 

V.  THE  PTEROBRANCHIA.     (Fics.  299,  300.) 

The  pterobranchia  have  been  shown  to  have  a  structure  so  much  like  that  of 
the  enteropneusta  in  respect  to  the  location  of  the  functional  mouth,  medullary 
plate,  gill  pouches,  cephalic  caecum,  ccelomic  chambers,  and  excretory  ducts, 
that  the  fundamental  features  in  the  morphology  of  both  groups  must  be,  with- 
out doubt,  interpreted  in  the  same  manner.  If  this  is  done,  then  it  is  apparent 


J5t. 


FIG.  299. — Diagrams  of  Cephalodiscus.     A,  Neural  surface;  B,  side  view,  in  optical  section. 

that  the 'features  which  are  more  specially  characteristic  of  the  pterobranchia, 
such  as  the  six  pairs  of  appendages,  the  short,  branchial  region,  the  U-shaped 
intestine  with  the  anal  end  bent  toward  the  posterior  neural  surface,  and  with 
the  genital  ducts  opening  in  the  neural  surface  of  the  branchial  region,  give 
a  decidedly  arachnoid  character  to  the  adults,  and  strengthen  and  confirm 
the  interpretation  we  have  given  for  the  enteropneusta. 

Cephalodiscus  may  therefore  be  regarded  as  a  naked  arthropod-like  animal 
with  a  closed  neurostoma,  the  location  of  which  is  indicated  by  the  pit  in  the  ante- 
rior portion  of  the  neural  plate  (Fig.  299,  n.st.},  and  by  the  cephalic  caecum,  di, 
that  probably  united  the  primitive  stomodaeum  with  the  enteron.  The  cephalic 
disc  with  its  procephalon,  thoracic  appendages,  and  thoracic  neuromeres,  m.b., 
represents  the  cephalothorax;  the  excretory  ducts,  opening  to  the  exterior  by  the 
proboscis  pores  and  collar  pores,  p.  p.  and  c.  p.,  represent  respectively  the  cephalic 
and  thoracic  ducts  of  arthropods;  and  the  single  pair  of  gill  sacs,  g.p.,  represent  a 


440 


POLYZOA. 


pair  of  invaginated  respiratory  appendages  of  the  vagus  region.  The  genital 
organs  and  ducts  have  approximately  the  same  location  as  in  many  arachnids 
and  Crustacea,  that  is,  just  behind  the  vagus  region. 

The  thoracic  appendages  of  Cephalodiscus  develop  in  an  approximately 
regular  order,  from  before  backward,  like  those  of  arthropods.  (Fig  300,  C,ZX)  In 
Rhabdopleura,  the  arms  probably  represent  a  single  pair  of  enlarged,  antenna- 
like  cephalic  appendages.  (Fig.  300,  A,B.) 

Attachment  is  effected,  as  in  the  tunicates,  by  a  postoral  haemal  outgrowth 
probably  representing  a  special  modification  of  the  posterior  part  of  the  cephalic 
navel.  (Figs.  299,  300.) 


FIG.  300. — Diagrams  of  Rhabdopleura.     A,  Adult,  from  neural  surface;  B,  from  side,  in  optical  section;  C,  D, 
larvae  of  Cephalodiscus,  from  the  neural  surface. 

It  will  be  recalled  that  the  cephalic  navel  is  a  center  of  convergent  growth,  and 
that  the  location  of  the  center,  and  the  nature  of  the  events  that  take  place  there 
is  largely  dependent  on  the  volume  of  the  yolk  sphere  and  the  rate  at  which  the 
growing  tissues  spread  over  and  enclose  the  haemal  surface.  The  conditions 
created  at  the  closing  area,  where  the  advancing  lines  of  equal  growth  and  dif- 
ferentiation tend  to  meet  and  annul  one  another,  may  result,  at  the  cephalic  end, 
in  an  ingrowing  tube,  the  opening  of  which  becomes  the  haemastoma,  and  at  the 
caudal  end,  as  in  the  tunicates  and  Pterobranchia,  in  a  stolon-like  outgrowth  that 
retains  an  indefinite  power  of  growth  and  that  becomes  the  seat  of  successive 
generations  of  new  buds. 

VI.  THE  POLYZOA. 

The  polyzoa  likewise  may  best  be  interpreted  as  descendants  of  primitive 
arthropods  of  the  cirriped  type.  They  do  not  develop  a  clearly  marked  metameric 


THE    ENTOPROCTA. 


441 


structure,  and  they  are  disguised  by  the  loss  of  important  larval  organs,  the  result  of 
an  extensive  degenerative  histolysis;  but  in  the  structure  and  development  of  the 
larvae  and  in  their  attachment  and  metamorphosis,  indications  of  arthropod  affini- 
ties may  be  recognized. 

The  entoprocta  have  undergone  the  least  modification,  and  one  of  them, 
Pedicellina,  will  best  serve  to  illustrate  the  salient  characters  of  the  group. 

The  young,  as  in  so  many  other  acraniates,  undergo  the  earlier  stages  of 
development  within  the  brood  pouches  of  the  atrial  or  vestibular  chamber.  There 
is  a  total  and  nearly  equal  cleavage,  and  the  flattened,  hollow  blastula  is  infolded 


an. 


t.c. 


FIG.  301. — Development  of  an  endoproctous  polyzoan  (Pedicellina).  A-D,  Formation  of  the  telocoele,  and  the 
early  larval,  or  naupula,  stages;  E-G,  mode  of  attachment,  and  metamorphosis;  H-I,  early  and  late  larval  stage, 
seen  from  the  neural  surface.  The  most  significant  features  are  the  absence  of  a  true  gastrula ;  the  conspicuous 
mantle,  its  formation  of  a  vestibular,  or  atrial,  chamber  that  encloses  the  appendages  and  the  whole  neural  surface 
of  the  body;  the  attachment  of  the  larva  by  means  of  a  large  cephalic  stalk;  the  rotation;  and  the  degeneration 
of  the  prosencephalon.  Semi-diagrammatic.  (In  part  afte*  Hatscheck,  Harmer,  and  Barrois.) 

at  the  caudal  end  to  form  a  telocoele.  (Fig.  301,  A.)  The  telopore  closes  and  an 
independent  ectodermic  infolding  at  the  head  end  unites  with  the  enteron,  forming 
the  primitive  stomodaeum  and  neurostoma,  n.st.  A  typical  gastrula  stage,  and  a 
blastopore,  therefore,  does  not  occur.  The  larva  is  a  naupula,  not  a  trochosphere, 
as  shown  by  the  prominent  labrum,  by  the  longitudinal  ciliated  band  represent- 
ing the  thickened  margin  of  a  branchial  fold,  and  by  the  teloccelic  method  of 
forming  the  germ  layers.  The  enteron  is  U-shaped,  with  the  concave  side  turned 
toward  the  neural  surface.  There  is  a  conspicuous  apical  disc,  d,  and  a  large 
preoral  ganglion,  or  forebrain,  that  is  probably  united  by  circumoral  commissures 
with  the  rudiments  of  a  ventral  nerve  cord.  The  latter  appears  as  two  thicken- 


442  THE    POLYZOA. 

ings  separated  by  a  deep  infolding,  one  next  the  anterior  wall  of  the  stomodaeum, 
the  other  in  front  of  the  rectum,  t.g.  The  two  ganglia  may  be  regarded  as 
representing  the  postoral  and  the  anal  group  of  ganglia  of  the  nauplius. 
(Fig.  272.) 

On  either  side  of  the  stomodaeum  is  an  excretory  duct  (Fig.  301,  H.  I.  c.d.), 
that  corresponds  approximately  with  the  cephalic  excretory  duct  of  the  nauplius. 
The  genital  cells  and  genital  ducts  appear  to  arise  from  the  deep  infolding  between 
the  postoral  and  postanal  ganglia,  in  a  position,  therefore,  that  corresponds  to 
the  location  of  the  same  organs  in  the  nauplius,  in  Cephalodiscus,  and  in  the  adult 
stages  of  many  arachnids. 

The  early  development  of  the  relatively  large  brain  and  nerve  cord  definitely 
locates  the  primitive  neural  surface,  and  gives  us  the  necessary  data  for  the  proper 
orientation  of  the  larva.  The  latter  comes  to  rest  in  the  typical  naupula  fashion, 
neural  side  down.  (Fig.  300,  E.)  A  large  cephalic  outgrowth  is  then  formed,  and 
as  it  elongates,  it  straightens  out  and  lifts  up  the  body,  which  meantime  turns  a 
half  somersault,  bringing  its  neural  surface  uppermost.  The  edges  of  the  pleural 
folds,  about  the  time  of  attachment,  unite  to  form  a  closed  vestibule,  or  atrial 
chamber,  at.  c,  within  which  the  appendages  make  their  appearance,  F.  During 
the  revolution  of  the  larva,  they  elongate,  rupture  the  vestibule,  and  finally  pro- 
trude from  it  in  the  same  manner  as  the  legs  of  a  cirriped,  or  the  arms  of  a  young 
crinoid.  (Figs.  274,  295.) 

During  the  early  stages  of  revolution,  the  brain  and  apical  disc  separate  from 
the  ectoderm  and  break  up  into  a  mass  of  loose  cells  that  nearly  fill  the  cavity  of 
the  cephalic  stalk.  (Fig.  301,  F.)  They  appear  to  increase  in  numbers  and  to 
receive  accessions  from  other  sources.  The  anterior  wall  of  the  enteron,  or  vesti- 
bule according  to  Harmer,  then  breaks  down,  and  its  walls,  surrounding  an  ill 
defined  space,  merge  with  the  degenerating  cells  in  the  stalk.  Later  some  of  the 
cells  undergo  still  further  degeneration,  whether  by  being  bodily  enclosed  in  the 
cavity  of  the  enteron  as  seems  probable,  or  not,  does  not  appear.  In  either  case 
the  rupture  in  the  walls  of  the  enteron  closes  over  and  the  amorphous  mass  of 
degenerating  cells  decreases  in  volume.  Some  cells  appear  to  persist  in  the 
stalk  as  star-shaped,  connective  tissue  cells. 

The  point  that  appears  to  be  clearly  established  is  the  disappearance  of  the 
forebrain  and  apical  disc  by  a  process  of  histolysis,  that  seems  also  to  affect  other 
adjacent  tissues,  including  the  anterior  wall  of  the  enteron.  The  fate  of  the  free 
cells  thus  produced  is  not  clear.  It  is  obvious  that  the  whole  process,  even  in  the 
way  it  affects  the  forebrain  and  apical  plate,  is  very  similar  to  those  which  occur 
on  the  haemal  surface  of  the  thorax,  and  in  the  cephalic  navel,  or  dorsal  organ,  of 
cirriped s  and  parasitic  copepods,  and  it  occurs  in  a  corresponding  region,  that  is, 
on  the  anterior  haemal  surface  of  the  head,  at  the  point  where  attachment  takes 
place.  (Figs.  274,  282,  283.) 

It  will  be  recalled  that  the  cephalic  navel  of  arthropods  is  an  area  on  the 
anterior  haemal  surface  of  the  cephalothorax,  where  extensive  degeneration  of 


THE    ECTOPROCTA. 


443 


embryonic  tissues  takes  place.  The  degenerating  cells  are  derived  from  two 
principal  sources,  the  haemal  blastoderm  and  the  haemal  musculature  of  the  anterior 
part  of  the  thorax.  They  concentrate  at  a  definite  point  on  the  haemal  surface  of 
the  head,  where  they  are  invaginated  en  masse  into  the  yolk,  and  thence  into  the 
enteron  where  they  are  absorbed. 

The  haemal  blastoderm  cells  are  always  invaginated  into  the  yolk  and  ab- 
sorbed. The  degenerating  lateral  plate  cells  of  the  thorax  certainly  do  not,  as  a 
rule,  undergo  this  fate,  although  it  is  difficult  at  times  to  distinguish  between 
them  and  those  derived  from  the  haemal  blastoderm.  The  vast  majority  of  them 
either  degenerate  in  situ,  become  free  blood-corpuscle-like  cells,  or  regenerate  into 
scattered  muscle  cells.  See  Chapter  XIII,  p.  230. 

The  cephalic  navel  of  arthropods  is  regarded  as  furnishing  the  initial  condi- 
tions that  lead  to  the  formation  of  a  haemostoma.  Pedicellina  is  the  only  case 


FIG.  302. — Larvae  of  polyzoa  in  the  naupula  stage,  illustrating  the  degenerative,  or  pauperitic  development 
of  the  first  generation.  A,  Pedicellina;  B,  Cyphonautes;  C,  Lepralia;  D-E,  Ctenostomidae ;  F,  Bugula.  (After 
Barrois,  Prouho,  and  Harmer,  slightly  modified.  Semi-diagrammatic. 

known  to  me  where  the  seat  of  degeneration  appears  to  be  in  temporary  communi- 
cation with  a  definitely  formed  enteron. 

Conclusion. — The  main  features  in  the  structure  and  development  of  the 
entoprocta  agree  with  those  of  primitive  arthropods,  and  not  with  those  of  the 
molluscs,  or  annelids,  or  with  any  other  worm-like  animals.  In  place  of  the 
gastrula,  blastopore,  and  trochosphere  that  are  typical  of  annelids,  we  have  the 
telocoele,  telopore,  and  naupula  that  are  typical  of  the  arthropods. 

The  larva  resembles  that  of  cirripeds,  in  passing  its  early  stages  of  develop- 
ment within  a  brood  pouch;  in  its  subsequent  mode  of  attachment  and  rotation; 
in  the  mode  of  growth  of  its  cephalic  stalk  and  mantle,  and  in  the  histolytic 
changes  in  the  region  of  the  cephalic  navel;  and  in  the  location  of  the  genital  cells 
and  principal  ganglia. 

The  Ectoprocta. — The  ectoprocta  may  be  regarded  as  modified  derivatives 
of  the  more  primitive  entoprocta.  They  present  an  extraordinary  diversity  of 


444 


THE    POLYZOA. 


larval  forms,  methods  of  metamorphosis,  egg  formation,  budding,  and  degenera- 
tion. It  is  not  our  purpose  to  discuss  these  interesting  but  exceedingly  intricate 
processes;  we  merely  wish  to  point  out  what  we  consider  to  be  the  chief  morpho- 
logical features  of  this  particular  division  of  the  acraniates. 

One  of  the  controlling  factors  in  the  morphology  of  the  ectoprocta  is  an  ex- 
treme exaggeration  of  those  initial  defects  in  germinal  material  that  have  been 
observed  in  all  the  acraniates.  The  result  is  that  the  first  generation  of  zoids  are, 
at  the  very  outset,  unusually  deficient  in  the  various  organs  that  are  normally  pres- 
ent at  those  periods.  These  defective  embryos  fail  to  develop  beyond  the  larval 
stages,  as  practically  all  their  organs  undergo  degenerative  histolysis.  After  the 
attachment,  practically  nothing  is  left  of  the  original  larva  but  a  shapeless  sac  filled 
with  a  mass  of  indifferent  cells.  A  second  factor  in  their  morphology  is  that  a  new 
generation  of  zoids  arises  from  a  bud-like  infolding  on  the  haemal  surface  of  the 
first  larva.  This  new  product  is  not  to  be  regarded  as  the  completion  of  the 

development  of  the  first  zoid,  but  as  a 
new  zoid,  representing  a  second  genera- 
tion that  arises  from  the  formless  rem- 
nants of  the  first.  But  while  the  first 
generation  of  larvae  conform  in  the  main, 
as  far  as  they  go,  with  the  larvae  of  the 
entoprocta,  the  second  generation  de- 
velop  into  a  new  type,  due  to  the  fact  that 
its  primary,  or  oro-anal  axis,  is  bent 
double,  in  exactly  the  opposite  direction 
from  that  in  the  entoprocta.  That  is, 
in  the  ectoprocta  the  body  is  bent  so  as 
to  bring  the  caudal  end  to  a  point  just  in 


-in.br. 


elongating     and     making    convex     the 

primitive  neural  surface,  and  greatly  shortening  the  haemal  surface,  thus 
reversing  the  conditions  in  the  entoprocta.  (Compare  Figs.  301  and  303.) 
In  the  ectoprocta,  the  resulting  elongation  of  the  body  is  therefore  in  a  haemo-neu- 
ral  direction,  and  it  appears  to  be  brought  about  in  the  same  manner  as  in  Phoronis 
(Fig-  3°5)>  ^7  the  evagination  of  the  middle  section  of  the  enteron  through  the 
space  between  the  divergent,  postoral  nerve  cords.  The  latter,  owing  to  the  very 
unequal  growth  of  the  haemal  and  neural  surfaces,  appear  to  be  transferred  to  the 
haemal  surface,  but  in  reality  undergo  but  little  change  of  position.  The  primitive, 
preoral  ganglion,  or  forebrain,  apparently  fails  to  develop,  or  it  subsequently  atro- 
phies, as  in  the  entoprocta  and  phoronida.  Consequently  the  so-called  circum- 
oral  nerve  ring  of  the  ectoprocta  does  not  represent  the  original  nerve  ring  connect- 
ing the  pre-  and  postoral  ganglia,  but  the  ventral  cords  and  their  transverse  com- 
missures. (Fig.  303,  B.) 

The  lophophore  on  this  interpretation  may  be  regarded  either  as  a  single  pair 


THE    BRACHIOPODS. 


445 


of  greatly  enlarged  cirrate  appendages,  comparable  with  those  of  brachiopods, 
or  as  the  laterally  extended  row  of  many  minute  appendages. 

It  will  also  be  observed  that  the  entoprocta  are  attached  by  stalk-like  out- 
growths from  the  haemal  surface  of  the  head,  in  typical  acraniate  fashion  (Fig. 
301,  J),  while  the  adult  ectoprocta  of  the  second  generation,  like  Phoronis,  are 
attached  by  the  evaginated  neural  surface.  (Fig.  303,  A.) 

VII.  THE  BRACHIOPODS. 

Our  knowledge  of  the  development  of  the  brachiopods  is  fragmentary  and 
the  data  we  do  possess  are  lacking  in  detail  and  precision.  The  evidence,  so  far 
as  it  goes,  indicates  that  the  brachiopods  belong  with  the  acraniates  and  that 
their  structure  is  best  interpreted  as  a  modification  of  the  arthropod  type.  In 
fact,  they  appear  to  have  retained  some  of  the  characteristic  features  of  the  cirri- 
peds  in  a  less  modified  form  than  any  other  group  of  acraniates,  while  their  own 
distinctive  features  constitute  a  natural  transition  to  the  condition  realized  in  the 


A 


FIG.  304. — Diagrams  of  a  brachiopod.     A,  Seen  in  optical  section;  B,  from  the  neural  surface. 

phoronida  and  polyzoa.  As  in  cirripeds,  the  eggs  pass  through  their  early  stages 
of  development,  in  some  cases  the  whole  larval  development  (Stringocephalus, 
Zittel)  in  brood  pouches  formed  by  folds  in  the  mantle  chamber. 

Cleavage  is  total  and  almost  equal,  giving  rise  to  a  hollow  blastula  that  is  in- 
folded to  form  a  teloccele.  The  telopore  closes,  the  mesoderm  separates  from  the 
entoderm  as  two  ccelomic  chambers,  and  the  body  is  divided  by  two  transverse 
constrictions  into  what  appear  to  represent  the  cephalic,  thoracic,  and  abdominal 
regions. 

The  details  of  the  method  of  attachment  and  of  the  subsequent  metamorphosis 
are  not  clearly  understood,  but  they  appear  to  be  essentially  the  same  as  in  Pedicel- 
Una.  A  haemal  outgrowth  serves  for  the  permanent  attachment  of  the  larva  and 
a  voluminous  mantle  fold  encloses  the  body  in  a  typical  atrial  chamber.  The 
mantle  secretes  a  thick  shell,  resembling  in  its  somewhat  complex  minute  structure, 
the  simpler  forms  of  the  mantle  skeleton  in  the  cirripeds. 

The  two  valves  are  usually  spoken  of  as  dorsal  and  ventral,  but  morpholog- 


446  THE    PHORONIDA. 

ically  they  are  better  designated  as  either  cephalic  and  caudal,  or  anterior  and 
posterior,  corresponding  approximately  with  the  rostrum  and  carina  of  the 
cirripeds. 

The  stomodseum  arises  at  a  comparatively  late  period  and  is  without  doubt 
formed  on  the  primitive  neural  surface,  as  indicated  by  the  location  of  the  nerve 
ring  itself  and  by  the  position  of  the  simple  sac-like  heart  on  the  opposite  side  of 
the  enteron,  h. 

The  genital  cells  are  located  in  the  walls  of  the  mantle,  one  pair  in  the  cephalic, 
g.o.1,  the  other,  g.o.2,  in  the  caudal  lobe,  recalling  the  arrangement  of  ovaries  and 
testis  in  the  cirripeds. 

The  nervous  system,  in  spite  of  the  relatively  large  size  of  the  animals,  is 
rudimentary  in  the  extreme,  the  central  portion  consisting  of  a  slender  circumoral 
ring  with  a  small,  postoral  ganglion. 

The  forebrain,  considered  as  a  nerve  center,  may  be  regarded  as  practically 
absent,  since  the  preoral  portion  of  the  ring  consists  of  little  more  than  a  slender 
commissure.  The  fact  is  significant,  in  view  of  the  absence  of  the  preoral  ganglion 
in  the  polyzoa  and  phoronida  owing  to  its  histolytic  degeneration  during  the 
metamorphosis,  and  in  view  of  the  extremely  rudimentary  condition  of  the  fore- 
brain  in  all  other  members  of  the  acraniates. 

VIII.  THE  PHORONIDA. 

In  the  ectoprocta  we  can  only  infer  from  the  condition  that  obtains  in  the 
adult,  that  the  metamorphosis  of  the  second  generation  of  zoids  takes  place  in  the 
manner  described  above,  for  the  embryonic  processes  by  which  this  result  is 
attained  are  not  clearly  defined.  In  the  phoronida,  however,  where  the  adult 
condition  is  apparently  very  similar  to  that  in  the  ectoprocta,  the  successive  steps 
in  the  metamorphosis  of  the  larvae  are  sharply  differentiated  and  are  sufficiently 
well  known  to  supply  this  deficiency. 

The  eggs  are  nearly  yolk  free  and  undergo  the  early  stages  of  development  in 
the  recesses  of  the  lophophore.  The  cleavage  is  nearly  equal,  forming  a  hollow, 
nearly  spherical  blastula  that  is  infolded  at  one  side  to  form,  apparently,  a  nearly 
typical  gastrula  (Fig.  305,  A),  the  blastopore  remaining  open  as  the  mouth,  and  the 
infolded  cells  forming  the  permanent  enteron.  The  latter  extends  in  a  caudal 
direction,  the  anus  forming  at  the  point  where  it  unites  with  the  ectoderm.  The 
mesoderm  arises  as  isolated  cells  at  various  points  from  the  walls  of  the  blasto- 
derm and  especially  in  the  vicinity  of  the  anus.  It  is,  apparently,  not  formed  in 
conjunction  with  the  endoderm,  nor  does  it  separate  from  the  later  as  definitely 
formed  ccelomic  chambers. 

There  is  a  prominent  apical  plate,  a.pl.,  that  probably  represents  the  begin- 
ning of  the  forebrain,  or  supra-oesophageal  ganglion. 

A  thick,  longitudinal  band,  ab,  is  formed  at  an  early  period,  that  contains 


THE    FUSIFORM   CELLS. 


447 


the  anlagen  of  the  larval  appendages  and  probably  such  portions  of  the  postoral 
nerve  cords  as  are  represented. 

The  preoral  lobe  corresponds  roughly  to  the  labrum  and  procephalon,  and 
the  region  covered  by  the  diagonal  ciliated  band,  to  the  thorax  of  the  naupula. 
The  abdominal  region,  as  in  the  ectoprocta,  is  represented  by  an  imaginal  disc-like 
infolding  that  grows,  in  a  measure,  independently  of  the  rest  of  the  larva,  C,  ab. 
After  a  while,  the  thickened  floor  of  the  infolding,  that  has  become  irregularly 
folded,  ruptures  its  amnion-like  covering,  and  is  violently  everted,  carrying  a 
U-shaped  fold  of  the  enteron  with  it.  Meantime  the  anal  end  of  the  larva  is 
drawn  in  a  haemal  direction,  the  diagonal  band  ruptures,  the  forebrain,  preoral 


FIG. 


305. — Diagrams    illustrating   the    development    and    metamorphosis   of    Phoronis.      (In   part,   after  Selys- 

Longschamps . ) 


and  marginal  lobes  break  down,  and  the  fragments  pass  into  the  enteron  through 
the  mouth.  The  short  permanent  appendages,  which  are  lined  with  a  prolongation 
of  the  coelomic  epithelium,  assemble  in  the  oral  region,  forming  the  basis  for  the 
lophophore. 

The  primitive  cerebral  ganglion,  or  forebrain,  disappears  with  the  preoral 
lobe,  and  the  permanent  ganglia  appear  to  be  derivatives  of  the  ventral  cords, 
transferred  to  the  haemal  surface  with  the  permanent  appendages.  As  in  the 
ectoprocta,  these  ganglia  are  connected  with  each  other  by  a  circumoesophageal 
band  that  probably  represents  the  remnants  of  the  transverse  commissures,  which 
have  been  greatly  elongated  by  the  haemal  migration  of  their  appropriate  ganglia. 

Fusiform  Cells. — At  about  the  close  of  the  metamorphosis,  many  peculiar 
spindle,  or  fusiform  cells  make  their  appearance,  that  probably  represent  the  rem- 
nants of  the  disintegrating,  or  the  unformed,  thoracic  musculature.  In  their 
histological  structure,  general  appearance  and  distribution,  they  agree  closely 
with  the  fiber  cells  of  Limulus  and  of  many  other  arthropods.  See  Chapter 
XIII,  p.  232. 

In  Phoronis,  they  are  mingled  with  the  vaso-peritoneal  tissue,  or  float  freely 
in  the  cavity  of  the  body.  They  have  been  regarded  as  modified  blood  cells,  but 


448  THE    CH^TOGNATHA. 

their  origin  and  fate  has  not  been  definitely  determined.  According  to  Selys 
Longchamp,  they  have  a  distinct  longitudinal,  usually  spiral  striation,  and  ap- 
parently a  small  eccentric  nucleus.  When  alive  they  are  flexible,  but  become 
hard  and  brittle  when  fixed.  Their  color  and  appearance  is  suggestive  of  mus- 
cular substance.  Similar  cells  occur  in  Lingula  (Yatsu)  and  are  said  to  occur  also 
in  the  annelids  (Nereis).  Their  significance  in  the  annelids  is  not  apparent.  In 
the  arthropods  they  are  associated  with  special  forms  of  muscle  building  and  with 
muscle  degeneration.  They  are  significant  in  the  acraniates  (phoronida  and 
polyzoa) ,  for  they  are  indicative  of  the  occurrence  of  extensive  muscular  degenera- 
tion, and  of  the  secondary  character  of  the  anatomical  structure  of  these  animals. 
Selys  Longchamp  figures  a  deep  infolding  that  is  formed  in  the  middle  of 
the  haemal  surface  at  the  close  of  metamorphosis.  It  is  not  described  in  detail, 
but  it  appears  to  be  the  seat  of  extensive  degeneration,  similar  to  that  in  the 
cephalic  navel  or  "dorsal  organ"  of  arthropods.  (Fig.  305,  G,  c.nv.) 

IX.  THE  CH^ETOGNATHA. 

The  chaetognatha  are  clearly  to  be  regarded  as  the  modified  descendants  of 
primitive  arthropods,  for  they  retain,  even  in  the  adult  stages,  some  of  the  more 
important  characters  of  that  group.  The  head  (Fig.  306,  A.B.),  with  its  rudimen- 
tary appendages,  mantle  (prepuce),  and  organs  of  special  sense,  represents  the 
modified  remnants  of  the  nauplius;  while  the  trunk,  which  is  a  voluminous  but 
very  simple  caudal  outgrowth  from  it,  represents  the  imperfectly  developed  tho- 
racic and  abdominal  tagmata,  /,  Brc.,  and  ab.  They  show  no  recognizable  divis- 
ion into  metameres,  and  are  used  mainly  for  locomotion  and  for  the  retention  of 
sexual  cells.  There  is  no  distinct  larval  stage,  no  period  of  fixation,  and  no 
striking  metamorphosis;  the  adult  is  to  be  regarded  merely  as  a  sexually  mature 
naupula  adapted  for,  and  leading  throughout  its  whole  life,  a  pelagic  existence. 

Development. — -The  eggs  are  relatively  large  (2  mm.  in  Sagitta),  transparent, 
and  contain  a  considerable  amount  of  yolk.  They  are  discharged  in  the  early 
morning,  and  develop  very  rapidly,  the  young  escaping  from  the  egg  membranes 
between  6  and  8  o'clock  on  the  evening  of  the  same  day.  Cleavage  is  total  and 
nearly  equal,  resulting  in  the  formation  of  a  spherical  blastula  with  a  small  cleav- 
age cavity.  The  so-called  "  gastrula  "  is  in  reality  a  typical  teloccele,  and  is  formed 
at  the  caudal  end  by  the  infolding  of  the  teloblasts  and  primitive  germ  cells.  Two 
primitive  sexual  cells  may  be  recognized  at  an  extremely  early  stage  on  the  posterior 
neural  surface  of  the  blastula,  D,  g,  in  a  position  that  corresponds  with  their  early 
location  in  many  different  arthropod  embryos.  (Lernaea,  hymenoptera,  coleop- 
tera,  etc.) 

The  apical  infolding  carries  with  it  the  mesentoblasts  and  germ  cells,  the  latter 
finally  lying  at  the  anterior  extremity  of  the  teloccele,  E.  With  the  closure  of  the 
telopore,  which  appears  to  take  place  on  the  posterior,  neural  surface  of  the  embryo, 
the  mesoderm,  endoderm,  and  germ  cells  begin  to  separate  from  one  another. 
The  endoderm  forms  the  walls  of  the  primitive  gut,  F,en,  and  the  mesoderm  forms 


THE    CH^TOGNATHA.  449 

sac-like  diverticula  on  either  side,  m.s.  In  the  next  stage,  G,  the  mesoderm  has 
divided  into  two  pairs  of  mesodermic  chambers,  one  for  the  head,  c.m.s.,  and  one 
for  the  trunk,  th.ms.  The  anterior  end  of  the  primitive  gut  unites  with  an  ecto- 
dermic  infolding  at  the  cephalic  apex,  that  gives  rise  to  the  mouth  and  stomodaeum, 
n.s.t.;  the  posterior  end  extends  backward  and  ultimately  unites  with  the  caudal 
apex  at  the  point  where  the  anus  is  formed  later.  Meantime  the  two  primitive 
germ  cells  have  divided  into  four  cells,  two  on  either  side  of  the  free  end  of  the 
enteron.  The  two  anterior  ones  are  the  anlagen  of  the  ovaries,  ov.,  and  the  two 
posterior  ones  of  the  testis,  /. 

In  the  next  stage,  H,  the  mesocceles  break  down,  the  mesoderm  forms  a  practi- 
cally solid  mass,  and  the  post-anal  section  of  the  body  develops.  During  the  early 
cleavage  stages,  a  large  nucleus  appears  in  association  with  the  cells  that  give  rise 
to  the  mesoblasts  and  germ  cells,  C,  x.  Later  it  is  transferred  to  the  primitive 
germ  cells,  and  finally  breaks  up  into  fragments  and  disappears.  It  appears  to 
represent  the  "yolk  nuclei"  so  characteristic  of  arthropod  eggs  containing  a  large 
amount  of  yolk. 

In  the  newly  hatched  young,  /,  three  regions  may  be  recognized,  the  head, 
Pr.c.,  consisting  of  what  appear  to  represent  the  rudiments  of  one,  possibly  three 
pairs  of  appendages.  It  is  partly  enclosed  by  two  lateral  folds,  mt.t,  that  may  be 
regarded  as  the  remnants  of  a  bivalve  shell  of  the  naupula. 

The  trunk,  Br.c.,  contains  very  large,  paired  ventral  ganglia,  m.br..  and  the 
ovaries,  ov.  A  membranous  partition  separates  the  trunk  from  the  caudal  region, 
a.b.,  in  which  are  located  the  testes,  t. 

The  three  divisions  of  the  body  indicated  at  this  early  period  are  not  com- 
parable with  metameres;  they  represent  three  tagmas,  or  three  imperfectly  devel- 
oped body  regions,  head,  thorax,  and  abdomen,  such  as  are  commonly  seen  in  many 
primitive  arthropods,  or  in  the  embryonic  stages  of  the  more  highly  developed 
forms.  In  the  chaetognatha,  either  these  regions  have  never  been  definitely  divided 
into  metameres,  or  if  so,  the  metameres  have  disappeared  through  the  regressive 
or  degenerative  processes  that  are  so  prevalent  in  the  acraniates. 

The  Adult. — The  newly  hatched  young,  without  any  strongly  marked  larval 
stage,  pass  directly  into  the  adult  form.  The  most  noteworthy  fact  in  their  devel- 
opment is  the  relatively  enormous  size  of  the  ventral  ganglion  in  the  younger,  as 
compared  with  the  older  stages,  indicating  a  certain  amount  of  degeneration  in 
the  history  of  the  group. 

The  integument  may  be  very  thick  and  surprisingly  complex  for  animals  of 
such  a  low  grade  of  development.  It  consists,  in  some  genera,  of  a  complicated 
irregular  network  of  interlacing  trabeculae,  recalling  the  vacuolated  or  cancellous 
ectoderm  of  the  cirripeds,  branchiopods,  and  tunicates. 

Excretory  organs  are  not  definitely  known  to  occur,  although  the  ducts  opening 
near  the  base  of  the  mandibles,  d,  may  be  connected  with  such  organs.  They  are 
regarded  by  MoltchanofT  as  the  nephridia-like  ducts  of  the  first  metamere. 

There  is  no  heart,  and  no  circulatory  organs,  and  there  are  no  indications  of 
29 


450  THE    CH^ETOGNATHA. 

a  cephalic  navel.  The  body  and  the  alimentary  canal  are  straight,  that  is,  there 
is  no  marked  neural  or  haemal  curvature,  and  no  multiplication  by  budding,  agree- 
ing in  these  respects  with  the  cephalochorda  and  the  enteropneusta. 

The  trunk  of  the  adult  is  elongated,  spindle-shaped,  flattened  on  the  neural 
and  rounded  on  the  haemal  surface,  with  broad  pleural,  or  thoracic  folds,  of  various 
forms  in  different  genera.  The  caudal  portion  is  provided  with  a  telson-like 
terminal  lobe. 

The  head  of  the  adult  undergoes  but  little  change  over  that  seen  in  the  young 
larva,  the  most  conspicuous  features  being  the  prepuce-like  mantle  folds,  represent- 
ing the  bivalve  shell  of  the  nauplius,  pr,  and  the  pair  of  large  muscular  lobes  or 
mandibular-like  appendages,  md.  They  are  provided  with  a  group  of  sensory, 
or  glandular  follicles,  /<?.,  and  armed  with  stout  movable  spines  consisting  of  a 
chitenoid  material,  apparently  perforated  with  typical  pore  canals  and  enclosing 
a  conspicuous  pulp  cavity.  The  structure  of  the  follicles  and  spines  gives  the 
appendages  a  decidedly  arthropod  appearance. 

The  Endocranium. — The  mandibles  and  their  spines  are  moved  by  power- 
ful adductor  muscles,  in  which  is  imbedded  what  appears  to  be  a  small  median 
fibroid,  or  cartilaginous  plate,  en.c. 

It  was  this  plate  that  Grassi  referred  to  as  an  organ  of  unknown  significance, 
but  which  might  possibly  prove  to  be  a  "precious  jewel"  in  the  eye  of  the  mor- 
phologist. 

In  his  description  of  the  muscles  in  question,  he  states  (p.  42.):  "Esso  e 
dentro  1'involucro  chitinoide  del  complesso  mediano.  Questo  musculo,  siccome 
dissi,  ha  forma  d'arco  concavo  anteriormente;  in  un  suo-seno,  sotto  al  punto  di 
massima  concavita  delParco,  riposa  un  corpicciolo  ovale,  appiattito  nel  senso 
dorso-ventrale,  fatto  di  cellule  neucleate  piuttosto  piccole  ed  a  contorni  piu  or 
meno  decisi;  esso  e  coperto  di  fibre  muscolari  da  ogni  lato,  eccetto  il  dorsale 
e  1'anteriore,  dov'e  separate  dalP  esofago  per  1'involucro  chitinoide  del  musculo. 
Questo  involucre  e  la  musculatura  lo  separano  dalla  commissura  nervosa  boccale. 
Ho  riscontrato  1'  organo  in  discorso  in  tutte  le  specie,  eccetto  la  Claparedi;  piu 
voluminose  sono  le  specie,  piu  compare  grosso.  Per  quanto  indagassi  non  riuscii 
ad  intenderne  la  significazione  fisiologica  e  morphologica.  Posso  congetturare 
soltanto  che  si  tratti  di  un  organo  o  nascente  o  rudimentale,  e  percio  di  una  pietra, 
forse  preciosa  agli  occhi  del  morphologo." 

The  plate  in  question  no  doubt  represents  a  small  endocranial  cartilage,  or 
sinew,  serving  for  the  attachment  of  the  mandibular  muscles.  It  has  a  special 
significance  for  us  because  we  have  seen  that  a  similar  cartilage  is  imbedded  in  the 
mandibular  muscles  of  many  primitive  arthropods,  as  in  Branchipus,  Apus,  and 
many  others,  and  that  it  forms  the  starting  point  for  the  cartilaginous  endocra- 
nium  of  the  higher  arthropods  and  of  the  vertebrates.  See  Chapter  XVII,  p.  312. 

Nervous  System. — The  chaetognaths  are  the  only  acraniates  that  retain  in  the 
adult  a  comparatively  large  forebrain,  with  the  full  equipment  of  stomodaaal 
ganglia,  nerves  and  cephalic  sense  organs,  B,  f.br. 


THE    NERVOUS    SYSTEM. 


451 


It  sends  nerves  to  three  sets  of  cephalic  sense  organs,  to  the  stomodaeum, 
mantle,  and  to  the  oral  appendages  and  their  follicles.  It  is  connected  with  the 
ventral  ganglion  by  double  circumoesophageal  commissures,  and  probably  includes 
the  rudiments  of  the  primitive  forebrain  and  one  or  more  segments  of  the  dien- 


J-br. 


pu 


FIG.   306. — Figures  illustrating  the  structure  and  development  of  Sagitta.     (In  part,  after  Hertwig,  Grassi,  and 

Epatiewsky.)     Semi-diagrammatic. 

cephalon.  In  its  general  appearance,  it  resembles  the  brain  of  a  primitive  arthro- 
pod more  than  that  of  any  other  invertebrate.  This  is  shown  by  the  internal 
arrangement  of  the  commissural  fibers,  medullary  substance,  and  nerve  cells,  as 
well  as  by  the  arrangement  of  the  principal  sensory  nerves,  and  the  distribution 
of  the  voluminous  stomodaeal  nerves  and  ganglia.  The  lateral  stomodaeal  ganglia, 


452 


CH^TOGNATHA. 


l.st.g.,  are  more  widely  separated  from  the  forebrain  than  in  most  arthropods, 
but  otherwise  they  have  a  similar  relation  to  the  stomodaeum  and  forebrain. 
They  are  united  with  each  other  by  the  same  highly  characteristic  cross  com- 
missure, st. co.,  that  passes  over  the  anterior  haemal  surface  of  the  stomodaeum, 
and  that  forms  such  an  important  landmark  in  the  brains  of  insects,  arachnids, 
and  phyllopods.  Moreover  the  lateral  stomodaeal  ganglia,  judging  from  the 
figures  of  Doncaster,  appear  to  arise,  as  they  do  in  the  arthropods,  independently 
of  the  forebrain,  from  lateral  thickenings,  or  evaginations  of  the  walls  of  the 
stomodaeum.  See  Chapter  IV,  p.  60. 

Cephalic  Sense  Organs. — The  cephalic  sense  organs  consist  of  two  compound 
lateral  eyes,  I.e.,  a  parietal  eye,  p.e.,  and  the  frontal  or  olfactory  organs  ("corona 
ciliata"),  co.c.  These  important  sense  organs  have  the  same  general  arrange- 
ment, innervation  and  structure  that  is  so  characteristic  of  the  cephalic  sense 
organs  of  all  primitive  arthropods.  Chapter  VIII,  p.  125. 

The  lateral  eyes,  I.e.,  consist  of  three  retinulae,  or  three  small  optic  cups, 
united  to  form  a  single  organ.  The  rhabdoms  are  terminal  and  nearly  upright, 
that  is  they  are  directed  outward  toward  the  optical  center  of  each  cup  and 
toward  a  centrally  located  refractive  body  consisting  of  three  sharply  denned 
segments  These  eyes  represent  the  compound  lateral  eyes  of  arthropods  in  a 
rudimentary  or  elementary  condition. 

The  parietal  eye  ("Fossetta  Retro cerebrali "  of  Grassi),  including  its  two 
appendages,  is  a  tri-lobed  sac,  lying  on  the  posterior  median  margin  of  the  fore- 
brain.  Except  for  the  absence  of  the  black  pigment  that  ordinarily  makes  the 
parietal  eye  so  conspicuous,  it  is  very  similar  to  the  parietal  ocellus  of  the  nauplius 
of  Apus,  Branchipus,  or  Lernaea. 

The  eye  sac  consists  of  a  median  portion  opening  freely  to  the  exterior,  and 
representing  the  unpaired  ocellus  with  its  short  epiphysis  and  pore,  while  the  two 
paired  ocelli  lie  in  the  diverging  blind  sacs  on  either  side.  The  globules  and 
glistening  granules  described  by  Grassi,  that  are  contained  in  these  sacs,  no  doubt 
represent  the  vesicular  retinal  cells  filled  with  the  white  pigment  so  commonly 
present  in  the  degenerate  parietal  eye,  e.g.,  Limulus  and  Petromyzon.  The  eye 
lies  directly  on  the  posterior  surface  of  the  brain,  as  in  Apus,  and  appears  to  be 
connected  with  it  by  short  but  indistinct  nerves. 

The  Olfactory  Organ. — Between  the  parietal  and  lateral  eyes  are  two  large 
nerves,  co.n.,  one  on  either  side,  that  represent  the  frontal-organ  nerves,  or  the 
olfactory  nerves  of  arachnids,  phyllopods  arid  entomostraca.  They  terminate 
diffusely,  that  is,  in  widely  distributed,  subcutaneous  branches,  as  is  characteristic 
of  these  nerves  in  the  above  mentioned  forms,  in  a  large  sensory  area  enclosed  by 
a  prominent  ciliated  groove,  " corona  ciliata"  of  Grassi.  This  area,  the  surround- 
ing ciliated  groove,  and  the  appertaining  nerves  (two  pairs  ?)  represent  the  frontal 
or  olfactory  organs  of  the  crania ta.  See  Chapter  X,  p.  160. 

The  Ventral  Ganglion. — The  ventral  ganglion  is  relatively  large  and  complex. 
Its  minute  anatomy  has  not  been  carefully  described,  but  it  appears  to  have  the 


CONCLUSION.  453 

typical  structure  of  the  ventral  nerve  cord  in  primitive  arthropods.  Whether  or 
no  it  consists  of  distinct  neuromeres,  secondarily  united,  will  probably  be  mani- 
fest on  a  more  careful  examination  of  its  transverse  commissures. 


Conclusion. — The  chaetognatha  are  unquestionably  primitive  arthropods, 
somewhat  degenerate.  They  are  adapted  to  a  permanent  pelagic  existence  and 
reach  sexual  maturity  without  passing  much  beyond  the  nauplius  stage.  Their 
relation  to  the  arthropods  is  shown  by  the  presence  of  a  typical  teloccele;  ccelomic 
pouches;  the  early  appearance  and  the  location  of  the  sex  cells  in  the  segmenting 
egg;  the  division  of  the  body  into  head,  thorax,  and  abdominal  regions;  the  loca- 
tion of  the  ovaries  in  the  thoracic  region,  and  the  testis  in  the  abdominal  region  of 
the  adult;  the  character  of  the  jaws;  the  lateral  or  pleural  folds;  the  bivalve  mantle 
folds  of  the  head  (prepuce);  the  prevalence  of  chiten;  the  fibro-cartilaginous  plate 
in  the  jaw  muscles  (endocranium) ;  the  structure  of  the  brain  and  the  arrangement 
of  stomodaeal  nervous  system;  and  finally  the  structure  and  distribution  of  the 
lateral  and  parietal  eyes  and  "  frontal  organs." 

The  history  of  the  chaetognatha  is  profoundly  significant,  for  it  shows  us  a 
group  of  animals  that  combines  in  a  convincing  manner  some  of  the  important 
anatomical  characters  of  adult  arthropods,  such  as  those  above  mentioned,  with 
the  more  striking  embryonic  characters  of  typical  acraniates,  such  as  the  telopore 
and  teloccele  and  ccelomic  pouches.  Their  structure  and  development  justifies 
the  conclusions  already  reached  in  other  ways,  that  the  teloccele  and  ccelomic 
pouches  are  secondary,  not  primary  characters,  and  that  they  have  been  acquired 
from  arthropod  ancestors,  partly  as  a  result  of  degeneration,  loss  of  yolk,  and  a 
consequent  rapid  process  of  early  development. 

The  chaetognatha  resemble  the  cephalochorda  and  the  enteropneusta  in  their 
straight,  elongated  body  and  intestine,  but  differ  from  them  in  the  presence  of  an 
open  neurostoma  and  the  absence  of  a  haemostoma  and  gill  clefts.  They  are 
definitely  excluded  from  the  nematodes,  annelids,  molluscs,  and  rotifers — with 
which  they  have  been  affiliated  by  various  authors — by  the  absence  of  the  gastrula 
and  trochosphere  stages,  by  their  highly  modified  development,  as  well  as  by  the 
peculiar  structure  of  their  brain,  stomodaeal  ganglia,  and  cephalic  sense  organs. 
They  are  clearly  acraniates,  but  are  not  closely  affiliated  with  any  other  sub- 
division of  the  group. 


CHAPTER  XXV. 
SUMMARY  AND  CONCLUSION. 

We  have  shown  in  the  preceding  chapters  that  the  great  trunk  line  of  animal 
evolution  is  the  vertebrate-ostracoderm-arthropod-ccelenterate  stock.  The 
recognition  of  this  fact  is  of  great  importance,  for  it  enables  us  correctly  to  locate 
several  other  important  phyla,  whose  position  in  a  natural  system  of  classification 
it  has  been  heretofore  impossible  to  determine.  For  the  first  time  it  opens  to  us 
the  great  creative  period  in  the  evolution  of  vertebrates;  lays  before  us  in  detail 
the  successive  stages  in  the  upbuilding  of  their  physical  structure  and  functional 
organization;  reveals  the  important  factors  that  create  and  control  the  process, 
and  the  critical  events  incident  to  its  consummation. 

This  reconstruction  of  the  phylogeny  of  the  animal  kingdom  adds  enormously 
to  our  perspective  of  evolution,  both  as  to  the  length  of  time,  and  the  number  and 
variety  of  graded  animal  forms  involved.  For  the  first  time  it  places  us  in  a  posi- 
tion to  study  the  rate  and  direction  of  organic  evolution  on  a  grand  scale  and 
to  observe  in  action  the  forces  that  direct  and  control  the  process,  for  all  the  great 
systems  of  organs  that  find  their  fullest  expression  in  the  vertebrates  were  in  a 
nascent  condition  in  the  arthropods;  here  their  qualitative  material  basis,  their 
relative  locations,  and  their  modes  of  growth  were  established,  and  the  conditions 
were  already  present  that  made  possible  the  characteristic  structures  of  man. 

This  new  point  of  view  shows  us  that  the  primary  creative  factors  in  organic 
evolution  lie  within  the  organism,  and  that  growth  itself  not  only  creates  the  con- 
ditions that  produce  the  framework  of  living  things,  but  marks  out  the  boundaries 
within  which  organic  evolution  is  possible.  External  environment,  natural  selec- 
tion, and  heredity  are  of  little  or  no  importance  in  this  process.  They  cannot  be 
considered  as  active  factors  in  evolution  till  after  the  underlying  framework  of  the 
organism  has  been  created. 


I.  THE  EVOLUTION  OF  A  CREATIVE  ENVIRONMENT. 

Cosmic,  Organic,  and  Social  Environments. — It  is  evident  that  we  may  not 
consider  environment  as  something  apart  from  and  independent  of  the  things 
environed,  for,  as  we  have  seen,  the  mere  process  of  growth,  or  of  continued  being, 
literally  creates  new  environments  for  all  the  constituent  parts,  whether  we  are 
dealing  with  proteids,  or  protoplasm,  or  a  group  of  cells,  or  a  vast  community,  or 
society  of  organisms;  and  the  new  environments,  likewise,  literally  create  new 
structures  and  new  organisms.  Structure,  organization,  and  environment  there- 

454 


THE   EVOLUTION    OF   A   CREATIVE   ENVIRONMENT.  455 

fore,  are  evolved  simultaneously  and  inseparably,  and  they  become  more  and 
more  complex,  interlocked  with  one  another  and  with  their  innumerable  constit- 
uents, with  the  lapse  of  time. 

It  is  important  to  distinguish,  more  sharply  than  is  usually  done,  between  the 
various  kinds  of  environment,  and  the  parts  they  play  in  the  production  of  new 
structures  and  organisms;  between  the  environments  that  create,  and  those  that 
are  prohibitory,  or  exclusive,  or  merely  permissive. 

Consider,  for  example,  a  simple  case,  such  as  ice.  There  are  three  essential 
factors  involved  in  its  production,  or  creation:  i.  The  inherent  nature  of  the 
hydrogen  and  oxygen  of  which  it  is  composed;  2.  the  relation  of  the  two  elements 
to  each  other  as  to  time,  space,  and  quantity,  and  3.  the  conditions  as  to  pressure, 
temperature,  etc.  When  all  three  factors,  i.e.,  materials,  time,  space  and  quan- 
tity relations,  and  environment,  are  in  a  definite  adjustment,  ice  appears,  or  is 
produced,  or  created.  If  some  animal,  or  other  agent,  devours  or  destroys  the 
ice  as  fast  as  it  is  created,  or  prevents  the  act  of  creation,  we  may  add  a  fourth 
factor  as  essential  to  the  creation  or  the  existence  of  ice,  namely  4.  the  absence  of 
an  excluding,  or  destructive  agent. 

The  first  and  second  factors  include  the  materials  and  their  distribution; 
the  third  and  fourth  are  external,  and  are  primarily  distinct  from  the  materials 
or  their  distribution;  they  constitute  the  environment.  Only  one  factor  may  be 
assumed  to  be  permanent,  or  at  any  rate  relatively  permanent,  that  which  con- 
stitutes the  quality  or  nature  of  the  materials,  or  simply  the  materials  themselves. 
All  other  factors  are  constantly  changing  or  fluctuating.  The  second  and  third 
factors  may  be  properly  considered  creative  factors  in  the  sense  that  when  they 
prevail,  ice,  a  different  thing  from  what  previously  existed,  appears.  In  no  sense 
can  ice  be  said  to  have  been  present  in,  or  to  have  pre-existed  in  either  the  ma- 
terials, H  and  O,  or  in  the  creative  conditions.  The  fourth  class  of  factors  may 
be  neutral  or  permissive,  exclusive  or  prohibitory,  but  under  no  conditions 
creative. 

Thus,  reduced  to  its  simplest  terms,  the  command  of  materials  and  of  their 
relations  to  each  other  as  to  time,  space,  and  quantity,  and  the  command  of 
environment,  constitute  creative  power. 

All  aggregations  of  material  create  for  the  aggregate,  and  for  each  of  its  com- 
ponents, new  directive  and  controlling  conditions.  This  is  true  whether  we  are 
dealing  with  the  aggregation  of  elements  to  form  water,  or  proteids,  or  mixtures  of 
proteids.  The  aggregation  of  cosmic  materials  to  form  the  earth  has  produced 
new  conditions  that  created  the  land,  and  sea,  and  atmosphere,  rivers,  mountains, 
valleys  and  soil,  and  gave  to  each  in  turn  its  power  to  direct  and  control,  and  to 
create  anew.  All  vital,  organic  growth  is  of  this  nature,  except  that  its  income  and 
outgo  are  more  accurately  balanced  and  its  sphere  of  activity  more  minutely 
localized. 

The  geologist  interprets  the  growth  of  the  earth  by  its  change  of  form  and 
by  the  distribution  of  its  materials,  and  seeks  to  correlate  its  structure  at  a  given 


456  SUMMARY   AND    CONCLUSION. 

time  with  the  conditions  which  at  that,  or  some  previous  time,  prevail,  convinced 
that  one  is  the  formal  expression  of  the  other.  We  have  used  the  same  method 
in  the  interpretation  of  organic  growth.  We  have  shown  that  the  mere  process  of 
radial  and  apical  growth,  or  the  aggregation  of  organic  materials  around  a  given 
center,  or  along  a  given  line,  or  surface,  automatically  creates  regularly  graded 
zones  of  unlike  conditions  that  are  coincident  with  the  distribution  of  unlike 
materials  or  organs  We  conclude  therefrom  that  the  basic  structure  of  plants 
and  animals  is  automatically  created  by  the  process  of  growth  itself,  or  that 
growth  automatically  creates  special  local  conditions,  which  are  expressed  in  the 
structures  that  appear  at  those  points. 

The  principal  factors,  therefore,  that  create  organized  structures  are  primarily 
internal  and  are  sharply  localized;  they  are  the  result  of  the  environment  of  its 
several  parts,  and  they  change  with  the  process  of  growth.  The  medium  external 
to  the  organism  as  a  whole,  its  cosmic  environment,  such  as  the  sea  water  and  its 
contents,  pressure,  temperature,  light,  gravity,  etc.,  is  practically  unaffected  by 
local  growths  and  remains  approximately  constant. 

Historically  speaking  then,  the  evolution  of  the  external  environment 
did  not  keep  pace  with  the  evolution  of  the  internal  environment.  Primarily 
the  external  environment  was  purely  inorganic,  or  cosmic,  broadly  permissive, 
or  neutral.  In  the  early  stages  of  organic  evolution  there  was  no  dependence  of 
one  organism  on  another,  no  social  organization,  no  social  environment,  no  or- 
ganic competition,  or  selection,  or  elimination,  for  all  alike  drew  their  materials 
from  the  surrounding  inorganic  media. 

As  primitive  organisms  became  more  complex,  the  balancing  points  of  internal 
environments  became  more  precisely  located,  and  were  expressed  in  more  stable, 
more  sharply  denned  differences  in  the  structure  of  the  resulting  forms.  The 
growth  of  individual  organisms  was  accelerated,  or  short  circuited  by  the  fusion, 
or  union,  or  absorption,  of  one  form  by  another,  giving  simultaneous  rise  to  organic 
nutrition,  sexual  reproduction,  social  environment,  and  social  competition. 

Thus  we  again  reach  the  conclusion  that  the  continual  aggregation  of  like 
units  to  form  a  homogeneous  whole  is  an  impossibility,  for  aggregation  creates  un- 
like conditions,  that  create  new  things,  new  organs,  new  organisms,  new  societies, 
and  new  organizations  of  old  societies. 

II.  CRISES  IN  ORGANIC  EVOLUTION. 

The  evolution  of  organisms  does  not  proceed  at  a  uniform  rate,  but  at  a 
variable  one;  now  slow,  now  fast,  retreating,  diverging,  advancing;  now  by  in- 
numerable minute  steps,  now  by  leaps  and  bounds;  because  the  ever  shifting 
relations  of  part  to  part,  organ  to  individual,  and  individual  to  society,  are  of 
unlike  nature  and  of  unequal  value. 

These  inequalities  in  the  potential  value  of  organic  readjustments  form  the 
true  basis  of  a  natural  system  of  classification.  They  create  the  critical  periods 
in  organic  evolution;  they  produce  actual  gaps  in  the  mosaic  of  adult  forms  which 


CRISES    IN    ORGANIC    EVOLUTION.  457 

constitutes  the  ideal  genealogical  tree  of  the  animal  kingdom;  their  advents  create 
the  real  subdivisions,  and  mark  the  starting  points  of  divergent  evolution  for 
phyla,  classes,  and  for  the  innumerable  smaller  branches.  The  gaps  in  a  natural 
system  of  classification  therefore  do  not  always  mark  the  periods  of  lost  records 
or  the  areas  of  densest  ignorance. 

Hence  organic  readjustments,  according  to  their  import,  mark  the  periods  of 
relatively  rapid  phyletic  changes  that  are  of  value  in  classification.  Let  us  con- 
sider some  of  those  organic  changes  that  are  recognizably  correlated  with  differ- 
ences in  bodily  form,  and  that  have  been  instrumental  in  fixing  or  guiding  the 
course  of  evolution  in  the  arthropod-vertebrate  stock. 

A.  The  Evolution  of  Metamerism  and  Bilateral  Symmetry. — It  has  been 
shown  that  a  local  exaggeration  of  radial  growth  results  in  apical  growth,  and  that 
under  the  existing  conditions  apical  growth  inevitably  creates  bilateral  symmetry 
and  a  double  series  of  graded  unlike  conditions  and  structures,  one  series  extend- 
ing in  a  cephalo-caudal  direction  giving  rise  to  metamerism,  the  other  in  a  neuro- 
haemal  direction,  giving  rise  to  the  graded  series  of  conditions  and  organs  on  the 
right  and  left  half  of  each  metamere.  (See  Fig.  157.)  These  conditions  are  prob- 
ably resident  in  all  coherent,  organic  growth,  for  we  see  essentially  the  same 
conditions  repeated  wherever  apical  growth  prevails,  whether  in  plants  or  animals. 

It  was,  no  doubt,  some  local  exaggeration  of  radial  growth  in  a  ccelenterate 
ancestor  that  initiated  the  craniate  stock.  The  craniates  began  as  minute  ani- 
mals (trocosphere  stage)  consisting  of  a  relatively  large  head  that  was  built  on  a 
radiate  plan,  and  that  represented  the  ancestral  ccelenterate  body.  From  this 
primitive  head,  a  local  outgrowth  arose  that  gave  rise  to  a  new  body.  The  latter 
gradually  increased  in  volume,  and  as  a  result  of  its  characteristic  mode  of  apical 
growth,  bilateral  symmetry,  metamerism,  and  the  difference  between  the  neural 
and  haemal  surfaces,  became  gradually  and  permanently  established. 

The  old  body,  or  primitive  head,  decreased  relatively  in  volume  and  was 
finally  represented  almost  solely  by  its  nervous  elements,  the  primitive  forebrain, 
and  by  the  oral  opening  into  the  alimentary  canal.  The  new  trunk  increased  in 
volume  by  the  spasmodic  production  of  new  groups  of  metameres,  alternating 
with  prolonged  periods  of  phylogenetic  inactivity.  Each  new  group  of  metameres 
always  differed  from  the  preceding  ones,  and  after  a  varying  period  of  greater 
or  less  organic  independence,  was  incorporated  with  them  as  a  subordinate  part 
of  an  increasingly  complex  head.  Even  in  its  most  elaborate  condition,  as  seen 
in  the  higher  vertebrates,  the  head  still  shows  distinct  traces  of  the  successive  gen- 
erations of  metameres  of  which  it  is  composed. 

Each  notable  increase  in  the  size  of  the  body  due  to  the  addition  of  a  new 
group  of  metameres,  disturbed  the  pre-existing  organic  equilibrium,  bringing 
about  the  shifting  of  the  organs  of  locomotion,  excretion,  circulation,  and  digestion, 
to  regions  farther  and  farther  back  in  the  body.  Outgrowths  of  nerve  fibers  from 
the  old  sensory  and  nervous  centers  then  established  coordinating  relations  with 
them. 


458  SUMMARY   AND    CONCLUSION. 

The  special  characteristics  of  apical  growth  in  the  craniate  stock  are  the  precise 
limitation  to  its  extent;  the  sharp  differences  between  the  metameres  of  different 
groups,  or  generations;  and  the  differences  between  the  various  parts  of  the  same 
metameres.  This  is  specially  notable  in  the  arthropods  where  many  of  the  sub- 
phyla,  such  as  the  insects,  arachnids,  etc.,  are  notable  fora  small,  definite  number 
of  metameres,  and  for  the  sharply  graded  step-like  differences  between  them. 
This  is  in  marked  contrast  with  the  leaf-like,  homogeneous  body  of  the  platyhel- 
menthes;  the  voluminous  and  elaborate,  but  short  bodied  mollusca;  the  inde- 
finitely elongated  series  of  similar  metameres  characteristic  of  the  annelids;  and 
finally  with  the  small  size,  feeble  definition  of  organs,  but  unlimited  power  of 
budding  so  commonly  manifested  by  the  acraniates. 

One  of  the  striking  features  of  apical  growth  that  is  manifest  through  the 
entire  range  of  the  arthropod-vertebrate  stock  is  the  vigorous  and  persistent 
power  of  producing  new  metameres  that  are  varied  in  character,  and  that  have 
a  well  marked  power  of  mutual  adaptability.  It  is  this  adaptability  that  ultimately 
leads,  especially  in  the  older,  more  anterior  metameres  of  the  higher  forms,  to  the 
almost  complete  disappearance  of  metamerism,  and  to  the  substitution  for  it  of 
a  linear  arrangement  of  unlike  functions  and  organs  where  the  location  of  an 
organ  in  the  series  is  determined  by  its  right  of  historic  precedence  and  by  the 
degree  to  which  the  performance  of  its  functions  depends  on  location. 


On  the  other  hand,  the  acraniates  are  universally  characterized  by  the  lack 
of  this  vital  vigor,  and  the  contrast  between  the  method  of  growth  and  differei,  tia- 
tion  in  these  two  great  subdivisions  of  the  animal  kingdom  is  most  instructive. 
Although  the  acraniates  apparently  started  at  the  same  time  as  the  craniates, 
with  a  very  similar  structure,  and  under  similar  external  conditions  they  gave 
rise  to  a  multiplicity  of  feeble,  defective,  often  degenerate  subphyla,  whose  most 
characteristic  features  are  the  degenerate  or  extremely  small  size  of  the  neuro- 
muscular  systems,  the  feeble  power  of  apical  growth,  and  the  indistinct  metamer- 
ism. We  may  attribute  this  lack  of  organic  definition  to  some  inherent  defect  in 
the  constituent  materials  common  to  them  all,  and  which  lies  quite  beyond  our 
reach.  Another  great  difference  between  the  two  groups  is  the  absence  of  a  fixed 
internal  environment  in  the  acraniates,  due  to  the  absence  of  an  impervious  exo- 
skeleton.  Without  it,  development  is  apparently  more  diffuse,  or  vegetative,  and  is 
marked  by  an  unlimited  power  of  budding.  At  the  same  time  there  is  clearly  some 
fundamental  defect  in  their  neuromuscular  system  which  checks  its  development,  or 
leads  to  its  almost  complete  degeneration. 

B.  Asymmetry  as  a  Creative  Factor. — While  bilateral  symmetry,  with 
its  accompanying  arrangement  of  unlike  parts  in  linear  and  transverse  series, 
is  apparently  an  inherent  product  of  apical  growth,  and  is  the  normal  condition 
in  the  arthropod-vertebrate  stock,  it  is  subject  to  modifications  of  unknown  origin 
that  produce  various  degrees  of  asymmetry.  Where  there  is  a  measurable  quanti- 


ASYMMETRY  AS  A  CREATIVE  FACTOR.  459 

tative  difference  between  corresponding  right  and  left  organs,  the  other  parts  of 
the  body  promptly  respond  by  a  change  of  form,  or  position,  which  tends  to  restore 
the  lost  equilibrium,  producing  thereby  a  more  obvious  deformity  or  asymmetry. 

It  is  often  assumed  that  asymmetry  is  the  result  of  a  sessile  or  parasitic  mode  of 
life.  But  it  is  altogether  more  probable  that  the  reverse  is  true,  for  comparatively 
few  sessile  or  parasitic  animals  are  asymmetrical,  and  extremely  asymmetrical 
forms  would  be  likely  to  find  a  sessile  or  parasitic  mode  of  life  the  only  one  open  to 
them,  because  these  conditions  are  less  exacting. 

The  initial  cause  of  asymmetry  is  usually  some  event  that  occurs  within  the 
egg  itself,  probably  at  a  very  early  period.  It  has  been  shown,  for  example,  that 
under  normal  conditions,  such  an  ancient  and  stable  type  as  Limulus  produces  a 
surprisingly  large  number  of  asymmetrical  embryos.  When  one  side  is  entirely 
absent,  the  remaining  side  is  thrown  out  of  its  original  straight  line  into  the  form 
of  a  bow,  a  half  circle,  or  a  semi-spiral.  These  forms  may  live  several  months 
and  appear  to  be  perfectly  healthy,  but  apparently  they  never  develop  beyond 
the  larval  stages. 

Extreme  asymmetery  in  the  embryonic  development  of  some  cirriped-like 
arthropod  probably  initiated  the  great  class  of  echinoderms,  for  the  echinoderm 
larva,  which  in  its  structure,  metamorphosis,  and  mode  of  attachment,  resembles 
that  of  a  cirriped,  is  at  first  quite  symmetrical.  When  the  organs  on  one  side 
degenerate,  or  fail  to  develop,  the  remaining  side  bends  till  its  two  ends  meet  and 
form  a  ring;  the  segmental  organs  are  then  arranged  along  the  radii  of  a  circle, 
instead  of  in  parallel  lines.  These  changes  are  similar  to  those  that  occur  in 
Limulus  embryos,  except  that  they  are  carried  farther  and  give  rise  to  animals 
capable  of  surviving  in  their  new  form. 

Thus  such  a  negative  character  as  the  absence  of  one  side  of  the  body  has 
created  a  new  condition,  a  new  organic  environment,  that  has  in  turn  created  a 
new  type  of  radiate  structure,  and  at  a  single  stroke  initiated  the  evolution 
of  a  new  class  of  animals.  Organic  readjustment,  during  this  crisis,  was  no  doubt 
extremely  rapid.  But  after  the  essential  changes  took  place,  elaboration  toward 
a  more  active  neuro-muscular  existence  practically  ceased.  The  history  of  this 
ancient  phylum  indicates  that  it  never  completely  recovered  from  the  effects  of 
this  radical  metamorphosis,  for  no  other  large  group  of  animals,  with  an  equal 
grade  of  organic  complexity,  shows  such  a  low  grade  of  neuro-muscular  adjust- 
ment to  its  environment. 

C.  Chiten  and  the  Exoskeleton  as  Creative  Factors. — Chiten  is  one 
of  the  most  characteristic  features  of  the  arthropods,  and  like  the  cellulose  wall 
of  the  plant  cell  is  a  creative  factor  of  very  great  significance. 

It  is  tough  and  flexible,  but  inelastic;  it  is  capable  of  great  hardness  and 
forms  for  the  entire  external  surface  of  the  body  a  water-  and  air-proof  covering, 
unaffected  by  any  chemical  changes  likely  to  occur  in  the  surrounding  media. 
More  perhaps  than  any  other  factor  it  controls  the  form  of  the  body,  its  method 
of  growth,  the  distribution  and  attachment  of  muscles,  the  character  of  the  appen- 


460  SUMMARY   AND    CONCLUSION. 

dages,  and  of  the  sensory  and  respiratory  organs.  Its  chief  significance  therefore 
lies,  not  in  its  occasional  and  purely  incidental  usefulness,  especially  in  the  higher 
forms,  as  a  material  to  construct  supplementary  organic  instruments,  or  weapons 
of  offense  and  defense,  but  in  its  compelling  influence  on  form,  and  in  its 
creation  of  an  internal  organic  environment  distinct  from  that  of  the  surrounding 
medium. 

Chiten  appears  at  a  precisely  defined  period  in  arthropod  embryos,  as  the 
result  of  some  chemical  transformation  that  takes  place  on  the  outer  surface  of 
ectoderm  cells.  Its  presence  is  at  once  recognized  by  the  way  in  which  it  pre- 
vents the  penetration  of  stains  and  other  chemical  reagents.  There  is  no  reason 
to  doubt  that  it  appeared  in  the  ancestral  arthropods  with  corresponding  rapidity, 
and  that  it  had  an  immediate  and  persistent  transforming  effect  on  those  animals 
in  which  it  occurred.  From  the  earliest  period,  therefore,  every  member  of 
the  arthropod  stock  has  been  a  practically  closed  mechanism.  Its  sensory, 
respiratory,  and  excretory  relations  with  the  exterior  were  necessarily  confined  to 
precisely  located,  and  definitely  constructed  points,  or  openings.  The  location  and 
attachment  of  muscles,  and  the  movements  of  one  external  part  on  another,  were 
controlled  by  the  location  of  flexible,  hinge-like  joints  in  the  armor;  and  no 
notable  increase  in  volume  or  in  organic  activity  could  take  place  without  special 
provisions  for  the  circulation  of  a  blood-like  plasma  that  should  serve  at  the  same 
time  as  a  uniform  internal  environment,  distinct  from  the  external  one,  and  that 
was  suitable  both  for  cell  growth  and  as  a  means  of  transporting  nutritive  and 
waste  substances  to  their  place  of  consumption  or  point  of  discharge. 

The  presence,  therefore,  of  a  chitenous  exoskeleton  in  the  arthropods  may 
be  regarded  as  one  of  the  primary  causes  of  their  slow  growth  in  volume  and  of 
the  early  historic  appearance  of  precisely  located  sensory,  motor,  excretory,  cir- 
culatory, and  respiratory  organs;  and  of  their  sharply  defined,  closely  knit,  and 
potentially  sound  organization. 

These  conditions  are  in  sharp  contrast  with  those  in  the  naked,  soft  bodied 
ccelenterates,  and  in  many  worms,  molluscs,  and  acraniates,  where  the  surface 
of  the  body  is  more  uniformly  exposed  to  external  agents,  and  the  external  medium 
has  great  freedom  of  access  to  the  interior. 

The  sudden  loss  of  a  chitenous  exoskeleton,  after  a  longer  or  shorter  period 
of  control,  is  no  less  significant  than  its  initial  formation.  When  some  com- 
paratively insignificant  change  in  its  chemical  composition,  or  in  its  mode  of 
growth,  led  to  a  radial  change  in  its  physical  properties,  or  to  its  complete  dis- 
appearance, the  body  was  again  released  from  its  control  and  a  new  set  of  form- 
creating  factors  arose,  but  on  a  different  level  from  the  old.  The  change  from  a 
chitenous  armor  to  one  of  cellulose  was  a  powerful  factor  in  the  creation  of  the 
tunicate  type;  and  the  almost  total  absence  of  chiten  in  Balanoglossus,  Cephalo- 
discus,  Amphioxus  and  the  polyzoa;  the  substitution  for  it  of  a  heavy  armor  of 
isolated  calcareous  plates  in  the  echinoderms;  and  the  predominance  in  it,  of 
a  heavy  calcareous  deposit  in  the  cirripeds,  were  all  factors  of  great 


INCREASING    VOLUME    OF    THE    YOLK    SPHERE   AND    BRAIN.  461 

importance  in  controlling  the  mode  of  life,  the  method  of  growth  and  the  direc- 
tion and  progress  of  evolution  in  these  phyla. 

In  the  craniates,  the  characteristic  chitenous  exoskeleton  of  the  arthropods 
is  doomed  to  extinction,  owing  to  conditions  created  by  its  own  mode  of  growth. 
It  has  been  shown,  for  example,  that  in  Limulus  a  peculiar  and  exceptionally 
vigorous  method  of  growth  in  the  skeletogenous  tissues  gives  rise  to  a  system 
of  subdermal,  interlocking  trabeculae  that  probibits  the  subsequent  periodic 
removal  of  the  exoskeleton.  A  condition  is  thus  produced  that  compels  the  per- 
manent retention  of  the  chitenous  products  within  the  tissues  of  the  animal,  and 
which  initiates  the  formation  of  a  new  type  of  exoskeleton  that  is  largely  sub- 
dermal,  cellular,  and  fragmented.  This  permits  a  new  mode  of  growth  for  the 
animal  as  a  whole,  differing  from  the  old  in  much  the  same  way  that  the  growth 
of  an  endogenous  stem  differs  from  that  of  an  exogenous  one;  and  it  ultimately 
liberated  the  arthropod  stock  from  the  bondage  of  an  increasingly  restrictive, 
burdensome,  and  menacing  armor.  This  new  type  of  skeleton  itself  practically 
disappears  in  the  higher  vertebrates,  giving  place  to  the  new  framework  of  carti- 
lages and  bone  that  constitute  the  internal  skeleton. 

D.  The  Increasing  Volume  of  the  Yolk  Sphere  as  a  Creative  Factor.— 
The  local  retardation  of  growth  caused  by  the  presence  of  yolk  in  the  developing 
ovum  has  long  been  recognized  by  embryologists,  but  they  have  not  recognized 
the  form  controlling  conditions  created  by  apical  growth  on  a  spherical  surface. 

The  gradual  increase  in  the  size  of  the  yolk  sphere  throughout  the  arthropod 
and  lower  stages  of  the  vertebrate  stock  creates  a  new  set  of  conditions  that  greatly 
modifies  the  process  of  development;  for  the  growth  of  an  embryonic  metamere 
over  the  surface  of  a  yolk  sphere  is  a  different  problem  from  the  growth  of  a  new 
metamere  added  to  the  apex  of  a  mature  animal.  One  spreads  film-like  in  mer- 
cator  projection  over  an  approximately  plane  surface,  the  other  grows  as  a  solid 
body  round  a  central  point.  When  the  size  of  the  yolk  sphere  is  increased,  the 
number  of  metameres  so  affected  is  increased,  and  the  effect  on  each  metamere 
will  depend  on  its  location  in  the  series,  that  is,  whether  it  lies  at  the  head  or  tail 
end,  whether  it  has  to  grow  round  the  equator,  or  round  the  poles  of  the  sphere. 
In  this  way  the  size  of  the  yolk  sphere  controls  the  structure  and  mode  of  growth  of 
the  heart,  the  belly  navel,  and  germ  wall,  and  has  created  the  phenomenon  of 
concrescence. 

Thus,  owing  to  the  difference  in  the  location  of  metameres  on  a  yolk  sphere 
of  variable  dimensions,  inevitable  differences  arise  in  the  conditions  under  which 
these  metameres  are  compelled  to  grow,  and  these  differences  are  increased,  or 
exaggerated,  with  the  increasing  volume  of  the  yolk  sphere.  These  differences 
in  conditions  coincide  to  a  large  extent  with  the  morphological  and  physiological 
differences  that  characterize  the  corresponding  regions  of  the  body,  and  may  be 
assumed  to  be  the  causes  that  have  brought  them  about. 

E.  The  Increasing  Volume  of  the  Brain  as  a  Creative  Factor. — A  con- 
spicuous feature  in  the  evolution  of  the  arthropods  is  the  steady  increase  in  the 


462  SUMMARY   AND    CONCLUSION. 

volume  of  the  sensory  and  nervous  centers  located  in  the  more  anterior  metameres 
of  the  head  and  trunk,  and  the  corresponding  decrease  of  the  lateral  plates,  to- 
gether with  the  alimentary,  locomotor,  and  mesodermic  elements  belonging  to 
the  same  metameres. 

Structural  changes  of  very  great  moment  are  inevitably  brought  about  by  these 
conditions.  Owing  partly  to  the  presence  of  yolk,  from  which  all  the  growing 
tissues  can  draw  their  sustenance  without  the  intervention  of  an  alimentary  canal, 
the  nervous  system  attains  a  very  considerable  volume,  and  becomes  functional 
long  before  the  stomodaeum  is  called  into  action.  This  delay  in  its  development, 
its  unfortunate  location,  and  its  comparatively  delicate  epithelial  walls,  places  the 
stomodaeum  under  a  heavy  handicap  in  its  competition  for  space  with  the  nervous 
system.  The  result  is  that  the  increasing  size,  precocity,  and  more  intimate  union 
of  the  cephalic  neuromeres  during  early  embryonic  development,  leads  to  a  gradual 
narrowing  of  the  nerve  ring  surrounding  the  oesophagus,  making  in  many  arthro- 
pods, for  a  longer  or  shorter  period,  a  fluid,  or  semi-fluid  diet  (i.e.,  blood-sucking, 
parasitic,  or  scavenging)  more  and  more  imperative.  The  same  increase  in 
volume  and  in  precocity  of  the  cephalic  neuromeres  also  led  at  an  early  embryonic 
period  to  the  invagination  of  the  entire  brain,  so  that  with  the  closure  of  the  neural 
crests  and  .palial  fold,  the  old  mouth,  which  opens  into  the  floor  of  the  brain 
chamber,  became  completely  shut  off  from  the  outside  world,  and  could  no  longer 
perform  its  normal  functions. 

The  increasing  precocity  of  the  cephalic  neuromeres  and  the  diminishing 
volume  of  the  corresponding  lateral  plates  also  lead  to  the  formation  of  a  more  and 
more  prominent  head  fold,  and  to  the  transfer  of  the  oral  arches,  which  in  the 
arthropods  are  neural  or  lateral  in  position,  to  the  haemal  surface  of  the  head. 
(Figs.  32,  33,  157.)  There  they  converge  toward  the  old  dorsal  organ  and 
cephalic  navel  that  constitutes  the  center  for  the  formation  of  the  new  mouth. 

The  actual  closure  of  the  old  mouth,  the  opening  of  the  new  one,  and  the 
transfer  of  the  jaws  to  the  haemal  surface  of  the  head,  were,  therefore,  brought 
about  in  a  large  measure  by  the  action  of  the  same  forces.  These  events,  by  the 
final  upsetting  of  a  long  established  organic  equilibrium,  took  place  rapidly,  the 
consummation  of  one  event  probably  accelerating  the  other;  they  also  led  to  a 
rapid  readjustment  in  other  parts,  and  to  important  changes  in  the  mode  of  life, 
especially  in  the  mode  of  feeding  and  in  the  position  of  the  body  in  locomotion. 


F.  The  Creation  of  a  New  Environment  for  the  Eyes. — The  location 
of  the  vertebrate  eye  in  the  walls  of  a  hollow  brain  has  caused  much  discussion. 
We  have  shown  that  these  so-called  cerebral  eyes  did  not  originate  in  situ  as 
the  result  of  the  stimulating  effect  of  light  acting  on  the  brain  through  the  body 
walls  of  a  transparent  ancestor,  or  by  use,  or  natural  selection.  They  are 
merely  the  ancient  median  and  lateral  eyes  of  the  arthropods  in  a  new  position. 
They  have  been  forced  into  the  brain  chamber  by  an  accident,  as  it  were,  because 


THE    SIGNIFICANCE    OF  A   NATURAL   SYSTEM   OF    CLASSIFICATION.  463 

they  happened  to  be  located  near  the  margin  of  the  rapidly  infolding  neural  plate. 
One  may  observe  in  the  arthropod  stock  the  steady  approach  of  this  inevitable 
disaster  to  the  visual  organs,  brought  about  by  the  increasing  precocity  of  the 
embryonic  brain  and  optic  ganglia.  It  is  important  to  observe  that  under  the 
existing  conditions  there  is  no  half-way  position  for  the  eyes;  they  are  either  carried 
wholly  inside,  or  remain  wholly  outside  the  brain,  and  that  once  inside  there  is  no 
escape  for  them.  But  the  very  slight  difference  in  the  physical  conditions  that 
finally  precipitates  the  eyes  into  the  brain  cavity,  creates  at  once  an  immense 
difference  in  the  physical  conditions  under  which  the  eyes,  the  brain,  and  indeed 
the  whole  anterior  part  of  the  head  must  complete  its  development.  At  one  stroke 
the  lateral  eye  is  changed  from  the  superficial  type  seen  in  the  invertebrate  to  that 
which,  in  the  vertebrate,  lies  in  the  walls  of  the  cerebral  vesicle. 

The  actual  closing  of  the  old  mouth  and  the  opening  of  the  new  one,  the 
transfer  of  the  oral  arches  to  the  haemal  surface  of  the  head,  and  the  transfer  of 
the  lateral  eyes  to  the  interior  of  the  cerebral  vesicle,  brought  about  a  great  crisis 
in  the  evolution  of  the  arthropod- vertebrate  stock;  and  the  successful  consum- 
mation of  these  internal  organic  changes  constitutes  the  most  important  event  in 
the  evolution  of  the  animal  kingdom. 

The  organic  adjustments  referred  to  above  were  necessarily  rapid  in  their 
progress  and  revolutionary  in  their  effect.  But  the  evolution  of  the  conditions 
that  led  up  to  them  was  extremely  slow,  consisting  of  a  long  series  of  cumulative 
internal  events  that  had  no  immediate  bearing  on  the  use  or  the  character  of  the 
organs  that  in  the  end  were  most  vitally  affected.  In  the  arthropods,  we  may 
follow  in  detail  through  an  immensely  long  period  of  time,  and  in  a  long  series 
of  animals,  the  steps  that  led  up  to  this  inevitable  crisis.  The  rapid  succession 
of  readjustments  that  followed  gave  rise  to  a  sharply  defined,  remarkably  short- 
lived, transitional  phylum,  the  ostracoderms.  After  that  stage  is  passed,  the 
organs  in  question  remain  practically  stationary  through  the  whole  vertebrate 
series  from  fishes  to  man. 

THE  SIGNIFICANCE  OF  A  NATURAL  SYSTEM  OF  CLASSIFICATION. 

A  consideration  of  the  facts  discussed  above,  to  which  many  more  might 
be  added,  throws  a  new  light  on  variation  and  environment  and  on  the  meaning 
of  a  natural  system  of  classification. 

A  natural  system  of  classification  is  an  attempt  to  represent  in  graphic 
form  a  genealogical  tree  of  the  animal  kingdom,  and  in  so  far  as  it  is  a  true  record 
of  evolution  and  descent,  it  should  reflect  the  guiding  and  controlling  factors  that 
have  created  it. 

If  natural  selection  and  external  environment  are  the  important  factors 
in  creative  evolution  that  they  are  frequently  assumed  to  be,  then  there  should 
be  reflected  in  a  natural  system  of  classification  a  broad  correlation  between 
structure  and  environment  that  could  be  used  as  an  aid  in  the  making  of  it.  But 
this  is  not  the  case,  for  we  do  not  usually  divide  animals,  according  to  their  mode 


464  SUMMARY   AND    CONCLUSION. 

of  life  or  environment,  into  those  for  example,  that  live  in  fresh  or  salt  water,  or 
in  the  air,  or  on  the  land,  etc. 

If  variation,  from  whatever  cause,  is  invariably  minute  and  of  equal  specific 
value,  then  a  perfect  record  of  the  past,  that  is,  the  genealogical  tree,  should  show 
a  continuous  system  of  branches  composed  of  a  uniformly  graded  series  of  animals. 

But  while  it  may  be  possible  to  arrange  the  members  of  certain  small  sub- 
divisions of  the  animal  kingdom  into  minutely  graded  linear  series,  it  is  usually 
exceedingly  difficult  to  determine  which  end  of  the  series  is  the  base  and  which 
is  the  apex,  because  a  considerable  gap  usually  exists  between  the  offshoot  and  the 
presumably  parent  stock. 

The  speculative  zoologist  often  attempts  to  fill  this  gap  with  hypothetical 
forms  having  the  desired  intermediate  characters,  on  the  assumption  that  con- 
necting adult  forms,  as  numerous  and  finely  graded  as  on  any  of  the  modern 
terminal  branches,  once  existed,  but  are  now  either  extinct,  or  unknown,  or 
both  extinct  and  unknown.  Some  morphologists  assume,  for  example,  that  the 
vertebrate  stock  had  its  origin  in  such  forms  as  the  annelids,  echinoderms,  tuni- 
cates,  or  enteropneusta,  and  that  the  enormous  series  of  animals  necessary  to  fill 
the  gap  between  them  and  the  vertebrates  are  now  extinct  and  will  remain  for- 
ever unknown. 

But  there  is  no  evidence  to  show  that  the  true  genealogical  tree  is,  or  should 
be,  a  minutely  and  uniformly  graded  series  of  animal  forms.  On  the  contrary, 
we  have  shown  that  the  internal  creative  factors  are  of  very  unequal  value  and 
that  they  are  exceedingly  variable  at  different  times,  giving  rise  to  well  defined 
periods  in  phylogeny  during  which  many  large  and  important  changes  of  form  take 
place.  Hence  while  some  of  the  apparent  gaps  in  our  imperfect  genealogies  may 
be  filled  by  the  discovery  of  new  forms,  it  is  evident  that  the  ideal  phylogenetic 
tree  of  the  animal  kingdom  must  be  in  reality  a  loosely  articulated  system,  con- 
sisting of  slender,  interrupted,  or  vanishing  basal  stems,  expanding  into  top- 
heavy  branches.  In  other  words,  it  was  minutely  graded  in  certain  places  only, 
notably  on  the  older  terminal  branches.  There  the  differences  may  well  be  either 
the  minute  so-called  continuous  variations  of  Darwin,  or  the  sharper,  discon- 
tinuous mutations  of  De  Vries. 

But  the  larger  natural  subdivisions  of  the  animal  kingdom,  in  some  cases  at 
least,  appear  to  be  separated  by  real  gaps,  much  larger  than  any  known  mutations, 
and  that  are  not  due  to  defective  records.  They  represent  periods,  varying  in 
intensity  and  in  duration,  of  rapid  transformation  in  definite,  predetermined  di- 
rections, followed  by  periods  of  slow  development  along  more  varied,  but  less 
revolutionary  lines.  These  gaps  can  never  be  filled  by  the  discovery  of  new 
forms,  for  in  reality  they  represent  changes  of  pace  in  evolution,  or  periods  of 
greatly  accelerated  evolution,  which  in  perspective,  and  by  comparison  with 
other  periods,  appear  as  gaps. 

A  phyletic  metamorphosis  of  this  character  I  have  named  a  methallosis.1 

1  To  rush  after,  to  leap  from  one  ship  to  another. 


THE    SIGNIFICANCE    OF   A   NATURAL   SYSTEM   OF    CLASSIFICATION.  465 

It  may  be  defined  as  a  marked  change  of  pace  in  phytogeny;  or  a  rapid  succession 
of  important  embryonic  variations  due  to  the  upsetting  of  a  long-established  con- 
dition of  organic  equilibrium;  or  to  some  organic  change,  insignificant  in  itself, 
but  which  at  once  creates  new  conditions  for  the  growth  of  the  organs  so  affected, 
or  of  other  organs.  The  extent  of  a  methallosis  maybe  measured  by  the  extent  of 
the  variations  and  by  the  length  of  the  period  in  which  they  occur. 

During  ontogeny  there  are  well-marked  periods  of  accelerated  development, 
quite  independent  of  growth  or  increase  in  size,  during  which  there  is  a  rapid 
succession  of  profound  structural  changes,  followed  by  longer  periods  of  slow 
development.  These  periods  of  accelerated  development  are  well  known  as 
metamorphoses,  or  transformations.  When  seen  in  perspective,  they  appear 
as  gaps  separating  distinct  periods  of  life.  They  represent  no  doubt  the  onto- 
genetic  repetition  of  periods  of  accelerated  race  development,  the  accelerated 
period,  in  the  latter  case,  representing  the  real  gaps  between  the  larger  subdivisions 
of  the  animal  kingdom. 

If  the  variations  that  have  given  rise  to  new  animal  forms  are  indiscriminate, 
diverging  in  all  conceivable  directions  from  the  parent  stock,  then  the  actual 
genealogical  tree  that  is  the  result  of  such  indiscriminate  variation,  and  as  con- 
trolled by  natural  selection,  should  show  a  recognizable  correlation  between  the 
structure  of  a  given  group  of  animals  and  its  surroundings.  But  this  is  so  to  a 
very  limited  extent  only,  and  only  in  regard  to  minor  or  superficial  features  of 
organs  that  have  long  existed  under  other  conditions. 

The  underlying  basic  structure  of  the  organism  is  in  no  way  modified  by  the 
mode  of  life  or  by  the  surroundings,  for  all  segmented  animals,  whether  they  live 
in  the  air,  in  water,  or  on  the  land,  agree  in  their  mode  of  growth  and  in  the 
relative  positions  of  the  principal  organs,  such  as  the  central  nervous  system, 
alimentary  canal,  heart,  eyes,  olfactory  organs,  etc.  The  same  thing  is  true  of 
the  whole  great  class  of  vertebrates,  where  the  basic  structure  of  the  jaws,  gill 
arches,  the  brain,  and  principal  sense  organs,  is  immutable,  and  identical  for 
every  member  of  the  class.  This  established  structure  has  rigidly  defined  the 
possibilities  of  evolution  in  the  past  and  it  will  control  the  actual  development  of 
the  future. 

The  power  of  articulate  speech,  for  example,  depends  on  the  structure  of  the 
lips,  jaws,  tongue,  larynx,  and  the  respiratory  organs,  and  upon  a  definite  nervous 
association  of  these  parts.  It  is  a  highly  characteristic  faculty  of  man,  yet  the 
organic  basis  of  the  entire  complex  mechanism  preexists,  and  the  framework  is 
already  set  up  in  the  fishes,  where  the  rudiments  of  all  these  organs  are  known 
to  occur  and  where  they  have  in  the  main,  the  same  mode  of  growth,  inter- 
relations, and  nerve  connections  they  have  in  man. 

In  the  same  broad  sense,  the  basic  structure  of  the  arthropod  rigidly  de- 
termines that  of  the  fishes,  and  the  fishes  that  of  man.  The  main  highways 
of  evolution  are  therefore  mapped  out  by  the  initial  structure  of  the  most  remote 
ancestors.  To  that  extent,  evolution  is  direct,  orthogenic,  predetermined.  It 

30 


466 


SUMMARY   AND    CONCLUSION. 


Pro.C.       G 


H 


FIG.    307. 


THE   EVOLUTION    OF   VERTEBRATES. 


467 


r.    st.c 


Pe'.P 


ol 


FIG.    308. 


468  SUMMARY   AND    CONCLUSION. 

is  dependent  on  antecedent  structure,  and  that  is  automatically  created  in  the 
process  of  growth  and  organic  readjustment. 


There  are  therefore  various  aspects  of  evolution;  some  are  over-emphasized 
by  one  school  of  biologists,  and  ignored  by  others,  because  the  creative  factors 
have  widely  different  values  at  different  periods  of  evolution,  and  in  different 
fields  of  investigation. 

There  is  no  master  key  to  evolution.  It  does  not  always  move  in  the  same 
way;  not  always  by  continuous,  nor  always  by  discontinuous  variations;  it  is  not 
always  direct,  orthogenic,  determinate;  not  always  indeterminate;  heredity  is  not 
always  a  controlling  factor,  nor  does  it  always  control  in  the  same  way;  neither 
does  the  external  environment,  nor  use,  nor  disuse,  nor  natural  selection.  All 
are  real  factors  and  all  have  doubtless  played  some  part  in  the  grand  total  of 
results,  but  each  has  a  different  value,  more  here,  less  there,  and  these  values 
have  changed  with  the  progress  of  evolution.  They  are  as  varied  as  life  itself. 

FIG.  307,  308. — Diagrams  illustrating  the  principal  stages  in  the  evolution  of  segmented  animals  (syncephalata) . 
They  illustrate:  a.  The  spasmodic  increase  in  the  number  of  metameres,  the  advent  of  each  new  group  (tagma) 
marking  a  distinctly  higher  level  in  evolution,  of  class,  sub-class,  or  divisional  value,  b.  The  approximate  historic 
period  at  which  new  functions  and  organs,  demanded  by  the  new  internal  conditions,  make  their  appearance;  e.g., 
circulatory,  respiratory,  locomotor.  c.  The  initial  location  of  the  most  important  functions  and  organs,  d.  The  most 
important  changes  in  the  location  of  functional  centers,  due  to  the  transfer  of  organs  to  other  regions,  or  to  their 
degeneration  or  atrophy,  and  the  appearance  of  new  organs  elsewhere  to  take  their  place.  The  substitution  of 
new  organs  and  functional  centers  for  the  old  is  apparently  always  in  a  haemad,  or  caudad,  direction,  never  cepha- 
lad,  or  neurad.  The  most  striking  change  in  the  location  of  old  organs  is  the  transfer  of  the  appendages  and  as- 
sociated parts  (visceral  arches,  nerves,  muscles,  and  ganglia  in  an  anterior  haemal  direction,  the  process  beginning 
with  the  oral  or  anterior  thoracic  arches  of  primitive  crustacea  and  attaining  completion  in  the  mammals,  with 
the  transfer  of  all  the  branchial  arches  to  the  anterior  haemal  surface  of  the  head.  One  of  the  principal  causes 
of  this  change  in  the  position  of  organs  is  the  atrophy  of  all  the  organs  on  the  corresponding  haemal  surface  of 
the  head.  The  most  striking  illustrations  of  local  atrophy,  and  the  formation  in  the  younger,  more  posterior 

metameres,  of  new  organs  or  parts  of  organs  serving  the  same  purpose  are  shown  by  the  locomotor,  L,  sexual, 

excretory,  X,  digestive,  D,  and  circulatory  organs,  C.  e.  The  substitution  of  one  organ  for  another.  The  most 
striking  illustration  is  the  closing  of  the  old  mouth  and  stomodaeum,  and  the  formation  of  a  new  opening  into  the 
mesenteron  in  the  region  of  the  "dorsal  organ,"  i.e.,  on  the  haemal  surface  of  the  procephalic  and  anterior  dia- 
cephalic  region.  Other  examples  are  the  substitution  of  lungs  for  gills;  and  of  local  expansions  of  the  lateral  or 
pleural  folds  of  the  trunk,  that  serve  as  balancing,  supporting,  and  locomotor  organs,  in  place  of  the  cephalic 
appendages.  /  The  permanency  and  very  great  antiquity  of  the  more  anterior  cephalic  organs  is  strikingly  shown 
by  the  procephalic  structures,  such  as  the  median  and  lateral  eyes,  olfactory  organs  and  their  ganglia,  the  primi- 
tive "hemispheres,"  cerebellum  and  stomodaeum.  g.  The  most  striking  innovation  is  the  perforation  of  the  walls 
separating  gill  sacs  and  enteric  diverticula.  h.  The  rise  and  decline  of  metamerism.  Metamerism  is  never  complete 
or  perfect  at  any  phylogenetic  or  embryonic  period,  or  in  any  region.  It  attains  its  highest  expression  in  the  mid- 
body  region  of  the  higher  arachnids  and  is  but  very  incompletely  expressed  in  the  anterior  cephalic  and 
caudal  regions.  The  decline  of  metamerism  begins  in  the  higher  arachnids,  ostracoderms,  and  primitive  verte- 
brates, and  makes  its  appearance  first,  and  in  the  most  marked  degree,  in  the  oldest  or  most  cephalic  regions,  and 
more  on  the  haemal  than  the  neural  side.  That  is,  it  follows  the  primary  axes  of  growth  and  structural  differ- 
entiation, i.  Result.  Each  new  local  growth,  atrophy,  transfer,  substitution,  or  innovation,  of  parts  is  interlocked 
with  all  the  others,  inevitably  creating  new  conditions  pregnant  with  new  structures  and  activities.  The  process  is 
most  conspicuously  manifest  in  the  gradual  creation  of  the  mammalian  "head"  and  "body,"  with  the  old  struc- 
tural units  in  a  totally  new  and  different  organic  relation  from  the  initial  one.  The  final,  permanent  relation  of 
part  to  part,  and  of  the  new  to  the  old,  is  a  logical,  inherently  necessary  one,  the  most  important  internal  factors 
in  bringing  it  about  being  priority  of  origin,  the  imperative  demands  for  intercommunication  and  distribution  of 
products,  mechanical  balance,  coherency,  and  stability. 

The  following  capital  letters  signify  the  location  of  the  principal  functions:    C,  Cardiac,   or  circulatory; 


D,  digestive;  G,  gustatory;  L,  locomotor;  M,  masticatory;  R,  respiratory;  X,  excretory;   0  ,  sexual.     Other  letters 

as   before:     A,  nauplius  stage;  B,  ostracode;  C,  cladoceran;  D,  merostome;  E,  transitional;  F,  larval  fish;  G,  am- 
phibian; H,  mammalian. 


THE    VARIOUS   ASPECTS    OF    EVOLUTION.  469 

In  the  creation  of  the  great  phyla  of  the  animal  kingdom,  natural  selection, 
external  environment,  heredity,  use  and  disuse,  have  played  an  insignificant, 
subordinate  part.  They  may  check,  or  stimulate,  or  eliminate  in  a  quantitative 
manner,  but  they  are  not  primarily  creators  of  structure  or  of  organization. 

The  familiar  "Deus  ex  machina"  of  heredity  and  natural  selection,  may  be 
summoned  to  account  for  the  absence  of  organs  that  ought  to  be  present,  or  to 
account  for  the  elimination  of  mechanisms  that  will  not  work,  but  they  are  power- 
less to  explain  the  method  by  which  a  given  change  in  one  organ  or  organism 
creates  a  change  in  some  other  organ  or  organism. 

The  creative  power  of  internal  environment  is  always  present,  always  active, 
always  changing.  But  the  basic  chemical  elements  of  life  are  the  same  as  they 
always  have  been;  and  the  cosmic,  inorganic  environment  has  changed  but  little 
since  the  dawn  of  organic  evolution.  It  is  the  internal,  organic  environments, 
and  the  social  and  communistic  environments  that  have  gained  in  power  with  the 
progress  of  evolution,  and  that  are  the  most  important  expressions  of  it. 

The  foundations  of  organic  structure  are  therefore  laid  down  and  locked 
up  within  the  narrow  bounds  of  internal  environment,  forming  a  self  contained 
system  that  grows  and  creates  from  within,  and  which  is  essentially  unmodified 
by  changes  in  the  external  environment,  except  those  that  are  absolutely  pro- 
hibitive of  all  life. 

Social  and  communistic  environment  are  later,  secondary  products,  that 
lend  effectiveness  to  selection  in  proportion  to  their  own  evolution;  that  is,  accord- 
ing to  the  degree  to  which  the  life  at  large  of  one  group  of  organisms  is  interwoven 
or  interlocked  with  that  of  others. 

Creative  evolution  is  the  progressive  interlocking  of  one  activity  with  another, 
to  a  common  end,  each  moulding  the  other,  and  all  moved  by  the  mainspring  of 
a  common  external  medium.  It  is  expressed  in  a  perpetual  flow  of  newly  created 
organisms,  and  the  form  and  action  of  each  one,  and  of  its  constituent  parts,  can 
alone  indicate  the  nature  of  the  forces  that  have  created  them. 

Hence  comparative  morphology  and  phylogeny  must  always  constitute  the 
fountain  head  whence  comes  our  knowledge  of  creative  evolution.  Such  prob- 
lems as  the  phylogeny  of  vertebrates  are  therefore  the  most  important  ones  the 
biologist  has  to  deal  with,  for  on  their  solution  depends  our  conception  of  the  way 
in  which  evolution  actually  has  taken  place. 

Comparative  morphology  has  no  value  except  in  so  far  as  it  points  out  the 
historic  sequence  of  organic  forms  and  functions,  and  reveals  to  us  the  trend  of 
evolution  and  the  causes  that  direct  and  control  it.  In  that  morphology  stands 
supreme,  for  the  thin  red  trail  that  marks  the  orbit  of  evolution  is  the  only  index 
we  have  of  life  as  it  was  and  shall  be.  If  our  reconstruction  of  phylogenetic 
lines  is  hopelessly  wrong,  then  indeed  will  ontogeny  be  a  false  guide,  and  we  shall 
be  left  the  hopeless  task  of  reconstructing  the  life  history  of  eons  from  the  contents 
of  a  test  tube,  or  the  products  of  the  breeding  pen;  or  tricked  into  the  hope  of 


47° 


SUMMARY   AND    CONCLUSION. 


stemming  the  tide  of  evolution  with  a  scalpel,  a  rule  of  conduct,  or  a  dietetic 
formula. 

It  is  not  surprising  that  the  large  expectations  of  the  outdoor  naturalist,  the 
cytologist,  the  experimental  evolutionist,  and  the  animal  breeder  have  not  been 
realized. 

The  cytologist  is  too  intent  on  the  raw  materials  of  life;  his  field  of  operation 
is  both  too  remote  and  too  narrow  to  give  either  measurable  detail  or  perspective. 
To  discover  the  immediate  causes  of  any  given  stage  in  the  evolution  of  the  nervous 
system,  or  of  the  endocranium,  by  a  study  of  chromosomes,  or  of  protoplasm, 
or  by  juggling  with  imaginary  hereditary  units  is  as  hopeless  a  task  as  it  would 
be  for  the  geologist  to  explain  the  delta  of  the  Ganges  by  an  appeal  to  the  co«i- 
position  of  cosmic  matter. 

The  naturalist  is  bewildered  by  the  amazing  detail  of  the  finished  product, 
and  so  much  absorbed  in  the  social  organization  of  the  present  moment,  or  in  the 
relation  of  one  plant,  or  animal  to  the  other,  and  to  the  environment  at  large,  that 
he  fails  to  acquire  an  adequate  historic  perspective. 

The  experimental  evolutionist,  for  a  few  hours,  or  months,  arbitrarily  nar- 
rows the  environment  of  an  organism,  and  measures  the  results,  if  any,  with 
instruments  of  precision,  or  with  the  aid  of  the  higher  mathematics;  but  he  gener- 
ally ignores,  or  looks  with  contempt,  on  the  vast  experiments  already  performed 
for  him,  where  the  laboratory  is  nature,  and  the  results  are  expressed  in  species, 
genera,  and  classes. 

The  comparative  morphologist  aims,  not  merely  to  trace  the  identity  of 
changing  structures  under  the  disguise  of  new  forms,  but  to  measure  the  rate  of 
these  changes,  and  to  seek  out  the  underlying  causes  that  have  brought  them 
about.  He  is  heavily  handicapped  by  the  lack  of  materials  that  can  be  precisely 
measured  or  controlled.  But  on  the  other  hand  there  is  a  certain  advantage 

FIG.  309. — Diagram  illustrating  the  phylogeny  of  the  principal  subdivisions  of  the  animal  kingdom.  The 
numbers  indicate  approximately  the  periods  at  which  some  of  the  more  important  events  in  the  evolution  of 
structures  and  functions  have  taken  place. 

51,  Fixation  of  balanced  internal  temperature;  50,  decline  of  yolk  volume,  and  perfection  of  uterine  gestation; 
49,  maximum  size  of  yolk  sphere;  48,  decline  of  exoskeleton  and  notochord  and  perfection  of  endoskeleton;  47; 
evolution  of  larynx,  cochlea,  ear-bones;  46,  decline  of  gills,  closure  of  gill  clefts;  45,  pectoral  and  pelvic  appendages, 
supporting  and  digitate;  44,  lungs;  heart,  three-chambered;  43,  second  migration  to  land ;  42,  elongation,  and  joint- 
ing of  pectoral  fins;  41,  air  bladder;  40,  median  fusion  of  paired  jaws;  fixation  of  maxilte;  39,  decline  of  meta- 
merism; 38,  decline  of  cephalic  appendages  and  rise  of  lateral  folds  and  paired  fins;  37,  merging  of  endo-  and  exo- 
skeletal  elements,  and  calcification  of  endo-skeleton ;  36,  vertebral  rings;  heart  two-chambered;  360,  decline  of 
predatory  life;  35,  gill  sacs  unite  with,  and  perforate  the  gut;  34,  leg- jaws;  three  pairs  transferred  to  the  haemal 
side;  33,  neuron  continuous;  tubular;  infolding  of  the  lateral  eyes;  32,  closure  of  neurostoma;  rise  of  haemos- 
toma;  31,  increased  size  of  forebrain  and  cephalic  sense  organs;  30,  increase  of  endo-skeleton,  endocranium,  noto- 
chord and  gill  cartilages;  29,  new  generation  of  caudal  metameres;  new  trunk  and  tail  flexible  laterally;  28, 
postanal  concrescence  of  the  germ  wall;  27,  increase  in  size  of  eggs  and  in  the  volume  of  yolk;  26,  exoskeleton 
thickened,  trabeculate,  cellular,  fragmented;  25,  notable  increase  in  size  and  improvement  in  locomotor  and 
predatory  organs;  carnivorous;  24,  first  invasion  of  land;  23,  respiratory  appendages,  forming  infolded  sacs; 
22,  parietal  eye  enclosed  in  forebrain  vesicle;  21,  endocranium;  20,  leg-jaws,  multiple,  paired,  neural;  19,  tagmatism, 
and  linear  distribution  of  functign;  18,  metamerism  perfected;  number  small;  17,  heart;  internal  circulation; 
16,  rise  of  excretory  organs;  15,  body  plasma;  separation  of  internal  and  external  media;  14,  exoskeleton,  water 
proof,  chitenoid,  non-cellular;  13,  coslom,  telocoel;  12,  metamerism  perfected;  3  to  7  in  number;  n,  pursuit  and  cap- 
ture; 10,  appendages  become  locomotor,  grasping,  respiratory;  9,  cephalic  sense  organs,  median  and  lateral  eyes 
and  olfactory  organs;  8,  bilateral  symmetry;  7,  gastrulation ;  6,  radiate  symmetry;  5,  neuron  circumoral;  4,  enteron 
opening  directly  to  exterior;  3,  early  evolution  of  tissues;  2,  evolution  of  cells;  i,  evolution  of  protoplasm. 


PHYLOGENETIC   TREE    OF   THE   ANIMAL    KINGDOM. 

v.  -V 


471 


Mammalia. 
V :     • .    / / 


Telocoel'/! 

Naupulj 
a,Rotirteta 


2rrotoz.oa 


1 

FIG.    309. 


472  SUMMARY   AND    CONCLUSION. 

inherent  in  the  very  size  and  remoteness  of  his  problems,  that  is  absent  in  the 
brief  laboratory  experiments  that  have  taken  place  under  the  eye  of  man.  His 
problems  must  be  viewed  from  a  great  distance,  but  one  that  gives  a  large  per- 
spective, and  draws  a  vast  range  of  structural  changes  into  a  single  horizon  where 
sporadic  details  disappear,  and  only  those  events  catch  the  eye  that  are  massed 
around  some  central  cause  or  are  ranged  with  monotonous  regularity  along  some 
common  line  of  physiological  upheaval. 


In  so  far  as  we  may  judge  from  the  teachings  of  morphology,  the  perfection 
of  physical  organization  is  reached  in  man.  The  structural  differences  between 
him  and  the  higher  mammals  are  insignificant;  but  the  potential  differences 
created  by  his  upright  gait,  his  use  of  the  hand  as  a  tool,  his  power  to  make 
his  experience  the  property  of  another,  to  look  into  the  past  at  what  occurred 
before  his  existence,  and  to  predict  what  will  occur  when  he  has  ceased  to  exist, 
are  faculties  of  great  creative  power,  which  in  a  true  biological  sense  constitute 
the  supreme  crisis  of  organic  evolution,  and  mark  the  advent  of  man  as  the 
beginning  of  a  new  class  of  animals. 

Organic  evolution  is  progressive  differential  growth,  in  more  intricate,  ever 
narrowing  environments.  It  is  primarily  concerned  with  obtaining,  preparing, 
and  distributing  the  raw  materials  of  life;  in  perfecting  the  coordinate  action  of 
the  great  bodily  functions;  and  in  establishing  more  economic  biologic  and  cosmic 
relations.  In  man,  growth  and  the  internal  organic  adjustments  to  the  above 
indicated  ends  have  reached  their  optimum  level,  and  progress  in  that  direction 
has  practically  ceased.  Further  progress  must  be  in  the  extension  of  man's  con- 
tact with,  and  control  over  nature  by  the  creation  of  instruments  of  power  and 
precision;  in  the  development  of  his  intellectual,  artistic  and  sympathetic  facul- 
ties; in  perfecting  the  social  organization  of  humanity,  and  in  the  creation  of  social 
consciousness. 


EXPLANATION  OF  THE  LETTERING. 

a .Anus;  bony  axial  rod  supporting  the  cephalic  appendages. 

ab Abdomen. 

ab.a Abdominal  or  branchial  appendages. 

ab.cl Ccelomic  chambers  of  the  abdomen. 

a.bm Abductor  branchial  muscle. 

ab.t Abdominal  temperature  organs. 

a.c Anterior  cornua;  atrial  or  peribranchial  chamber. 

a. card Anterior  cardinals. 

a.d Anterior  dorsal  plate. 

a.dl Anterior  dorso  lateral  plate. 

a. dp Row  of  teeth  on  anterior  margin  of  mandible. 

a.f Ant-orbital  foramen. 

ag Argentea. 

a.h.co Anterior  haemal  commissure. 

a.h.p Anterior  haemal  process. 

a.l Anal  lobe. 

a.l.m. Alary  muscle. 

a.m Anterior  marginal  line  of  canal  organs. 

an Anus. 

an.c Anterior  commissure. 

an.n.co Anterior  neural  commissure. 

an.p Anterior  neuropore. 

an.pl Anal  plate. 

a.n.s Anterior  neural  spine. 

ao Aorta. 

ao.ar Aortic  arch. 

ap Apical  plate;  appendages. 

a.p Anal  plate. 

a.pl Apical  plate. 

a.p.r Anterior  bony  entapophysis. 

a.s Anterior  sclerotic  plate. 

at Atrium;  antennae. 

at.c Atrial  chamber;  vestibule. 

at.d Excretory  duct  to  antennal  segment. 

at.r Anterior  triangular  recess. 

au.o Auditory  organ. 

a.v Anterior  median  ventral. 

a.v.l Anterior  ventro-lateral  plate. 

ba Basilar  plate  to  the  cephalic  appendages 

b.c Blood  cells. 

bd Bud. 

bl Balancers. 

bl.c Blood  corpuscles. 

b.nv Belly  navel. 

b.o Branchial  opening. 

473 


474 


EXPLANATION  OF  THE  LETTERING. 


b.pl 

br  ....... 

bran  .......... 

br.c  ........... 

br.ct..  ........ 

b.t.... 

b.t.hm  or  b.t.m 
b.th.n  ......... 

b.v  ............ 

c1"3  ........... 

c  ........  ...... 

c.ap  .....  ...... 

cap.b  .......... 

card.g.or  cd.g.  . 
card.m.  .  ...... 

card.s  ......... 

c.c.  ...  ........ 

c.ca  ........... 

c.d..   ......... 

c.cl  .......... 

c.cn  ........... 

c.co  .....  ...... 

cer  ............ 

cerbl  ____  ...... 

cer.p  ____  ...... 

c.f  ............ 

c-g  ........ 

chila  ........... 

ch.g  ............ 

ch.H.c  ......... 

ch.H.tr  ........ 

ch.l  ...........  . 

chl.b  ........... 

chlic  ........... 

ch.pl  ........... 

ch.t  ............ 

cl  .............  . 

c.l  ............. 

c.ms  ........... 

c.n  ............. 

c.nar  or  c.nv.  .  . 
cn.c  ............ 

cnl  ............. 

co  .............. 

co.c  ............ 

co.f  ............ 

co.n  ............ 

cort  ............ 

cox  ............ 


cr. 


Basilar  plate. 

Brain;  brain  chamber;  procephalic  lobes. 

Branchiae. 

Branchiocephalon. 

Branchial  cartilage. 

Branchial  temperature  organs. 

Branchio thoracic  or  hypobranchial  muscle. 

Branchio  thoracic  nerve. 

Blood-vessel. 

Lines  of  canal  organs. 

Cancellae;  corneal  membrane;  ccelom. 

Cephalic  appendages. 

Capsuligenous  bar  of  the  endocranium;  chilarial  bar. 

Cardiac  ganglion. 

Cardiomeres. 

Cardinal  sinus. 

Canalis  centralis;  colar  ccelom. 

Conical  pulp  cavities. 

Cephalic  disc.     Cephalic  duct. 

Procephalic  ccelom. 

Collar  nerve. 

Cephalic  caecum. 

Cerebrum. 

Cerebellum. 

Cerebral  peduncles 

Cranial  floor. 

.    .Corneagen  layer;  cephalic  ganglia;  caudal  neuromeres. 
......  Chilaria. 

Cheliceral  ganglion. 

......  Chelicero-hemisphere  cells. 

Chelicero-hemisphere  tracts. 

Cheliceral  lobe. 

Chilarial  cartilage  bar  of  endocranium. 

Chelicerae. 

....  Chilarial  pleurite. 
.  .  .  Chitenous  tubule. 

Cloaca. 

Cancellous  layer. 

Cephalic  mesoderm. 

.Procephalic  neuromeres;  cancellae. 
-Cephalic  navel;  dorsal  organ;  haemostoma. 

Neural  canal;  neuroccele. 

Canaliculi. 

Commissure. 

.  .Cerebral  commissure  cells;  corona  ciliata. 
.  .  Cerebral  commissural  fibers. 
.  .  Nerve  to  corona  ciliata. 
.  .  Cerebral  cortex. 

Coxal  spurs. 

....  Collar  pore. 

.  .  Branchial  cartilage  of  embryo 


EXPLANATION    OF    THE    LETTERING.  475 

c.r Root-like  cephalic  outgrowths. 

cran Cranium. 

cr.g Cranial  ganglia. 

cu.d * Cuvierian  duct. 

cut Cuticular  layer. 

c.w Circle  of  Willis. 

cx.g Coxal  glands. 

cx.t Coxo-tergal  muscles. 

d.  and  d1 Dorsal  plate;  dentine,  or  dentine-like  chiten. 

d.bl Dorso-branchial  line  of  canal  organs. 

d.c Sensory  and  glandular  ducts. 

d.end;  or  d.e Ductus  endolymphaticus. 

d.f Dorsal  fins. 

d.fd Dorsal  portion  of  anal  frill. 

di.c Dicephalon. 

di.enc Diencephalon. 

d.o Dorsal  organ;  dorsal  opening  in  cephalic  shield;  cephalic 

navel. 

dor.o Dorsal  organ. . 

d.s Dorsal  sclerotic  plate. 

dt.  and  dt1 .Denticles  or  denticle-like  spines. 

e.  and  e1 Mesethmoid;  enamel  layer  or  enamel-like  chiten;  ocelli. 

e.b.m. External  branchial  muscle. 

ec Ectoderm. 

ec.d Excretory  duct. 

ec.pa.e Ecto-parietal  eye. 

el External  labials  (Drepanaspis) ;  extra-laterals;  suborbitals. 

en1 Primitive  gastrula;  cephalic  endoderm. 

en . Endoderm  of  trunk. 

en.ch Endochondrites. 

encl Endoccele. 

en. ex Entocoxal  nerves. 

en.me.cl Endomesoccele. 

en.pa.e Endoparietal  eye. 

ent Entapophyses. 

ep.  or  eph Epiphysis;  parietal  eye  tube. 

e.t. Parietal  eye  tube. 

ex.p . .  r. . ," . . .  .External  pores. 

ext.g External  gills. 

f.br Forebrain. 

flab,  or  fl     Flabellum. 

f.p Fringing  plates. 

f.r Free  nerve  endings;  frontals;  fleshy  portion  of  the  rostrum 

g Gills;  gastrula;  germ  cells. 

g.ar Gill  arches. 

gast Gastrula 

g.c Germ  cell. 

g.ctr.  or  g.c Gustatory  center;  gustatory  tracts. 

g.c.tr General  cutaneous  tracts. 

g.cut General  cutaneous  nerves. 

g.g Gustatory  ganglia. 


476  EXPLANATION  OF  THE  LETTERING. 

g.h Cephalic  gastro-hepatic  gland. 

g.i Intraganglionic  infolding  of  the  middle  cord. 

g.iv Invagination  for  optic  ganglia. 

gm.c Germ  cells. 

g.n.c Gustatory  nerve  roots. 

g.o Gustatory  organs;  genital  organs. 

g.p Gular  plates. 

gr Groove  between  olfactory  organs. 

gt.p Gut  pouch. 

g.tr Gustatory  tracts. 

gust.n Gustatory  nerve. 

gust.o Gustatory  organ. 

g.w.  . Germ  wall. 

h Hypodermis. 

h1"2 Bony  plates  covering  the  hyoid  arches. 

H.as Hemisphere  association  cells. 

H. as.tr .  .Hemisphere  association  tracts. 

h.c Haversian  canals;  hepatic  caeca. 

he Hydroccele. 

h.co Haemal  commissures. 

h.l. Haemal  lamina  of  cephalic  shield. 

h.m Hyo-mandibular. 

h.m.n. Hyo-mandibular  line  of  canal  organs. 

h.n.c Haemal  nerve  cord. 

h.ne.n Hae mo-neural  nerve. 

h.n.m Haemo-neural  muscle. 

h.pr Haemal  processes. 

h.r1"5 Haemal  roots. 

h.ri Horn  ridges. 

h.s Tergal  plates  of  the  abdominal  half-segments. 

hst . Haemostoma. 

ht Heart. 

h.tr . Haemal  tracts. 

hy. ...  Hyoid. 

hyp Hypophysis. 

i Intestinal  nerves  (i-io). 

i.ent. Inter-entapophysial  muscle. 

i.g .  .  Inter-ganglionic  infolding  of  the  middle  cord. 

i.g.s Inter-ganglionic  spaces. 

i.Lrn. Gill  muscle. 

i.n.l Inner  neurilemma;  neuroglia. 

int Intestinal  nerve. 

i^og .Post-oral  or  mesocephalic  neuromeres. 

i.o.l . Infra-orbital  line  of  canal  organs. 

i.p. .  Internal  pores. 

i.pl Intestinal  plexus. 

i.r . . . .  .  Transverse  ridge-plate  on  inner  surface  of  the  premaxillae 

i.sh .  .  Inner  neurilemma  sheath. 

i-V.  .  .Infoldings  leading  to  neural  canal. 

iv.l.e.g .Invagination  for  lateral  eye-ganglion. 

iv.ol.l Invagination  for  olfactory  lobes. 


EXPLANATION    OF    THE    LETTERING.  477 

iv.op.g Invagination  for  optic  ganglion. 

1 Lacunae;  lateral  plate;  labrum. 

l.ab.m Longitudinal  abdominal  muscle. 

l.ab.n. . Longitudinal  abdominal  nerve. 

la.f Lateral  fold. 

l.ch Lemmatochord. 

l.c.n Lateral  cardiac  nerve. 

l.co Longitudinal  connectives. 

l.cord Lateral  cord. 

l.e Lateral  ethmoids;  lateral  eye. 

l.e.n Lateral  eye  nerve. 

Ley Lateral  eye. 

l.ey.g Lateral  eye  ganglion. 

l.ey.n .Lateral  eye  nerve. 

l.f! Lateral  fold. 

lg. .    .  .  Lung. 

Ig.b.  or  l.b Lung  book. 

l.h.tr Longitudinal  haemal  tract. 

1.1 Lateral  line  of  canal  organs. 

l.lo Lateral  lobes  to  the  olfactory  recess. 

l.l.ch Lateral  bands  of  the  lemmatochord. 

1m Lamellae. 

l.m.pl. Nerve  plexus  of  longitudinal  abdominal  muscle. 

l.n Lateral  nerve. 

Ins Lens. 

l.n.tr Longitudinal  neural  tract. 

l.p. Lateral   process;   lateral  subdermal  process  to  the  mandi- 

bular  plates. 

l.pl Small  bony  plates  in  the  fleshy  sides  of  rostrum. 

l.pl.t Lateral  plastro-tergal  muscle. 

l.tr Lateral  tracts  of  brain. 

m Mouth;  muscles. 

man Mandibles. 

max Maxillae. 

m.br . Midbrain. 

m.c Merochord;  segment  of  the  lemmatochord;  marginal  cells. 

m.cd.n Median  cardiac  nerves. 

m.ch Middle  chord;  median  nerve;  notochord. 

m.cl Mesenccele. 

md Mandibles. 

m.d Mullerian  duct. 

mes.c Mesocephalon. 

mesen.c Mesencephalon. 

met.c Metacephalon. 

meten.c Metencephalon. 

m.f Median  furrow. 

m.l Median  line;  marginal  line  of  canal  organs;  mantle  layer 

m.lch V Median  portion  of  the  lemmatochord. 

m.lo ,  * . . .  .  Maxillary  lobes. 

m.m Muscle  markings. 

m.n Median  nerve  (mouth). 


478  EXPLANATION    OF    THE    LETTERING. 

m.n.co Median  neural  commissure. 

m.oc Median  ocellus. 

mp JVJalpighian  tubules;  madreporite. 

m.pl Medullary  plate. 

m.r Marginal  wall  or  ridge. 

ms. Mesoderm. 

msc Mesenchyme. 

ms.en Mesentoderm. 

m.s.p.ent Mesoplastro-entapophyseal  muscle. 

m.t Motor  nerve  tubes. 

mt Mantle. 

mx Maxillae. 

mx.d Excretory  duct  to  maxillary  segment. 

mxl Maxillaria. 

n.  or  n.c Nerve  cord;  neuron;  neuromere. 

n.ch.c Neurochordal  canal. 

n.co Neural  commissure. 

n.cr Neural  crests. 

neph Nephridia. 

neu.p Neuropore. 

n.l Neural  lamina  of  cephalic  shield. 

n.n Neural  nerve. 

noto Notochord. 

n.p Neuropile;  neural  process;  neural  plate;  neuropore. 

np.d .Nephric  duct. 

n.r Nerve  ring. 

n.st Primitive  mouth;  neurostoma 

n.tr Neural  tracts. 

n.v Neural  vessel. 

o.b Opening  to  branchial  chamber. 

oc Ocelli;  occipital  plate. 

o.c Occipital  line  of  canal  organs. 

oc.f Occipital  foramen 

od Oviduct. 

oe (Esophagus. 

ol Primitive  olfactory  organ;  frontal  organ. 

ol.l Olfactory  lobes. 

ol.l.n Lateral  nerve  of  olfactory  organ. 

ol.m.n Median  nerve  of  olfactory  organ. 

ol.np Olfactory  neuropile. 

ol.o Olfactory  organ. 

ol.r  .  . Olfactory  recess. 

ol.v Ventricle  of  olfactory  lobes. 

o.m Circumoral  membrane. 

op Operculum. 

o.p Postorbital  valley. 

op.g    Optic  ganglion. 

op.neu  Optic  neurones. 

op.pl Optic  plate;  opercular  pleurite. 

op.s Opercular  segment. 

op.tr Optic  tracts. 


EXPLANATION  OF  THE  LETTERING.  479 

op.tr.h Optic  tract  to  hemispheres. 

o.r Lateral  eye  orbit. 

os Cardiac  ostia. 

o.sh Outer  sheath. 

ov Ovaries. 

p External  opening  of  parietal  eye  tube;  epiphysis;  anterior 

neuropore;    opening    to    endolymphatic    ducts;     penis 

thoracic  appendages. 

pa Parietals. 

p.ap Pectoral  appendage. 

pa.ey Parietal  eye. 

pa.ey.g Parietal  eye  ganglion. 

pa.ey.n1"2 Roots  to  parietal  eye  nerves. 

p.b Posterior  branchial  line  of  canal  organs. 

p.b.c. Peribranchial  chamber. 

p.c Paccinian-like  corpuscles. 

p.c Pore  canals;  proboscis  ccelom. 

pc.a Pectoral  arch. 

p.ca Pulp  cavity  of  denticle. 

p. card Posterior  cardinals. 

p.ce Prosencephalon. 

p.d Pericardium. 

p.dl. Posterior  dorsolateral  plate. 

p.d.p Posterior  mandibular  dental  plate. 

p.e Parietal  eye. 

p.e.n Parietal  eye  nerve. 

pec Pectines. 

ped.g Pedal  ganglion. 

p.e.p Pits  on  inner  surface  of  postorbital  plate;  posterior  parietal 

eye  pits. 

per.c Pericardial  nerve. 

p.e.t Parietal  eye  tubercle. 

p.g Polar  globules. 

ph Pharynx. 

p.h.co Posthaemal  commissures. 

pit.f Pituitary  foramen;  passage-way  for  primitive  stomodaeum. 

pl.ap. Pelvic  appendage. 

pi. ex Plastro-coxal  muscles. 

pl.ent. . Plastro-entapophyseal  muscle. 

pl.f Pleuralfold. 

pl.t. Plastro-tergal  muscle. 

p.m Posterior  marginal. 

p.m.v. Posterior  median- ventral  plate. 

p.n. Pedal  nerve. 

p.n.co Posterior  neural  commissure. 

p.np Posterior  neuropore. 

p.o.c Preoral  chamber. 

p.p .Posterior  process;  parietal  eye  plate;  proboscis  pore. 

pr Prepuce;  cephalic  mantle. 

pr.mx.  or  p.mx Premaxillae. 

pr.n  or  pro • Pronephros. 


480  EXPLANATION    OF    THE    LETTERING. 

proc Procephalon. 

prosen Prosencephalon. 

pr.s Primitive  sheath. 

prt Protoplasm. 

ps Posterior  scl erotics;  pancreas. 

p.s.o Primitive  sense  organs. 

pt Pituitary  gland  or  duct. 

p.t Primary  tentacles. 

p.t.r Posterior  triangular  recess. 

p.v Posterior   median    ventral  plate;    postorbital  line  of  canal 

organs. 

pv.l Pulsatile  vessel. 

p.v.t Mass  of  bony  trabeculse  below  the  postorbital  valley. 

r Ridges;  roots  to  spinal  nerves;  rostrum. 

r.d Visual  rods. 

r.l Rostral  line  ot  canal  organs. 

rost Rostrum. 

r.s Shelf  plate  on  the  inner  surface  of  the  rostrum. 

s Scales;  sulcus  on  the  floor  of  the  procephalic  lobes;  stomach. 

s.c. . . Superficial  canals;  sensory  canals. 

s.c.n Segmental  cardiac  nerve. 

s.c.no Subcortical  neuropile. 

s.d Sucking  disc. 

s.g Shell  gland. 

s.l Superficial  layer. 

s.l.c Cells  derived  from  sheath  of  lateral  cords. 

s.lch Sheath  to  lemma tochord. 

s.m.n Sheath  to  median  nerve. 

sn.c Sensory  canals. 

s.o Suborbital  plates;  sense  organs. 

s.o.l Supra-orbital  line  of  canal  organs. 

sp Spindle-shaped  dilatation  on  the  sensory  tubes;  subdermal 

process  on  lateral  margin  of  premaxillae. 
st. .  .  .Stomach;  stomodaeum. 

s.t.  .......  Sensory  tubes. 

st.c.  or  St. CO Stomodaeal  commissure. 

st.g. . Stomodaeal  ganglion. 

st.n Stomodaeal  nerves. 

sto. .  ...  .Stomodaeum;  stolon. 

t Terminal  joint  to  cephalic  appendages;  testis. 

t.b. .  Tongue  bar. 

t.cl Teloccele. 

t.cx. .  .Tergo-coxal  muscles. 

t.d. .  .Tear  duct;  thoracic  duct. 

telbl..  .Teloblasts. 

telen . .  .  Telencephalon. 

tel.o..  Telopore. 

t.g. .  .  .  Groove  (for  tentacle?). 

tg-prpl  -  Tergo-proplastral  muscles. 

th .Thyroid. 

th.ap  ...  Thoracic  appendages. 


EXPLANATION    OF    THE    LETTERING.  481 

th.n Thoracic  nerve  cord. 

th.so Mesoblastic  somites  of  thoracic  metameres. 

t.l Remnants  of  ingested  larval  tentacles. 

t.l.n Thoracic  temperature  organs. 

t.p Telopore. 

tr Trabeculae 

tr.c Trabeculae  cranii. 

t.st Tendinous  stigmata. 

u Point  of  union  of  vascular  canals. 

v Ventricle;  subneural  gland;  vestibule. 

v.c Vascular  canals. 

v.d Vas  deferens. 

v.dec Vagus  decussation. 

v.fd Ventral  wall  of  anal  frill. 

vg. Vagus  (yellow  body). 

vg.ap Vagus  appendages. 

vg.n Vagus  nerves. 

vg.nm Vagus  neuromeres. 

v.l Vascular  layer. 

v.m Ventral  margin. 

w.d Wolffian  duct. 

yk.c.h Yolk  cells  of  head. 

yk.c.t. Yolk  cells  of  tail. 

yk.nav Yolk  navel. 


INDEX. 


Acraniates,  393,  396,  405 

appendages,  398 

circulation,  401 

degeneration,  399 

development,  401 

heart,  401 

mantle,  400 

metamerism,  398 

nervous  system,  399 

sexual  organs,  401 

skeleton,  400 
Amphibia,  386 
Amphioxus,  393 
Anal  frill,  371 
Anaspida,  364 
Annelids,  403 
An ti arena,  367 
Appendages,  invaginated,  275 

loco  mo  tor,  249,  268 

respiratory,  264 

sequence  of  functions  of,  271 
Archenteron,  248 
Arches,  neural,  307 
Arthro-dira,  386,  388 
Aspidocephali,  358 
Asymmetry,  as  creative  factor,  458 
Ateleaspidae,  363 
Atrial,  folds,  31 

frill,  37 1 
Auditory  organ,  120 

Pit,  39 

Balancers,  261 
Bdellostoma,  oral  lobes,  260 
Birkeniidae,  364 
Blastopore,  243-404 
Bothroidal  cord,  328 
Bothriolepis  canadensis,  367 

atrial  frill,  371 

cephalic  appendages,  376 

exoskeleton,  367 

eyes,  375 

food,  379 

gills,  37 1 

hyoid  arches,  375 

locomotion,  379 

mandibles,  374 

mode  of  preservation,  377 

mouth,  375 

olfactory  organs,  375 

premaxillae,  373 

preoral  chamber,  373 

sensory  grooves,  376 

viscera,  372 
Brain,  function  of,  174 

meaning  of  term,  41 

minute  structure  of,  71 
Branchial  cartilages,  306 


Branchial  neuromeres,  67,  71 

warts,  115 

Branchiencephalon,  67,  69 
Branchiocephalon,  20 
Brachiopods,  445 
Branchipus,  16 
Bunodes,  29 

Canalis  centralis,  331 
Capsuligenous  bars,  316 
Cardiac  ganglion,  202 
Cardinal  veins,  199 
Cartilage,  branchial,  307 

neural,  306 
Caudal  navel,  243 
Cement  organ,  of  polypterus,  262 
Cephalaspidae,  358 
Cephalization,  26,  254 
Cephalic  functions,  arrangement  of,  8 
Cephalic  navel,  25,  250 
Cephalogenesis,  in  arachnids,  5 

in  arthropods,  3 
Cephalodiscus,  439 
Cerebellum,  19 
Cerebral  hemispheres,  54 
Chaetognatha,  448 
Cheliceral  lobes,  59 

neuromere,  59,  62 
Chewing  apparatus,  188 
Choroid  fissure,  151 

plexus,  19 
Circle  of  Willis,  199 
Circulation,  198 
Cirripeds,  408 
Coccosteus,  388 
Coelom,  407,  426 
Ccelomic  chamber,  23 
Ccelolepidae,  364 

Commissures,  43,  60,  72,  79,  80,  90 
Concrescence,  25,  243-404 
Cord,  minute  structure  of,  71 
Cornua,  40 
Cranial  flexure,  40 

ganglia,  17 

Craniates,  393~395>  4°3 
Creative  factor,  asymmetry  as,  458 

brain  as,  461 

chiten  as,  459 

exoskeleton  as,  459 

yolk  sphere  as,  461 
Cyclostomata,  383 

Degeneration,  277-279,  280,  281,  288 

Denticles  dermal,  303 

Dentine,  304 

Dermal  skeleton,  289 

Dicephalon,  15 

Diencephalon.  18,  57,  58 

483 


484 


INDEX. 


Dipnoi,  386 

Dorsal  organ,  18,  238 

Double  embryos,  281 

Echinoderms,  421 
larva  of,  422 
Ectoprocta,  443 
Elasmobranchii,  384 

Endocranium,  16,  20,  306,  312,  319,  436,  450 
of  limulus,  3 14 
of  scorpion,  317 
summary  and  comparison,  319 
of  Telyphonus,  319 
Endoskeleton,  306 
Enteropneusta,  431 
Entoprocta,  441 
Epibranchial  organs,  17 
Epiphysis,  133,  136 
Equilibrium,  191 

Evolution  of  bilateral  symmetry,  457 
cosmic,  454 

of  creative  environment,  454 
crices  in,  456 
of  metamerism,  457 
of  social  environment,  454 
Eye,  ectoparietal,  134 

environment  of  lateral,  462 
endoparietal,  134 
frontal,  125 
lateral,  127,  149 
ganglia,  154 
lens,  151,  153 
parietal,  125,  126,  129 
of  apus,  138 
of  branchipus,  136 
of  limulus,  131 
of  scorpion,  129 
of  vertebrates,  139 
of  ganglia,  142 
lens  of,  144 
placodes,  146 
tube,  133,  136 

Fiber  cells,  232 
Fins,  pectoral,  251 

pelvic,  251 
Fission,  longitudinal,  282 

transverse,  282 
Flabellum,  94,  113 
Flexure  cranial,  65 

forebrain,  55 

hind-brain,  69 
Fringing  processes,  269 
Frontal  eye,  125 

organs,  165 
Fusiform  cells,  447 

Ganglia,  cardiac,  202 

of  cord,  97 

cranial,  17,  81,  95 

habenulse,  142 

lateral  eye,  155 

pedal,  81,  95 

optic,  62,  154 

stomodaeal,  15,  60 
Gaskell,  34 
Gastrula,  34,  404,  433 

in  limulus,  227 
Gastrulation,  248 


Germ  disc,  224 

layers,  229 

wall>  23,  35,  228,  237 
concrescence  of,  35 
Gill  arches,  261 

external,  261 

pouches,  251 

sacs,  263 
Growth,  apical,  246 

causes  of  differential,  215 

interpretation  of  early  stages  of,  219 
Gustatory  apparatus,  186 

nerves,  84 
roots,  84 

organs,  in 

tracts,  84 
Gut  pouches,  266 

Haemal,  nerve  cord,  43' 
roots,  77,  8 r 

surface,  orientation  of,  27,  221 

tracts,  85 

Hasmostoma,  25,  40 
Head  cavities,  23 

subdivisions  of  vertebrates,  n 
Heart,  39 

beat,  207 

cardiac  ganglia,  200,  202 

cardiac  nerves,  200 

comparison  of  vertebrate  and  arachnid,  197 

development  of,  195 

experiments  on,  205 

lateral  cardiacs,  200 

location  of,  195 

pericardial  nerves  of,  200 

segmental  cardiacs,  200 
Hemisphere  association  cells,  58 

development  of,  55 
Herrick,  34 
Holocephali,  385 
Hydrocele,  426 
Hypophysis,  260 

Infundibulum,  18,  62 
Jaws,  250,  255-257 

Lateral  eye  of  vertebrates,  151 
ganglion,  154 

fold,  269 

line  organs,  121 

plates  of  mesoderm,  22 
Leg  jaws,  15,  250 
Lemmatochord,  323 

of  lepedoptera,  326 

of  limulus,  334 

of  scorpion,  329 
Lens,  parietal  eye,  144 
Lernasa,  branchiata,  408 
Limnadia  lenticularis,  409 
Lobes  olfactory,  13 

procephalic,  38 
Lobi  inferiores,  18-62 
Locomotor  appendages,  268 
Locomotion,  191 

Macrocystis,  301 
Marine  arachnids,  337 
origin  of,  338 


INDEX. 


485 


Median  fusion  and  antero-posterior  degeneration, 
277 

nerve,  43 

Merochord,  328,  330 
Merostomes,  337 
Mesencephalon,  18,  19,  65 
Mesencoele,  66 
Mesocephalon,  15 
Mesoderm,  12,  22,  426 

lateral  plates  of,  231 

sources  of,  230 
Metacephalon,  19 

Metameres,  formation  of,  in  limulus,  225 
Metamerism,  49 
Metencephalon,  67 
Middle  cord  or  lemmatochord,  12,  43,  323 

of  insects,  324 

of  limulus,  334 

of  scorpion,  328 

summary  or  comparison,  335 
Monstrosities,  274 
Mouth,  25,  26,  253,  406 

closure  of,  18,  31,  249,  251 

Nauplin,  408 

Naupula,  406,  408 

Navel,  cephalic,  238,  253,  255,  414 

Neostoma,  26,  238 

Nerves  of  branchiencephalon,  102 

branchio-thoracic,  105 

cardiac,  200 

circumoral,  42 

cutaneous,  105 

enteric,  103 

gustatory,  84 

haemo-neural,  102 

hypo-branchial,  105,  107 

lateral  line,  97 

longitudinal  abdominal,  104 

median,  42 

of  metencephalon,  98 

neural,  94 

olfactory,  162 

parietal  eye,  134 

peripheral,  44,  94 
arrangement  of,  46 
differentiation  of,  45 

of  rostrum,  43 

Vagal,  107 
Nerve  ends,  in 

roots,  76 

haemal,  77,  81 
neural,  81 

Nervous  system,  framework  of,  43 
specialization  of,  44; 
summary  of,  209 
Neural  arches,  307,  322 

sinus,  328 

surface,  44 

surfaces,  orientation  of,  27,  221 
Neuritemma,  325 
Neuroblast,  52 

Neuro-coelia,  summary  of,  92 
Neuroglia,  93,  325,  331 
Neuromeres,  49 

branchial,  cell  clusters  of,  73 
specialization  of,  44 

cephalic,  80 

cell  clusters  of,  80 

components  of,  94 


Neuropile,  cardiac,  204 

centres,  79 

Neuropore,  anterior,  130 
Neurostoma,  18,  26,  31,  249,  251,  253,  406 
Notochord,  323 

termination  of,  65 

Occipital  ring,  316 
Ocelli,  larval,  125,  128 
Olfactory  ganglia,  170 

lobes,  55,  160,  164,  171 

of  limulus,  164 
organ,  127,  160,  162 
of  apus,  167 
of  branchipus,  166 
of  limulus,  1 60 
of  phyllopods,  165 
nerves,  162 
placodes,  162,  168 
Optic  ganglia,  13,  62 
Oral  arches,  16,  40,  255 

development  of,  in  frog,  257 
surface,  44 

Orientation  of  neural  and  haemal  surfaces,  27,  221 
Ostracoderms,  337,  341,  348 
auditory  organs,  356 
cephalic  appendages  of,  350 
cutaneous  sense  organs  of,  356 
eyes  of,  355 
historical  review,  342 
muscle  markings  in,  346 
olfactory  organs  of,  356 
skeleton  of,  351 
subdivisions  of  body,  349 
trend  of  development  in,  341 
exoskeleton  of,  352 

Parietal  eye,  13,  125 

ganglion,  142 

nerve,  134 

roots,  134 

Peribranchial  chamber,  428 
Phoroinda,  446,  447 
Polyzoa,  440 
Primitive  cumulus,  224 

streak,  22,  246 
Procephalon,  12 
Procephalic  lobes,  13,  38 
Prosencephalon,  54 
Psamosteidae,  366 
Pteraspida,  364,  366 
Pterobranchia,  439 
Respiration,  191 
Rhabdospleura,  440 
Rostrum,  14,  29 

nerves  to,  43 

Saccus  vasculosus,  18 
Segmental  sense  organs,  17 
Sense  buds,  cell  division  of,  51 

primitive,  50 

organs,  13,  no 

auditory,  120 

branchial  warts,  115 

general  cutaneous,  no 

lateral  line,  121 

primary,  38 

slime  buds,  17,  116 

special  cutaneous,  in 

summary  of,  209 


486 

Sense  buds,  temperature,  no 
Skeleton,  branchial,  322 
dermal,  289 
of  Ateleaspis,  295 
of  Limulus,  296 
of  Astracoderms,  290 
of  Pteraspis,  293 
of  Tremataspis,  290 
Stomodaeal  commissure,  19 
infolding,  38 
ganglia,  15,  60 
nerves,  42 
Stomodaeum,  42,  61 
Swallowing  reflexes,  189 

Tagmata,  26 
Taste  buds,  17 
Tear  duct,  260 
Teleostomii,  386 
Teloccele,  425,  433 
Telopore,  22,  35,  246 


INDEX. 


Thymus,  263 
Thyroid,  263 
Tracts,  general  cutaneous,  87 

gustatory,  84 

lateral,  86 

longitudinal  haemal,  85' 

neural,  86 

Tremataspis,  32,  359 
Triple  embryos,  284 
Trochosphere,  34 
Tunicates,  415 

Variation,  274 
Vascular  area,  24,  236 
Vagus  appendages,  19 

decussation,  191 

nerves,  20 

neuromeres,  19-67 

region,  19 
Vertebrates,  381 
Visceral  arches,  263 


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